ALBERT

Notes: Dielectric measurements have been carried out on partially hydrated collagen in the frequency ranges 100 kHz-5 MHz, 100 MHz-1 GHz, and 8-23 GHz. In the low-frequency range, a dispersion was observed around 100 kHz which results from inhomogeneous conductivity of the samples. A dielectric relaxation was observed aroud 0.3 GHz using time-domain-spectroscopy techniques. This relaxation can be considered to originate from mobile side chains. Microwave measurements indicate that the water relaxation may extend into the 10-GHz region. An apparent discrepancy between the main water relaxation time and the average rotational correlation time of water as measured by nmr line widths was resolved by the assumption that a fraction of the water molecules is bound to the collagen with residence times on the order of 10-6 sec, whereas the remainder of the water is only weakly bound and exhibits rotational rates on the order of 10-10 sec.

Notes: A model for the time dependence of DNA conformational state probabilities is formulated in the form of first-order differential equations. This model is applied to investigate the renaturation and denaturation rates for T2 and T7 DNA as reported in the series of experiments by Record and Zimm. Qualitative agreement is found in denaturation and for series of renaturation experiments with the same initial condition. However, partial agreement with series of renaturation experiments having the same final condition is obtained only by including an initial bimolecular step with properly matched pairs of strands. Comparison of all experiments with the calculated rates yields 5 × 104 min-1 as the step rate for melting a single base pair.

Notes: The conventionally protected oligopeptides of the two homologous series Boc-(L-Ile)n-OMe and Boc-(D-aIle)n-OMe (n = 2-6) were synthesized in a standard stepwise fashion and their uv and CD spectra in 2,2,2-trifluoroethanol, and solid-state ir spectra were investigated. In addition, two oligomeric products derived from the NCAs of L-isoleucine and of D-allo-isoleucine and having a DP of 20 and 12, respectively, were studied in the solid state by x-ray and ir. No substantial differences between the properties of the diastereomeric oligomers in the solid state were noticed, a β-structure being very likely at least for the Boc-protected hexapeptides and the higher oligomers. In contrast, differences were observed between the spectroscopic properties of the diastereomeric oligopeptides, and especially of the hexapeptides, in trifluoroethanol solution. The different properties of the hexapeptides in solution were related to the existence, in the case of Boc-(L-Ile)6-OMe, of soluble molecular aggregates in which the peptide chains assume the β-conformation. These results provide an additional example of the influence of the configuration of asymmetric carbon atoms of the side chains on the conformational properties of peptide molecules in solution.

Notes: The 1H-nmr chemical shifts and the spin-spin coupling constants of the common amino acid residues were measured in solutions of the linear tetrapeptides H-Gly-Gly-X-L-Ala-OH in D2O and H2O, the influence of X on the nmr parameters of the neighboring residues Gly 2 and Ala 4 was investigated. The titration parameters for the side chains of Asp, Glu, Lys, Tyr, and His were determined. The pKa values obtained in D2O, with the use of pH-meter readings with a combination glass electrode uncorrected for istope effects, were 0.06 pH units higher in the acidic range and 0.10 pH units higher in the basic range than the corresponding pKa values in H2O. This suggests that the present data are suitable “random-coil” 1H-nmr parameters for conformational studies of polypeptide chains in D2O and H2O solutions.

Notes: This paper shows that backbone amide proton titration shifts in polypeptide chains are a very sensitive manifestation of intramolecular hydrogen bonding between carboxylate groups and backbone amide protons. The population of specific hydrogen-bonded structures in the ensemble of species that constitutes the conformation of a flexible nonglobular linear peptide can be determined from the extent of the titration shifts. As an illustration, an investigation of the molecular conformation of the linear peptide H-Gly-Gly-L-Glu-L-Ala-OH is described. The proposed use of amide proton titration shifts for investigating polypeptide conformation is based on 360-MHz 1H-nmr studies of selected linear oligopeptides in H2O solutions. It was found that only a very limited number of amide protons in a polypeptide chain show sizable intrinsic intration shifts arising from through-bond interactions with ionizable groups. These are the amide proton of the C-terminal amino acid residue, the amide protons of Asp and the residues following Asp, and possibly the amide proton of the residue next to the N-terminus. Since the intrinsic titration shifts are upfield, the downfield titration shifts arising from conformation-dependent through-space interactions, in particular hydrogen bonding between the amide protons and carboxylate groups, can readily be identified.

Notes: We present a method that can reduce conformational energy calculations for an arbitrary peptide consisting of n residues (n-peptide) to the complexity of a computation for (Gly)n. This reduction, and the concomitant savings in computer time, is accomplished by replacing all side chains, as well as the backbone CαHα and CαH2α groups, by “interaction centers.” The backbone CONH group is left intact in order to preserve its directional character. The interaction centers “see” each other, and the atoms of the CONH group via Boltzmann and space-averaged effective center-center and center-atom potentials, respectively. This averaged-interaction method is tested on the repeat tetra-, penta-, and hexapeptides of elastin, Val-Pro-Gly-Gly (VPGG), Val-Pro-Gly-Val-Gly (VPGVP), and Ala-Pro-Gly-Val-Gly-Val (APGVGV), using the stereoalphabet strategy for the energy calculations. The excellent qualitative and quantitative agreement we obtain with both full atom-atom calculations and extensive nmr data, coupled with the order-of-magnitude reduction in computer time, augurs well for the potential usefulness of the method.

Notes: Linear response theory in the decorrelation or random-phase approximation is used to calculate the absorption and CD spectra of model helical polymers, including single-stranded polyadenylic acid. The method, which makes use of infinite polymer selection rules for the linear response tensor, has the advantages that (1) only a few three-dimensional matrices need be inverted; (2) spectral band shapes of the polymer arise naturally from those of the monomer, as well as from the geometry-dependent interactions in the helix; and (3) the spectral dependence on geometrical factors of the helix is made transparent. It is found that the structure of the polymer CD spectrum depends critically on monomer bandshape. An asymmetric CD spectrum, similar to some experimental spectra, arises from either a Gaussian or a composite monomer band. Single-stranded polyadenylic acid spectra are sensitive to helix geometry in the region 200-240 nm, in reasonable agreement with experimental spectra. This sensitivity arises from the 207-nm monomer transition, and the results suggest that this region of the spectrum should be more fully exploited as a tool for helix geometry studies.

Notes: C24H34N2O9, orthorhombic, P212121; a = 39.432 (10), b = 14.061 (5), c = 4.850 (2) Å, M = 494 a.m.u., Z = 4, Dm = 1.22 g cm-3, Dx = 1.22 g cm-3, R = 0.13 for 1205 observed reflections after refinement with isotropic thermal factors. The urethane and amide bonds are in the trans configuration, as well as all the ester groups. The ϕ and ψ angles of the L-glutamyl residues fall in the β-structure region of the Ramachandran's plot; the molecule is rather flat with the amide plane almost parallel to the c axis along which two hydrogen bonds hold the molecules together to form long rows in a “parallel pleated-sheet” fashion.

Notes: The far-ir absorption spectrum of lysozyme was measured at room and liquid-nitrogen temperatures. Dried layers of single crystals of tetragonal lysozyme chloride with a diameter of 100-300 μm were grown on a silicon plate. Such single-crystalline samples were considered to have the following advantages in obtaining far-ir spectra: (1) surface scattering is reduced, (2) the protein molecules are closely packed, and (3) air-drying of the crystals reduces the number of water molecules without considerably changing the original configuration. The spectrum obtained consisted of a strong background absorption and a number of absorption peaks that were not clearly observed with the sample in the form of lyophilized powder. The peaks were ascribed to various delocalized vibrations of the main and side chains in the molecule. The peaks were also compared with the positions of Raman lines. The uniform background was assigned to the water molecules remaining in the crystals.

Notes: The duplex-to-strand transition of the self-complementary sequence dG-dC-dG-dC has been probed at the exchangeable and nonexchangeable protons and backbone phosphates by high-resolution nmr spectroscopy. The Watson-Crick imino and amino hydrogen-bonded protons, as well as the exposed amino protons, could be followed through the duplex-to-strand transition and provide information on base-pair stability at the tetranucleotide duplex level. The magnitudes of the experimental upfield nonexchangeable base-proton chemical shifts on duplex formation are consistent with calculations based on base-pair overlap geometries of the B-DNA type. The variation of the 31P chemical shifts in dG-dC-dG-dC with temperature appear to monitor changes in the ω,ω′ rotation angles about the O—P bonds in the postmelting transition temperature region. The complex formed between the antitumor anthracycline antibiotic daunomycin and the dG-dC-dG-dC duplex was probed at the nucleic acid and the antibiotic resonances as a function of temperature. The experimental complexation shifts of the observable daunomycin resonances have put constraints on possible overlap geometries between the intercalating anthracycline ring and adjacent base pairs.

Notes: Conformational analyses of cyclic tetrapeptides consisting of alternating cis and trans peptide units have been made using contact criteria and energy calculations. This study has been restricted to those structures having a symmetry element in the backbone ring, such as a twofold axis (d) or a center of inversion (i). There are five main results. (1) There are two distinct types of conformations, which are stereochemically favorable corresponding to each of twofold and inversion-symmetrical structures, designated as d1, d2 (for twofold symmetrical) and i1, i2 (for inversion-symmetrical). Among these, the i1 type has the lowest energy when glycyl residues occur at all four α-carbon atoms. (2) With the glycyl residue at all four α-carbon atoms, methyl substitution at the cis peptide nitrogen atoms is possible in all the four types, whereas the substitution at trans peptide nitrogen atoms is possible only for the i1 type. Thus only in the i1 type can all the nitrogen atoms be methylated simultaneously. The conformation of the molecule in the crystal structure of cyclotetrasarcosyl belongs to the i1 type. (3) When alanyl residues occur at all four α-carbon atoms, the possible symmetrical type is dependent on the enantiomorphic form and the actual sequence of the alanyl residues. (4) The methyl substitution at peptide nitrogen atoms for cyclic tetrapeptides having alanyl residues causes more stereochemical restriction in the allowed conformations than with glycyl residues. (5) The prolyl residue can be incorporated favorably at the cis-trans junction of both d and i types of structures. The results of the present study are compared with the data on cyclic tetrapeptides available from the crystal structure and nmr studies. The results show an overall agreement both regarding the type of symmetry and the conformational parameters.

Notes: Based on equilibrium binding studies, as well as on kinetic investigations, two types of interactions of Cu2+ ions with native DNA at low ionic strength could be characterized, namely, a nondenaturing and a denaturing complex formation. During a fast nondenaturing complex formation at low relative ligand concentrations and at low temperatures, different binding sites at the DNA bases become occupied by the metal ions. This type of interaction includes chelate formation of Cu2+ ions with atoms N(7) of purine bases and the oxygens of the corresponding phosphate groups, chelation between atoms N(7) and O of C(6) of the guanine bases, as well as the formation of specific intestrand crosslink complexes at adjacent G°C pairs of the sequence dGpC. CD spectra of the resulting nondenatured complex (DNA-Cu2+)nat may be interpreted in terms of a conformational change of DNA from the B-form to a C-like form on ligand binding. A slow cooperative denaturing complex formation occurs at increased copper concentrations and/or at increased temperatures. The uv absorption and CD spectra of the resulting complex, (DNA-Cu2+)denat, indicate DNA denaturation during this type of interaction. Such a conclusion is confirmed by microcalorimetric measurements, which show that the reaction consumes nearly the same amount of heat as acid denaturation of DNA.From these and the kinetic results, the following mechanism for the denaturing action of the ligands is suggested: binding of Cu2+ ions to atoms N(3) of the cytosine bases takes place when the cytosines come to the outside of the double helix as a result of statistical fluctuations. After the completion of the binding process, the bases cannot return to their initial positions, and thus local denaturation at the G·C pairs is brought about. The probability of the necessary fluctuations occurring is increased by chelation of Cu2+ ions between atoms N(7) and O of C(6) of the guanine bases during nondenaturing complex formation, which loosens one of the hydrogen bonds within the G·C pairs, as well as by raising the temperature. The implications of the new binding model, which comprises both the sequence-specific interstand crosslinks and the described mechanism of denaturing complex formation, are discussed and some predictions are made. The model is also used to explain the different renaturation properties of the denatured complexes of Cu2+, Cd2+, and Zn2+ ions with DNA.In temperature-jump experiments with the nondenatured complex (DNA-Cu2+)nat, a specific kinetic effect is observed, namely, the appearance of a lag in the response to the perturbation. The resulting sigmoidal shape of the kinetic curves is considered to be a consequence of the necessity of disrupting a certain number of the crosslinks existing in the nondenatured complex before the local unwinding of the binding regions (a main step of denaturing complex formation) may proceed.

Notes: A new analysis has been made on studies of the influence of imino acid content on the changes of collagen thermal stability (tm). It is shown that, for the interstitial vertebrate collagens, there is a strict regularity in the changes of tm depending on hydroxyproline content. No correlation is observed between tm and proline content. Also, no correlation between tm and hydroxyproline content is observed for invertebrate and basement membrane collagens. On the basis of the reported data, the dependence of tm on hydroxyproline content is considered to be not a correlation between tm and the total content of hydroxyproline, but only as the correlation between tm and the content of hydroxyproline occurring at the third position in the sequence (Gly-R2-R3)n. The results agree with the idea that the influence exerted by proline and hydroxyproline on the stabilization of the triple helix of collagen is different.

Notes: The conformation and conformational transitions of poly(His-Ala-Glu) have been investigated by ir, nmr, and CD measurements. The results obtained - as well as the results of our previous investigations by potentiometric titration and hydrodynamic techniques [Goren et al., Biopolymers (1977) 16, 1541-1555] - indicate that when dissolved in water, the co-polymer assumes a disordered conformation. On changing the pH of the solution, the states of ionization of the side-chain imidazole and carboxyl groups change in the same manner as in low-molecular-weight model compounds. Concomitantly, the overall shape of the macromolecule is altered, while the conformation of the polypeptide backbone changes from one disordered state to another but never assumes a regular form. In water/methanol and water/trifluoroethanol mixtures, transitions from a disordered state to the α-helix conformation were observed on increasing the alcohol content of the system. The conformational transitions followed pathways which differ from one another according to the experimental conditions employed. Conformational landmarks (intermediates) have been identified along these pathways.

Notes: The decrease in the limiting viscosity number [η] with temperature T for hyaluronic acid in nonalkaline solution and chondroitin 4-sulfate in neutral and alkaline solutions may be expressed in terms of the temperature coefficient of the Kratky-Porod persistence length a: d ln a/dT = -0.0040 (±0.0005). The result, while numerically somewhat smaller, resembles qualitatively that of cellulose derivatives. As in the latter case, standard conformational calculations underestimate the coefficient, which may be due to neglect of random occurrence of local conformational features of higher energy. In alkaline solution, large decreases in [η] of hyaluronic acid are accompanied by a positive temperature coefficient of [η]. This temperature effect is interpreted as an endothermal shift from the alkaline, low [η] form of the polymer to the neutral, high [η] form with increasing temperature.

Notes: We have considered whether or not the tertiary structure of a biomolecule is the same in a crystal (or an oriented film) as it is in solution. A methodology has been developed for comparing polarized absorption spectra obtained from a solid-state sample with those obtained from an oriented solute to further resolve this question. An electric dichroism instrument built in our laboratory was used to measure the solution dichroism signal which, along with the ordinary solution uv absorption spectra, yields polarized absorption spectra in the directions parallel and perpendicular to the applied electric field. These were then compared to polarized absorption data from oriented films of nucleic acids to determine whether the two sets of data could be rotated into coincidence. This rotation was accomplished using a computer program based on a nonlinear programming method. Four nucleic acids were studied and the film and solution data for three of these were found to be equivalent, requiring rotation through an angle of 3°-20°, depending on film humidity, to bring them into coincidence. For the fourth sample we were unable, perhaps because of signal-to-noise ratio limitations, to find a correlation. Flow dichroism and electric dichroism data were also found to be quite similar. Thus it is clear that the induced dipole moment is along the helical axis and that the physical, hydrodynamical, and electrical axes of the nucleic acid molecules are equivalent.

Notes: The effect of enzyme-inhibitor complex formation on the hydration properties of the macromolecular moiety was investigated on the model system of α-chymotrypsin and its Ser-195 tosyl derivative. The primary (A-shell) hydration of the native and modified enzyme was compared by sorption measurements. The secondary (B-shell) hydration water was investigated by differential scanning calorimetry. Tosylation is known to induce pronounced conformational changes in the chymotrypsin molecule. These structural modifications have the following effects on the hydration of the native enzyme.The water binding capacity of the protein surface is significantly increased, as shown by both the calorimetric and the sorption results. The amount of unfreezable water of primary hydration is increased by 50 mol H2O/mol chymotrypsin.The heats (ΔH) and entropies (ΔS) of the interaction of water with chymotrypsin are strongly reduced in the modified enzyme. This effect is interpretable by a reduction of the H bonding potential of the protein surface. Parallel to this decrease in δH, the heats of fusion of the secondary hydration water (Qfus) are significantly increased by tosylation (Qfus = 256.2 ± 7.8 and 294.2 ± 4.8 J g-1 H2O for the native and the tosylated enzyme, respectively). This increase in Qfus reflects an increase in the extent of H bonding in the B-shell hydration sphere.These changes in the hydration of the native enzyme, associated with the reaction: native chymotrypsin → tosylchymotrypsin, are interpreted by cooperative phase transitions of water molecules in the primary and secondary hydration water. One of these transitions was found to exhibit a significant, linear enthalpy-entropy compensation effect. The compensation temperature \documentclass{article}\pagestyle{empty}\begin{document}$ \hat{\beta} $\end{document} is 290.7 ± 2.8°K. This \documentclass{article}\pagestyle{empty}\begin{document}$ \hat{\beta} $\end{document} value agrees well with compensation temperatures reported in the literature for a series of biochemical reactions in aqueous solution (250-320° K). This agreement in \documentclass{article}\pagestyle{empty}\begin{document}$ \hat{\beta} $\end{document} may point to a common source of both compensation phenomena.

Notes: The conformations of melanostatin have been studied experimentally using CD spectroscopy and via calculations. In aqueous solution and 2,2,2-trifluoroethanol (TFE) there is no evidence that monomers of the tripeptide exist in an ordered (β-bend) structure. In water and TFE solutions (3-6 × 10-4M) the neutral molecules aggregate very slowly, taking about 3 days to attain equilibrium at room temperature. At equivalent concentrations in TFE, although not in water, the cationic molecules also slowly aggregate, although to a lesser extent. Calculations using rotational isomeric state theory give the most probable unperturbed end-to-end distance of the molecule at 9.3 ± 0.1 Å and indicate that a vast majority of the molecules exist in some extended conformation, end-to-end distance ≥6 Å. Only 0.4% of the molecules are calculated to have O…H separations compatible with a β-bend structure. An intramolecular hydrogen bond must have an energy at least 2 kcal/mol lower than that of an intermolecular hydrogen bond to solvent if a β-bend is to be experimentally observable.

Notes: Nps-[Glu(OBzl)]6-NHEt has been prepared by coupling Nps-[Glu(OBzl)]2-OH with HCl,H-[Glu(OBzl)]4-NHEt by means of dicyclohexylcarbodiimide. The ir spectra of its nujol mull show that the hexapeptide has the β-structure of antiparallel chains. When it is dissolved in dioxane or ethylene dichloride, the hexapeptide consists of a mixture of the β-form and the solvated σ-form, but the β-form can exist only above a certain critical concentration. The critical concentration is about 0.4g dl-1 in dioxane and 0.08g dl-1 in ethylene dichloride, and the content of β-form increases with increasing concentration above it. The CD of the dioxane and ethylene dichloride solutions shows concentration dependence in visible and uv regions.

Notes: Light scattering from the solutions of Nps-[Glu(OBzl)]6-NHEt in dioxane or ethylene dichloride has been measured at different concentrations, and a critical concentration of intermolecular association is found to exist, which is equal to the critical concentration of β-form formation. The Debye plot of light scattering leads to the molecular weight of aggregates at the critical concentration, which corresponds to an aggregation number 15 in dioxane and 53 in ethylene dichloride. In the latter solvent the aggregates further associate into a larger aggregate consisting of 330 molecules when the concentration is increased beyond the critical concentration. The content of β-form, which is a measure of number of hydrogen bonds, is derived from the ir data previously obtained. The results on the modes of intermolecular association and hydrogen bonding lead to possible structures of aggregates formed by both hydrogen bonds and other nonbonding side-chain interactions.

Notes: Low shear viscosities have been determined for a 1 mg/ml poly(L-lysine) solution as a function of added salt concentration in the region of the previously reported ordinary-extraordinary phase transition. The measured viscosities indicate that the polyions are far from completely extended at the transition. Estimates of the longest internal relaxation time for an equivalent free-draining Rouse-Zimm chain give τ ≃ 10-5 sec, similar to that of the rapid, angle-independent component previously observed in the dynamic light-scattering correlation function at the transition. An unusual peak and valley are observed in the curve of [η]0 versus [NaBr] in the transition region. Possible interpretations of these features, and their bearing on the nature of the extraordinary phase, are discussed.

Notes: The IR data for the R1 CO-O-CHR2-CO-NHR3 derivatives are interpreted in terms of a H…π interaction involving the N—H bond and the π orbitals of the ester function and giving rise to a high ν(C=O) frequency and a low ν frequency. The resulting molecular conformation corresponds to the angular values φ # -90°, ψ # 0°. The H…π interaction in MeCO-L-Lac-NHMe is highly destabilized by water and aprotic solvents but is retained in methanol. Considering the high ν(C=O) ester or amide frequency of the middle function in β-folded depsipeptide or peptide sequences, it may be supposed that the residue indexed i + 2 in β turns experiences a H…π interaction which has a stabilizing effect on β turns. Some examples concerning valinomycin and some model compounds are discussed.

Notes: The crystal structure of the valinomycin analog, cyclo-[(-D-Val-Hyi-Val-D-Hyi-)3-] (meso-valinomycin, C60H102N6O18) has been determined by direct x-ray diffraction procedures. The crystals are triclinic, space group P1, number of molecules per unit cell Z = 1, and cell parameters a = 11.831, b = 13.815, c = 14.889 Å, α = 109.54°, β = 116.10°, γ = 98.89°. The atomic coordinates for the C,N,O atoms were refined in the anisotropic thermal motion approximation and for the H atoms in the isotropic approximation to R = 0.07.The structure is centrosymmetric and has a threefold axis of pseudosymmetry. The depsipeptide chain is in the form of a bracelet stabilized by six identical intramolecular 4 → 1 hydrogen bonds between the amide C=O and NH groups. The ester carbonyls are oriented towards the symmetry axis, their O atoms forming an ellipsoidal molecular cavity. The isopropyl side chains are located on the molecular periphery. The structure found differs considerably from the conformation of the crystalline naturally occurring antibiotic, valinomycin, but completely resembles that of valinomycin and meso-valinomycin in nonpolar solvents. In the crystal, meso-valinomycin molecules form stacks. The molecular cavities situated in the stacks one above the other along the pseudo-C3 axis form a continuous channel, the internal surface of which is lined by O atoms. The possible conformations of depsipeptides of the valinomycin series and their mode of action in membranes are discussed in the light of the data obtained.

Notes: The difference between the theories of Manning, on the one hand, and of Odijk and Skolnick and Fixman, on the other, for the polyelectrolyte contribution to the persistence length of DNA is shown to arise entirely from a subtle geometrical error in the theory of Manning. The corrected theory of Manning predicts a negligible polyelectrolyte contribution in 1.0M NaCl and only 33 Å in 0.01M NaCl, thus giving a change in total persistence length by a factor of only 1.07 over that range, in agreement with Odijk. Pertinent data in the literature indicate that the persistence length must change by a factor of ≤ 1.6 between 1.0 and 0.01M NaCl, and very likely by less than a factor of 1.4. Evidently, the intrinsic rigidity of the uncharged double-strand filament dominates the bending rigidity at NaCl concentrations above 0.01M.

Notes: The crystal structures of L-3,4-dehydroproline, t-butoxycarbonyl-L-3,4-dehydroproline amide, and acetyl-L-3,4-dehydroproline amide have been determined. L-3,4-Dehydroproline is orthorhombic with a = 16.756, b = 5.870, c = 5.275 Å, and Z = 4; t-butoxycarbonyl-L-3,4-dehydroproline amide is orthorhombic with a = 6.448, b = 8.602, c = 21.710 Å, and Z = 4; acetyl-L-3,4-dehydroproline amide is monoclinic with a = 4.788, b = 10.880, c = 7.785 Å, β = 105.25°, and Z = 2. The final R value for the L-3,4-dehydroproline is 0.046 based on 529 reflections; for t-butoxycarbonyl-L-3,4-dehydroproline amide, 0.050 based on 792 reflections; and for acetyl-L-3,4-dehydroproline amide, 0.058 based on 632 reflections. The structures clearly establish that the free amino acid exists in the zwitterionic form in the crystalline state. The molecular conformations of the t-Boc and acetyl derivatives consist of two planes: one involving the primary amide and the other the remaining atoms of the molecule. The acetyl-L-3,4-dehydroproline amide contains a tertiary amide bond in the cis conformation. To the best of our knowledge, this is the first example of a cis bond in an acetyl derivative of an amino acid or peptide. At variance with the previously reported proline amides, which present φ and ψ values corresponding to those of a right-handed α-helical conformation (conformation A), the t-Boc and acetyl derivatives both have φ and ψ values corresponding to a collagenlike conformation (conformation F).

Notes: The structure of thermally denatured Type I collagen has been studied using laser light scattering. The results indicate that the diffusion coefficients of α-chains and β- and γ-components are 1.550 ± 0.08 × 10-7, 1.000 ± 0.05 × 10-7, and 0.835 ± 0.04 × 10-7 cm2/sec, respectively, at temperatures between 20 and 40°C. It is concluded from diffusion data that these species have hydrodynamic radii of about 13.8 nm (α-chain), 21.5 nm (β-component), and 25.7 nm (γ-component), consistent with previous studies of thermal denaturation by light scattering. It is also concluded, based on volume calculations, that a large volume increase occurs when the triple helix unfolds. Homodyne correlation functions for two component mixtures of α-chains and β-and γ-components appeared to decay exponentially. In all but one case discussed the correlation function could be fitted with a single component having a translational diffusion coefficient which was an intensity weighted average of the diffusion coefficient of each component present.

Notes: Circular dichroic spectra and oscillator strengths of the π-π transition near 190 nm are calculated for helical (Gly)6 and (Ala)6 at 30° intervals of the backbone torsion angles (φ,ψ) over the range -180° ≤ φ ≤ -60°, -60° ≤ ψ ≤ 180°, using the partially dispersive normal mode treatment of the dipole interaction model. Polarizabilities of atoms and the NC′O group are those determined semiempirically in previous studies. Calculations for (Ala)6 at (φ,ψ) angles corresponding to the α-helix, the poly(Pro) II helix, a collagen single helix, a poly-(MeAla) helix, and single β-helices are found to agree well with most of the available experimental data.

Notes: The interaction of the fluorinated antimalarial drug fluoroquine [7-fluoro-4-(diethyl-amino-1-methylbutylamino)quinoline] with DNA, tRNA, and poly(A) has been investigated by optical absorption, fluorescence, and 19F-nmr chemical-shift and relaxation methods. Optical absorption and fluorescence experiments indicate that fluoroquine binds to nucleic acids in a similar manner to that of its well-known analog chloroquine. At low drug-to-base pair ratios, binding of both drugs appears to be random. Fluoroquine and chloroquine also elevate the melting temperature (Tm) of DNA to a comparable extent. Binding of fluoroquine to DNA, tRNA, or poly(A) results in a downfield shift of about 1.5 ppm for the 19F-nmr resonance. The chemical shift of free fluoroquine depends on the isotopic composition of the solvent (D2O vs H2O). The solvent isotope shift is virtually eliminated by fluoroquine binding to any one of the nucleic acids. 19F-nmr relaxation experiments were carried out to measure the spin-lattice relaxation time (T1), 19F{1H} nuclear Overhauser effect (NOE), off-resonance intensity ratio (R), off-resonance rotating-frame spin-lattice relaxation time (T1ρoff), and linewidth for fluoroquine in the nucleic acid complexes. By accounting for intramolecular proton-fluorine dipolar and chemical-shift anisotropy contributions to the fluorine relaxation, all of the relaxation parameters for the fluoroquine-DNA complex can be well described by a motional model incorporating long-range DNA bending on the order of a microsecond and an internal motion of the drug on the order of a nanosecond. Selective NOE experiments indicate that the fluorine in the drug is near the ribose protons in the RNA complexes, but not in the DNA complex. Details of the binding evidently differ for the two types of nucleic acids. This study provides the foundation for an investigation of fluoroquine in intact cells.

Notes: In carbonate buffer at pH 10.5, a transparent solution of poly(L-lysine HBr) was obtained up to fairly high concentration of 3 g/dl at room temperature. The hydrodynamic behavior of the solution has been studied by sedimentation analyses and viscosity measurements. A dimer form for high concentrations and a monomer form for low concentrations were inferred. The dimer and monomer forms were assigned to a β-structure and α-helix, respectively, based on the CD and optical rotary dispersion spectra. Using CD spectroscopy, a reversible transition between α-helix and β-structure was observed as a function of either poly(L-lysine HBr) concentration or temperature. An aggregated form which was assigned to the antiparallel pleated sheet appeared at 50°C on the basis of its ir spectrum.

Notes: Observations of induced circular dichroism (CD) bands in chloroform solution demonstrate the formation of specific, asymmetric complexes of the aromatic ligands 2-pyridone and 2,6-dichlorobenzoic acid with cyclic dipeptides of the general formula cyclo(L-Pro-X). The induced CD changes sign with the configuration of X due to subtle influences of the side chain on the geometry of the complex. Computations of interaction energies suggest that a plausible model for the complex of an aromatic ligand with the -CONH- of the cis secondary amide is a nearly planar arrangement of six heavy atoms in a ring containing two hydrogen bonds. The observed CD is matched by that computed for a tilt of the aromatic ligand toward the side chain of X. Binding constants were determined from the induced CD as a function of ligand concentration. For dichlorobenzoic acid these are about 450m-1 for the secondary amide and 50m-1 for the tertiary amide. For pyridone the binding constant is about 45m-1 for either the secondary or tertiary amide. For comparison self-dimerization constants determined by vapor-pressure osmometry in chloroform solution at 25°C are 870, 350, 50, and 20m-1 for pyridone, benzoic acid, dichlorobenzoic acid, and cyclo(L-Pro-Gly), respectively.

Notes: Proton and phosphorus nmr have been used to investigate the double-helical structures of polyriboadenylic acid [poly(A)] formed in acidic solutions (pH 〈 6). The results obtained at low pH (∼4.5) are consistent with the model for the acid poly(A) double helix proposed by Rich [Rich, A., Davies, D. R., Crick, F. H. C. & Watson, J. D. (1961) J. Mol. Biol. 3, 71-86]. Other models that have been proposed are inconsistent with the nmr data. The nmr measurements have also been used to examine the conformation of poly(A) helix in the half-protonated state. Although the base-stacking arrangement of this state is similar to that observed in the more extensively protonated low-pH state, the phosphate backbone conformation is different from that found in either the neutral or low-pH structures.

Notes: Large molecular weight bacteriophage G DNA, about five times larger than T2 DNA, was used to test Zimm's theory [(1974) Biophys. Chem. 1, 279-291] for the effect of rotor speed on the sedimentation of large linear monodisperse DNA. Sedimentation profiles from neutral sucrose gradinets at low and high rotor speeds show G DNA sedimenting from 1.8 to 0.7 times as fast as T2 DNA. Experimental measurements indicate that the sedimentation coefficient of G DNA decreases with increasing rotor speed about as fast as predicted by theory.

Notes: We studied the effects of some organic cosolvents (monohydric alcohols and amides) on the reaction of hemoglobin with oxygen. We present evidence showing that our data can be analyzed within the framework of the Monod-Wyman-Changeux model and that the main effect of cosolvents is to alter the T ⇄ R conformational equilibrium of hemoglobin, without significantly affecting the intrinsic oxygen dissociation constants. Following a previously described phenomenological approach, the overall effects have been separated into effects related to the variation of the bulk dielectric constant of the solvent and effects not related to the variation of this constant.

Notes: The spectroscopic properties (uv, CD, nmr) of histidine, glycylhistidine, histidylglycine, glycylhistidylglycine have been investigated in water and methanol in the temperature range 200-320 K in order to obtain information about their conformational equilibria. This analysis has been carried out for the different ionic forms of the compounds, in order to evaluate the influence of the ionization state of the carboxyl, histidyl, and amino groups on the rotamer distribution of the histidyl side chain (as evaluated from proton nmr analysis) and on the overall molecule (as judged from CD spectra). On the basis of certain approximations and from the temperature dependence of the proton nmr resonance, the thermodynamic parameters (ΔH° and ΔS°) characterizing the conformational equilibrium of the hystidyl side chain have been evaluated for the different structures and ionization states. Relatively large entropy differences between the rotamers are obtained in some cases. The data of the sidechain rotamer population, as determined by nmr, have been analytically correlated with the CD data, and in the case of hystidine and histidylglycine in basic solution, first-approximation values for the ellipticity of the single conformers have been evaluated. Finally, in the example of glycylhistidine and histidylglycine in basic solution, it is shown how the data obtained from the different experimental approaches (nmr and CD), as well as from theoretical energy calculations, converge to characterize the most stable conformation in solution.

Notes: Photon-correlation spectroscopy is a powerful technique for measuring the translational diffusion coefficient of particles and macromolecules in solution. In the study described here, this technique was used to analyze a specific dimerization process involving the association of two tRNA molecules through complementary anticodons. The tRNAs used in the analysis were E. coli tRNA2Glu and yeast tRNAPhe. The experimental data on the concentration dependence of the observed diffusion constants are shown to agree well with theoretical predictions. From these data, the equilibrium constant of the association reaction was determined for dimers formed over a wide range of temperatures and in several different solution conditions. In solutions of 0.1M ionic strength at 22°C, the equilibrium constants vary from 1 × 105M-1 in the absence of magnesium to 1.5 × 106M-1 in 10 mM Mg+2. The enthalpy and entropy changes for dimer formation in the absence and presence, 5 and 10 mM, of magnesium have been obtained from the temperature dependence of the equilibrium constant. The results show that both ΔH and ΔS contribute to the free energy of binding and that their relative contributions are similar for each solution condition evaluated.

Notes: A conformational quantum-mechanical study of (Gly-Phe-Pro) and (Gly-Pro-Phe) repeating tripeptide sequences has been carried out with the PCILO method. Using appropriate molecules as a model, we investigated the conformational possibilities of each in situ residue. Computations have been done taking into account the two typical pyrrolidine ring puckering and the most favorable orientations of the phenylalanyl side chain. Major conclusions drawn from this study are that the phenylalanyl can be accommodated at both second and third positions in the sequence without preventing the formation of triple-helix conformation. However, the analogy observed between the rotational possibilities around the second residue of Gly-Pro-Pro and Gly-Phe-Pro indicates that phenylalanyl in the second position favors the triple-helix formation.

Notes: The structure of the nucleosome core particle in solution has been studied by neutron scattering using the full-contrast variation technique, which reduces the experimental spectra to three fundamental scatter functions holding information on shape and structure. Systematic calculations of the fundamental scatter functions expected from proposed core-particle models have been compared with the observed functions and show that the neutron-scattering criteria severely restrict the number of models which can be valid for the structure in solution. The best model for the core particle in solution has a hydrophobic histone core about which 1.7 ± 0.1 turns of DNA are wrapped at a pitch between 3.0 and 3.5 nm. This core contains most of the histone and has an average thickness of 4 nm and diameter 6.4-7.5 nm. While solution scattering is not able to specify uniquely the actual shape of the core to high resolution, all models which are possible for the shape of the core to a resolution justified by the data have been considered. It is clear that cylindrical or wedge shapes compatible with the above dimensions are valid structures. A hole probably penetrates the histone core, but the data do not allow a diameter greater than 1 nm. Available evidence suggests that about a quarter of the total histone is outside the core.

Notes: The relationship between published vicinal proton-proton coupling constants and the pseudorotation properties of the pyrrolidine ring in L-proline, 4-hydroxy-L-proline, 4-fluoro-L-proline, and several linear and cyclic model proline peptides is investigated. Compared to earlier studies, several important improvements are incorporated: (1) a new empirical generalization of the classical Karplus equation is utilized, which allows a valid correction for the effects of electronegativity and orientation of substitutents on 3JHH; (2) an empirical correlation between proton-proton torsion angles and the pseudorotational parameters P and τm is derived; and (3) the best fit of the conformational parameters to the experimental coupling constants is obtained by means of a computerized iterative least-squares procedure. Two pseudorotation ranges were considered, classified as type N (χ2 positive sign) and type S (χ2 negative sign). The conformational equilibrium is fully described in terms of four geometrical parameters (PN, τN, PS, τS) and the equilibrium constant K. The present results indicate that, in general, the geometrical properties found in x-ray studies of proline and hydroxyproline residues are well preserved in solution. Several novel features are encountered, however. It is demonstrated that the proline ring occurs in a practically 1:1 conformational equilibrium between well-defined N- and S-type forms. Introduction of an amide group at the C-terminal end has no observable effect on this equilibrium, but the formation of a peptide bond at the imino nitrogen site results in a pronounced, but not exclusive, preference for an S-type form which is roughly 1.1 kcal/mol more stable than its N-type counterpart. The hydroxyproline ring system in neutral or acidic medium displays a pure N-type state, but N-acetylation results in the appearance of a minor (S-type) conformation. Cyclic proline dipeptides similarly exist in a biased conformational equilibrium. The major form (77-88%) corresponds to the N-type conformer observed in the solid state; the minor S-form has not been observed before. In contrast, cyclic hydroxyproline dipeptides display complete conformational purity. Ranges of endocyclic torsion angles deduced for the various classes of pyrrolidine derivatives in solution are presented. Each torsion appears confined to a surprisingly narrow range, comprising about 4°-8° in most cases. In all, the proline ring is far less “floppy” than hitherto assumed.

Notes: Measurement of the equilibrium distribution of persistence length fragments of DNA in high concentration in the ultracentrifuge shows that the reduced osmotic pressure rises much faster than linearly. From analysis of the data in terms of the Zimm cluster integral we infer that the net interactions between helices are purely repulsive at all distances. A theoretical equation of state derived from scaled particle theory with one adjustable parameter is in excellent agreement with the experimental data so long as the salt concentration is not excessively low. The parameter represents the hard-core radius in a simplified approximation to the potential function for the electrostatic repulsion between helices. Its value depends on the salt concentration, and it shrinks at high salt to a radius in close agreement with direct structural estimates. At a particular value of the osmotic pressure that is only slightly salt dependent, the solution undergoes a reversible transition to a denser, turbid, optically anisotropic phase. The relation between DNA volume fraction, including the electrostatic radius, at the transition point and the effective asymmetry of the molecules as a function of salt is in approximate correspondence with various theoretical treatments. However, the experimental function extrapolates to the correct limit for spherical particles. The work needed to bring DNA to a high concentration is estimated. The results suggest that the phase transition is first order.

Notes: We have examined the NH stretching frequencies of N-acetyl-N′-methyl-L-alanineamide (blocked Ala), N-acetyl-N′-methylglycineamide (block Gly), and N-acetyl-N′-methyl-L-leucineamide (blocked Leu) in chloroform using irspectroscopy. Their spectrum of blocked Leu in carbon tetrachloride was also obtained. A major absorption band at 3450 cm-1 is attributed to the unperturbed NH stretching frequency. Another major band at 3437 cm-1 (for Ala) or 3432 cm-1 (for Leu) is attributed to conformations in which the NH stretching frequency is perturbed by the spatial proximity of the Cβ atom. An absorption band between 3300 and 3370 cm-1, which has in the past been assigned to the intramolecular hydrogen-bonded NH in the C7eq conformation, was found to be concentration dependent and could not be observed below 5 × 10-4M in chloroform; thus we find no evidence for a strongly hydrogen-bonded NH in the C7eq conformation in chloroform. An absorption band at 3416 cm-1 was observed in chloroform solutions of blocked Gly, and a similar absorption appeared as a shoulder on the 3437- and 3432-cm-1 bands of blocked Ala and blocked Leu, respectively, in the same solvent. These bands, occurring near 3416 cm-1, may be assigned to extended (C5) conformations [Avignon et al., Biopolymers 8, 69 (1969)]. In CCl4 the spectrum of blocked Leu remained concentration dependent below 2.8 × 10-4M, with the 3300-3370-cm-1 band progressively weakening and shifting to higher frequencies on dilution from higher concentrations. Analysis of the spectra indicates that there is considerable flexibility in the blocked single residues, in agreement with the results of conformational energy calculations.

Notes: Type I collagen fibrillogenesis in vitro has been studied by laser light scattering, and the results indicate that initiation of aggregation involves at least two steps. Step I of aggregation involves no change in the intensity of scattered light at an angle of 90° and is accompanied by a decrease in the diffusion coefficient. Step II is characterized by an increased intensity of scattered light and decreased diffusion coefficients. Theoretical calculations using the Stokes-Einstein equation for the translational diffusion coefficient and the Perrin equation for the frictional coefficient of a prolate ellipsoid indicate that the step I aggregates are 4D staggered linear dimers and trimers 570 and 845 nm long, whereas step II aggregates are greater than 950 nm in length. These dimensions are similar to those previously reported based on physicochemical measurements and electron microscopy. It is proposed that the rate and extent of fibrillogenesis in vitro is controlled by the concentration of the linear aggregates and that the effects of temperature and collagen concentration on fibrillogenesis previously observed are qualitatively explained in terms of their effects on the concentration of these aggregates.

Notes: The method hitherto used for estimating the electrostatic term in empirical intramolecular calculations of stable conformations of biologically important molecules and macromolecules and intermolecular calculations of molecular associations or packing energy in molecular crystals had been analyzed. It has been shown that the contribution of atomic hybridization moments is omitted in the calculation of electrostatic interactions from net atomic charges localized on nuclei which have been determined by standard quantum-chemical methods. This contribution plays an important part in determining electrostatic interactions, mainly in molecules containing atoms with lone pairs. Simultaneously, a modified method for calculating the electrostatic term comprising the interaction of the lone pairs, which are represented by atomic hybridization moments, has been proposed. The relationship between the atomic hybridization moment and the bond angle has been expressed for some typical configurations occurring in biologically important molecules. Finally, this new approach is illustrated by results of the conformational analysis of some model compounds for biomolecules and compared with the approach used so far for the estimation of the electrostatic interaction in empirical methods of calculation of the intra- and intermolecular energy.

Notes: Intensity fluctuations of laser light scattered from filamentous viruses Pf1 [length L (Å) × diameter d (Å) = 20,000 × 90], M13 (9000 × 90), potato virus X (5150 × 130), and tobacco mosaic virus (3000 × 180) in sucrose density gradients were measured with a photon correlation spectrometer over a range of scattering angles from 15° to 120°. The experimental data can be approximated by two exponential decays, “slow” and “fast.” The slow decay rate constant ts-1 corresponds to the translational diffusion D of the virus, i.e., ts-1 = K2D, where K is the magnitude of the scattering vector. The amplitude of the slow component, i.e., translational diffusion, remains greater than that of the fast component, even at high KL. The fast decay rate constant tf-1 is also proportional to K2 for viruses such as Pf1, M13, and even potato virus X. In the companion paper, we shall attribute the amplitude enhancement of the translational diffusion to the coupling of its anisotropy to the rotational diffusion modes. In order to explain the excessive decay rates in the fast component, we need to consider the bending mode of rodlike viruses, especially in the longer viruses such as M13 and Pf1, in addition to the usually expected rotational diffusion modes.

Notes: We have compared four theoretical effects of rodlike macromolecules with the fast components, i.e., components other than translational diffusion, of our experimental data, which are presented as amplitude autocorrelation functions of electric field scattered from dilute solutions of monodisperse rodlike viruses with lengths from 3300 Å for tobacco mosaic virus to 20,000 Å for Pf1. The four effects are (1) the optic anisotropy treated by Aragón and Pecora, (2) coupled translational-rotational diffusion due to anisotropy in translational mobility recently reformulated by Gierke, (3) anisotropic rotational diffusion with respect to the direction of translational displacement first discussed by Berne and Pecora, and (4) the bending mode of a rod by Fujime and Maruyama. We show that both the first and second effects are required to explain the enhancement of amplitude of the translational diffusion at the expense of fast components. The experimental decay rates of the fast component exceed that of the rotational diffusions. In order to explain the excessive decay rate in the fast component, we need to include a minute amount (∼1%) of bending mode of rodlike viruses, especially in longer viruses such as M13 and Pf1.

Notes: The previous report that poly(L-glutamic acid) exhibits doubled resonances in the helix-coil transition region by either proton or carbon-13 nmr resolves the question of whether or not this behavior is limited to uncharged polypeptides in organic solvents, as had been previously thought. In the present work, we show that the underlying principle causing this anomalous double-peak behavior is due to molecular-weight polydispersity of the sample. The molecular-weight range in which this phenomenon is observed is largely dependent on the values of σ, the nucleation or cooperativity factor. The principles developed are shown to encompass all classes of polypeptides in a very natural way and to explain the key experimental data in the literature.

Notes: Empirical conformational energy calculations with the use of ECEPP energy functions have been carried out for linear dipeptides H-X-L-Pro-OH, with X = Gly, L-Ala, D-Ala, L-Leu, D-Leu, L-Phe, and D-Phe, in different states of protonation of the end groups. The results of these calculations are compared with the previously reported experimental equilibrium populations for the cis and trans isomers of the X-Pro bond in the different species. For all the protonation states of the seven dipeptides, the calculated nonbonded interactions and the conformational entropy term lead to a preference of the trans forms over the cis isomers by at least 1 kcal/mol. The electrostatic interactions stabilize the cis conformations in all species except the cationic forms of the D,L-peptides, and it could further be shown that only the carbonyl group of X and the two end groups contribute significantly to the total electrostatic energy. One of the principal results of the experimental studies, i.e., the occurrence of 5-15% cis-proline in all the peptides with an uncharged C-terminus, was corroborated by our investigation of the cationic species. A detailed assessment of the electrostatic contribution to the total energy of the different conformations of H-Gly-L-Pro-OH indicates that the standard ECEPP parameters tend to overestimate the electrostatic interactions in aqueous solutions of the X-Pro dipeptides.

Notes: The secondary structure of the lac repressor protein proposed by Chou et al. has been modified to include the recent revisions in sequence. In addition to the Chou and Fasman method, five other methods were used; they include those of (1) Lim, (2) Ptitsyn and Finkelstein, (3) Burgess et al., (4) Bunting et al., and (5) Wu and Kabat. Any two individual methods gave results differing sharply from one another. Three or more methods were in agreement for 91, 39, and 126 residues in helix, in β, and in combined coil plus turn conformations, respectively; there were such agreements for a total of 256 of the 360 residues. Agreements in the amino-terminal third of the molecule were found for 68% of the residues, whereas in the remainder of the molecule only 53% of the residues showed such agreements. Only two helix-breaking and two β-breaking tripeptides were inconsistent with the composite predictions by three or more methods. The large number of disagreements among the results for different methods indicates that only very limited information is provided by each method and that the basis on which they operate is not clear. There is no a priori reason for a composite prediction to be more reliable than any individual prediction, and such a procedure does not permit the determination of an unambiguous secondary structure. Since these methods were applied to lac repressor before any three-dimensional crystallographic structure was known, the methods may ultimately be evaluated should such a structure become available.

Notes: The conformational changes and binding behavior of tetranactin on complexation with sodium, potassium, rubidium, cesium, and ammonium ions were investigated by the measurements of proton magnetic resonance, ir, and Raman spectra. It has been clearly shown that alkali cations coordinate to the oxygen atoms of both the carbonyl group and the tetra-hydrofuran ring, but the ammonium ion coordinates only to the oxygen atom of the tetrahydrofuran. Among the alkali cations the potassium ion most strongly coordinates to the tetrahydrofuran oxygen atoms. The complexation with larger cations induces an expansion of the cavity of the macrocyclic ring of tetranactin and smaller cations contract the cavity. The evidence is revealed by the coupling constants of the methylene protons and the frequency separation between the carbonyl stretching vibrations of the ir- and Raman-active modes. The conformations of the cation complexes in the solid are maintained in solution but that of the cation free form is not.

Notes: This report presents a quantitative test of the ability of the counterion condensation theory to describe the proton-induced lowering of DNA melting temperature. From a general approach of Record et al. [Record, M. T., Anderson, C. F. & Lohman, T. H. (1978) Q. Rev. Biophys. 11, 103-178], we have obtained an expression that may be computer-fitted to the experimental data by numerical minimization of χ2. To do this, in addition to the assumptions made by Record et al., it was necessary to suppose that the interchange between protons and sodium is independent of pH and, due to the absence of data, take the enthalpies of protonation as thermally independent over the experimental temperature range. The dependences of the enthalpy of denaturation at neutral pH on sodium concentration and on G + C content were taken from literature. In the fitting process we have used 250 melting temperatures obtained at different pH and sodium concentrations for various natural DNAs. The theoretical expression gives a good quantitative description of the G + C and sodium concentration influences on the phenomenon but is only qualitative with respect to the dependence of dT/d log[Na+] on the pH. The adjusted pK values for the bases in denatured DNA agree with those for isolated deoxynucleosides. Interchange between sodium and protons is found to be less than 1:1. Calculated protonation enthalpies are ill-defined because of their low numerical influence. In short, it appears that the theory gives a good description of most of the aspects of the phenomenon even if it has some shortcomings, perhaps due to the great number of assumptions.

Notes: We studied the kinetics of replacement of O2 by CO in hemoglobin in the presence and absence of organic cosolvents (methanol, ethanol, iso-propanol, n-propanol, formamide, acetamide, N-methyl-formamide) and at 10 and 25°C. Quantitative analysis of the results indicates that these cosolvents do not affect the intrinsic binding constants of ligands to the heme when hemoglobin is in the R conformation. The present results confirm the previously reported suggestion that the effects of the above cosolvents on the oxygen affinity of hemoglobin are related to effects on the T ⇄ R conformational equilibrium.

Notes: Preparations have been made of acid-soluble collagens whose telopeptides have suffered different levels of proteolytic attack. The collagens with more intact telopeptides form fibrils more rapidly than those with degraded telopeptides. In addition, we have shown that a high molecular weight aggregate rich in the carboxyterminal CNBr peptide, α1CB6, can be found in cyanogen bromide digests of fibrils formed from intact collagen. A similar aggregate is found in CNBr digests of native tendons. The aggregate formed in fibrils assembled in vitro can be stabilized by reduction, and its generation is strongly dependent on the presence of intact telopeptides. The latter point is the most objective evidence that to reproduce the characteristics of native fibrils in vitro, the collagen telopeptides must be preserved from proteolysis.

Notes: We present an alternative to the common lattice model for nonspecific DNA-protein interactions by using ligands that translate freely along the polynucleotide instead of binding to distinct lattice sites along the polynucleotide chain. The general model we present corresponds to a one-dimensional continuum gas and is referred to as the “continuum model” to distinguish it from the general lattice model. Explicit expressions are obtained for the binding isotherm equation for two version of the continuum model, including the effects of binding-site exclusion and attractions between bound ligands. Theoretical results are compared to those obtained from the McGhee-von Hippel (1974) analysis of the lattice model with cooperative interactions between ligands occupying more than one lattice site. Practical applications of the continuum model are illustrated by analyzing (i) the noncooperative binding to single-stranded DNA by RNase (Jensen and von Hippel, 1976), and (ii) the highly cooperative binding to poly(rA) by a proteolyzed fragment of the gene 32 protein of phage T4 (Lonberg et al., 1981).

Notes: Intercalation-site geometries are generated for a tetramer duplex extracted from B-DNA. Glycosidic angles and puckers of the deoxyribose sugar groups bonded to base pairs BP1 and BP4, namely, those at either end of the tetramer duplex, are assumed to be those of B-DNA to insure continuity. All possible geometrical conformations for combinations of C(2′)-endo, C(3′)-endo, C(2′)-exo, and C(3′)-exo sugar puckers are determined for the tetranucleotide backbone. Those with minimum energy are selected as candidates for intercalation sites. Calculations reveal two pairs of physically meaningful families of intercalation sites which occur in two distinct regions, I and II, of helical angles which orient BP2 relative to BP3 and with the helical axis disjointed between these base pairs. For each site I and II within BP2 and BP3, there are two distinct backbone conformations, A and B, connecting BP3 to BP4 or BP1 to BP2 which do not disrupt backbone conformations connecting BP2 to BP3. Hence two pairs, IA and IB, and IIA and IIB, of intercalation sites exist in which the sugar puckers along the backbone of the tetramer alternate from C(2′)-endo to C(3′)-endo on the backbone (5′p3′) connecting BP2 to BP3. The glycosidic angles of the C(3′)-endo sugar χ3γ are, coincidentally, 80° ± 2° for both conformations γ = A and B connecting BP3 to BP4 along the phosphate backbone (5′p3′). Consistent with the theoretical results, the experimental unwinding angles can be grouped into two categories with absolute values of 18° and 26°. The theoretical unwinding angles for sites IA and IB of 16° and for sites IIA and IIB of 20° occur for a displacement of -0.8 Å in the helical axes of BP2 and BP3 and for a 100% G·C composition, with a decrease depending on the amount of A·T base pairs present. Ratios of theoretical unwinding angles of sites I and II, which range from 0.75 to 0.84 for the two principal sites, compare well with the experimental value of 0.71. The theoretical results, in agreement with experimental observation, provide a new interpretation of the nature and conformation of the possible binding sites. Conformations obtained from these studies of intercalation sites in a tetramer duplex are used to rationalize the well-known neighbor-exclusion principle. The possibility of violation of this principle is demonstrated by the existence of two families of physically meaningful conformations. Conformations of unconstrained dimer duplexes are also obtained, one of which corresponds to the experimental crystal structure of ethidium-dinucleoside complexes, but these cannot be joined to the B-DNA structure. Backbone conformations of the tetramer duplex can be constructed until the base-pair separation reaches 8.25 Å, which may limit the molecules that can intercalate.

Notes: As a continuation of previous papers [Biopolymers (1976) 15, 879; (1978) 17, 1508], the low-frequency dielectric relaxation of DNA solutions was studied with a four-electrode cell and the simultaneous two-frequency measurement. Below a critical concentration, the dielectric relaxation time agrees with the rotational relaxation time estimated from the reduced viscosity and is almost independent of DNA concentration Cp, and the dielectric increment is proportional to Cp. The critical concentration is approximately 0.02% of DNA for molecular weight Mr 2 × 106 and 0.2% for Mr 4.5 × 105 in 1 mM NaCl. Dielectric relaxations are compared for samples before and after deproteinization, and the protein contamination is found to have a minor effect on the dipole moment of DNA. The effect of a mixed solvent of water and ethanol on the dielectric relaxation of DNA is well interpreted in terms of changes in viscosity and the dielectric constant of the solvent, assuming that the relaxation arises from rotation of the molecule with a quasi-permanent dipole due to counterion fluctuation.

Notes: The structure of water and its interaction energy with a fragment of B-DNA composed of 12 base pairs and of the corresponding 24 sugar and 22 phosphate units and Na+ ions (one at each phosphate group) are analyzed using Monte Carlo simulations. The sample of water molecules, at the simulated temperature of 300 K, is composed of 447 water molecules. The results are discussed either in terms of statistical analyses over the 2,000,000 simulated conformations (after equilibration) or with reference to an “average configuration.” Comparison is made to a simulation previously presented for the same system but without counterions. Isotherm at different relative humidity, hydration, and reactivity scales for different sites, the hydration number at each site, the structure of intraphosphate and interphosphate hydrogen-bonded filaments of water are reported and discussed. The stabilization of the B-conformation induced by the solvent with counterion (“ion-induced compression effect”) is analyzed on the base of the above findings. A preliminary model to predict conformational transition in DNA is presented. The analyses reported are very detailed to allow refined interpretations of spectroscopic (infrared, Raman, and nmr) and scattering (x-ray and neutron beam) data on DNA insolution.

Notes: The conformational response of calf thymus DNA to solvent conditions altered by varying amounts of ethanol and NaCl has been monitored by circular dichroism (CD). These measurements, which extend over a much greater range of conditions than previously examined, reveal (above critical concentrations of ethanol and salt) a condensed form of the macro-molecule with unusually large positive ellipticity in the 250-300-nm region [the Ψ(+) state]. Mere increase in NaCl concentration at constant 35% (v/v) concentration of ethanol suffices to convert such Ψ(+) samples - via a series of intermediate forms with CD spectra resembling those of A-DNA, then B-DNA - into Ψ(-) states having anomalously large negative ellipticity similar to the well-known Ψ(-) forms produced by above-critical concentrations of poly-(ethylene oxide) and salt. These ethanol/salt-induced transitions are all completely reversible and can occur without formation of any visible precipitates. We suggest that they represent predominantly tertiary structural changes of B-form DNA molecules analogous to the changes which occur in several other systems where Ψ(+) ⇌ Ψ(-) interconversion has been reported. A “skein-of-yarn” model for the condensed tertiary (and quaternary, i.e., aggregated) state of the DNA affords one possible explanation for the inversions of ellipticity in all these cases. Such a model accords well with the accepted description of cholesteric liquid crystals.

Notes: Conformational energies of α- and β-D-glucopyranoses were computed by varying all the ring bond angles and torsional angles using semiempirical potential functions. Solvent accessibility calculations were also performed to obtain a measure of solvent interaction.The results indicate that the 4C1 (D) chair is the most favored conformation, both by potential energy and solvent accessibility criteria. The 4C1 (D) chair conformation is also found to be somewhat flexible, being able to accommodate variations up to 10° in the ring torsional angles without appreciable change in energy. Observed solid-state conformations of these sugars and their derivatives lie in the minimum-energy region, suggesting that the substituents and crystal field forces play a minor role in influencing the pyranose ring conformation. Theory also predicts the variations in the ring torsional angles, i.e., CCCC 〈 CCCO 〈 CCOC, in agreement with the experimental results. The boat and twist-boat conformations are found to be at least 5 kcal mol-1 higher in energy compared to the 4C1 (D) chair, suggesting that these forms are unlikely to be present in a polysaccharide chain. The 1C4 (D) chair has energy intermediate between that of the 4C1 (D) chair and that of the twist-boat conformation. The calculated energy barrier between 4C1 (D) and 1C4 (D) conformations is high - about 11 kcal mol-1.

Notes: Earlier determinations of density gradient proportionality constants β0, density distributions ρ(r), and the effect of pressure on density gradients in the analytical ultracentrifuge have been of limited precision and usefulness in the study of proteins and polypeptides. Reasons for these difficulties are that numerous intermediate relationships were required in the calculations, and the density ranges studied were generally above 1.2 g/ml. Relations are derived in the present paper to directly compute β0(ρ) values and β0′(ρ) values from the original data without any intermediate expansions or approximations. Data are presented for CsCl, CsBr, and Cs2SO4 and compared with literature values. Density distributions are computed for all three salts under a wide variety of experimental conditions of density, column length, and angular velocity. These values of ρ(r) and re are obtained by a numerical iterative technique. Values obtained by this new method are compared with values obtained using closed-form expressions. The effects of pressure on the composition density gradient for the three salts given above are calculated and found to be significant for Cs2SO4 solutions.

Notes: We present evidence for structures of two ordered forms of polyxanthylic acid based on ir spectroscopy, pH titrations, and thermal transitions. Over the pH range ∼6-9.5, the structure is a four-stranded helix with alkali metal ions specifically complexed in the central channel. These internal counterions stabilize the structure by complexing with carbonyl oxygens and by partial screening of electrostatic repulsion caused by ionization of the xanthine residues in this pH range. Below pH 5, the structure is quite different and much more stable. Our data are consistent with a six-stranded helix in which both carbonyl oxygens and both NH protons are hydrogen bonded.

Notes: 1H-Nmr was used to measure the rate of cis-trans interconversion of X-Pro bonds in linear and cyclic oligopeptides. k(cis → trans) = 2.5 × 10-3 s-1 at 25°C was found for the zwitterionic form of H-Ala-Pro-OH, in good agreement with earlier measurements. Replacement of Ala by Phe, Tyr, or Trp resulted in a 10-fold slower interconversion rate, whereas after substitution of Ala by His or Glu, the rate decreased only slightly. Independent of the residues X, the interconversion rate was increased by a factor of ca. 20 when the peptide chain was elongated by addition of Ala to the C-terminal Pro. An additional increase by a factor of 6 was observed when going from the protected linear peptide CF3CO-Gly-Gly-Pro-Ala-OCH3 to the closely related cyclic compound c[-Gly-Gly-Pro-Gly-Ala-]. These data are evaluated with regard to their possible use in future studies on the role of X-Pro cis-trans isomerization in the kinetics of protein folding.

Notes: A molecular theory of protein secondary structure is presented that takes account of both local interactions inside each chain region and long-range interactions between different regions, incorporating all these interactions in a single Ising-like model. Local interactions are evaluated from the stereochemical theory describing the relative stabilities of α- and β-structures for different residues in synthetic polypeptides, while long-range effects are approximated by the interaction of each chain region with the averaged hydrophobic template. Based on this theory, an algorithm of protein secondary structure prediction is proposed and examples are given of “blind” predictions made before the x-ray structural data became available.

Notes: To increase our understanding of peptide-water interactions, we are simulating the behavior of water molecules in the intermolecular channels of [Phe4Val6]antamanide dododecahydrate crystals. There is good overall agreement between the positions predicted using two alternative potential functions and those that have been observed by x-ray diffraction. Detailed differences between the predictions for the two potential functions are discussed.

Notes: Experimental measurements of disulfide bond stability at various stages of protein folding are considered in terms of the effective concentrations of the thiol groups relative to each other; values of up to 107M are observed, so that intramolecular interactions within the interior of a protein are much more stable, and provide greater stability to the folded conformation, than those on the surface or in a flexible segment. Intramolecular interactions can have substantially lower free energies than intermolecular, for solely entropic reasons; this implies that polar interactions, such as hydrogen bonds and salt bridges, can provide net stabilization to a folded conformation, in spite of the unfolded protein having intermolecular interactions with the solvent. These considerations can account for the lower free energy and enthalpy of the folded state and are useful for considering protein flexibility.

Notes: Near- and far-uv CD spectra of microtubule protein preparations have been examined to study the possible role of protein conformation in relation to the kinetics of the self-assembly of these proteins into microtubules in vitro. Although tubulin can form conformations with high helical content under apolar solution conditions, this transformation is apparently not involved in self-assembly. There is no major perturbation of tubulin near-uv CD by reagents and solution conditions favoring assembly. Thus, in these preparations, tubulin, as dimer and as oligomer with MAPs, is effectively in the conformation in which it undergoes self-assembly. This conclusion is consistent with a hybrid model of assembly of microtubule protein involving direct incorporation of oligomeric species as an alternative to the condensation polymerization of tubulin dimer as the exclusive assembly mechanism.

Notes: The ligand-modulated kinetics of the autoproteolysis of thermolysin and the high-molecular-weight products of the reaction provide evidence for the conclusion that separation of the two structural domains is most probably the first step on the unfolding pathway of the protein under native conditions.

Notes: The in vivo pathway of folding and subunit assembly of a trimeric bacteriophage protein has been studied by characterizing precursors to the native protein and by analyzing temperature-sensitive mutations that kinetically block the pathway. The native trimer is formed via an intermediate composed of three partially folded chains, the protrimer. At 39°C, temperature-sensitive mutations prevent the formation of both the native trimer and the protrimer, possibly by destabilizing earlier intermediates. However, the mutations do not affect the stability of the native protein, formed at 30°C. Thus, these mutations identify amino acid residues involved in interactions that determine the folding pathway.

Notes: As a model compound for the growing chain in the activated-NCA type of polymerization of α-amino acid N-carboxyanhydride (NCA), 3-[ω-acetylglycyl-poly(α-amino acid) acyl]-α-amino acid NCA (called the prepolymer) having various degrees of polymerization (DPs) was synthesized by the polymerization of Phe, Val, Glu(OEt), and Asp(OBzl) NCA in the presence of AcGly NCA by the tertiary amine. Activated (S)-Phe, Val, Glu(OEt), and Asp(OBzl) NCA were added to the terminal cyclic group of the corresponding (S)- or (R)- prepolymer, and the enantiomer selectivity in the reaction was investigated. With prepolymers having DPs ranging from 1 to 15, the addition reaction always took place preferentially between species having the same configuration, and the degree of the enantiomer selection increased with increasing DP of the prepolymer. With prepolymers having DP = 1 and 2, we found contributions from the chiral terminal unit and the chiral penultimate unit to the enantiomer selection, respectively. Prepolymer having DP = 5 was shown to take a β-type conformation, which led to higher enantiomer selection; and prepolymers having DP = 10 and 15 were shown to take an α-helix conformation, which led to much higher enantiomer selection than did the β-type conformation. In the present investigation the mechanisms of terminal-unit control, penultimate-unit control and conformational control of the enantiomer selection in the activated-NCA type of polymerization were clearly observed.

Notes: The helix-coil transition and conformational structure of poly(8-bromoadenylic acid) [poly(8BrA)] have been investigated using 1H- and 13C-nmr, CD, and ir spectroscopy. The results have been compared with the structure of the related 5′-mono- and polynucleotides. The chemical shifts of H(2′), H(3′), C(2′), and C(3′) nmr signals show an interesting correlation with both the puckering of ribose ring and glycosidic bond torsion angle. Poly(8BrA) shows an upfield shift of the C(3′) signal and a downfield shift of the H(3′) signal compared to the chemical shifts in poly(A). These shifts are consistent with a C(3′) endo-syn conformation for poly(8BrA). A similar effect has been reported previously and is also observed here on the C(2′) and H(2′) signals when the preferred conformation is C(2′)endo-syn (e.g., in 5′-8BrAMP). The chemical-shift parameters thus act as a probe for studying syn ⇄ anti and N ⇄ S equilibria in solutions. The three-bond 1H-′13C coupling constants between H(1′) and C(8) and C(4) have been measured in poly(8BrA) and 5′-8BrAMP and their structural implications have been discussed. The observed preference of a C(3′)endo-syn conformation for poly(8BrA), coupled with other evidence, throws doubt on the validity of a correlation previously reported whereby a syn conformation is associated with a C(2′)endo ribose pucker. The backbone conformation of randomly coiled poly(8BrA) is very similar to the structures found in polyribonucleotides: poly(A) and poly(U). All three polymers show strong preferences for the backbone angles found in RNA helices. The CD spectrum of poly(8BrA) has a striking relationship to that of poly(A). The signs of all extrema are inverted, and the magnitudes are related by a constant factor. We suggest that these differences result from a change in the angle between coupled transition moment vectors in the two polymers. Infrared spectra of poly(8BrA) in H2O and D2O solution are reported for the frequency range below 1400 cm-1. The antisymmetric 〉PO2- stretching vibration is observed at an unusually low frequency in the helix (1214 cm-1). The symmetric 〉PO2- stretch occurs at ∼1095 cm-1 but is not resolved from a ring vibration near this frequency. A conformationally sensitive band, characteristic of helical RNA structures, is observed at 817 cm-1 and disappears when the helix is melted. This observation confirms the conclusion that ordered poly(8BrA) has a regular helical structure with an RNA backbone conformation. A stereochemical explanation is provided for the failure of poly(8BrA) (or other syn polymers) to form double helices with anti-polyribonucleotides.

Notes: We studied the kinetics of O2 release by oxyhemoglobin caused by sodium dithionite, in the presence and in the absence of organic cosolvents (monohydric alcohols and formamide) at 10°C. This study was performed by using standard stopped-flow techniques coupled with microprocessor-based data acquisition. We have fitted the experimental data to a mathematical expression obtained on the basis of a two-state model that takes into account the kinetic heterogeneity between α- and β-chains and the presence of αβ-dimers in oxyhemoglobin solutions. Results indicate that the cosolvents mainly affect the allosteric parameter L, i.e., the T ⇄ R conformational equilibrium of hemoglobin, leaving the intrinsic deoxygenation rates of both R and T states almost unaltered. The L values obtained in the present work are in excellent agreement with analogous values previously estimated from oxygen equilibrium measurements.

Notes: Porcine submaxillary mucin (PSM) is a glycoprotein composed of a protein core and frequent, short oligosaccharide side chains. We report static and dynamic light scattering experiments and intrinsic viscosities for PSM in aqueous solvent systems. In 0.1M NaCl solution, the data suggest PSM exists as large, internally branched, highly hydrated, polydisperse aggregates that slowly dissociate to give a stable species of weight-average molecular weight (Mw) 7.4 × 106. In 6M GdnHCl solution, the noncovalent bonds between PSM molecules are broken, giving a highly elongated molecule of Mw = 2.0 × 106. The irreversible nature of this dissociation suggests that the forces that stabilize the native aggregates of PSM in 0.1M NaCl are specific in nature. On reduction of PSM with mercaptoethanol, the polydispersity decreases and Mw also decreases to 9 × 105. A discrete change is observed in the solution properties of PSM in 0.1M NaCl at a concentration of 2mg/mL, manifested by a sudden decrease in the translational diffusion coefficient, an increase in viscosity number, and a decrease in slope of the osmotic compressibility. We tentatively propose that a weak and reversible secondary association process occurs at this concentration, although a purely hydrodynamic interaction cannot be ruled out.

Notes: The uv linear dichroism of calf thylus DNA has been studied at different degrees of orientation both in flow-oriented ethanol-water solutions and in a stretched aqueous host of poly(vinyl alcohol) (PVA). The reduced dichroism (LDR) curves in the region 250-290 nm for DNA in PVA films at 75 and 100% relative humidity (r.h.) are in fair agreement with the curves calculated for the A- and B-forms of DNA, based on the fiber structures and the π-π* transitions of the free bases. This suggests that DNA adopts its A and B conformations in PVA at 75 and 100% r.h. In ethanol, on the other hand, a deviation from the A-form spectrum shows that the conformation of DNA in the solution can differ from the fiber structure. At shorter wavelenghts, a positive contribution to LDR is explained in terms of an out-of-plane polarized n-π* transition.

Notes: A vibrational analysis has been performed for a double-helix-single-strand junction. A Green's function technique has been used in treating the junction as a defect on an otherwise perfect system of infinite chain homopolymers. We calculate that the hydrogen-bond stretching at the junction is amplified by a factor of two relative to the interior of the double helix, B poly(dG)-poly(dC). Breathing modes localized near the junction have also been predicted at 77 and 94 wave numbers. The calculated results are shown to be consistent with predictions from recent nmr studies.

Notes: Nmr studies of the protected and free tetrapeptide Gly-Pro-Gly-Gly were carried out in β-turn-supporting solvents, that is, in CDCl3 for Z-Gly-Pro-Gly-Gly-OMe and in Me2SO-d6 for H-Gly-Pro-Gly-Gly-OH. Comparisons with specifically α-deuterated analogs gave complete assignments of the NH and methylene regions. Analysis of chemical shifts, coupling constants, and the temperature dependence of chemical shifts show that the peptide adopts a type II β-turn conformation. This turn is stabilized for the protected tetrapeptide by two hydrogen bonds between (i) C=O (Gly1) and NH(Gly4), and (ii) urethane function NH and methyl ester C=O.

Notes: Ab initio minimal and split-valence basis set calculations have been performed on compounds that are involved in retro-inverso modifications, i.e., gem-diaminoalkyl and malonyl structures. These calculations are compared with empirical force field calculations and the minor differences discussed. All calculations agree that the preferred helical conformation of the isolated gem-diaminoalkyl and malonyl derivatives of residues found in the retro-inverso modified peptides is 5-8 kcal/mol lower than the Ceq7 conformation preferred by the isolated peptide residues. Population analysis and contour plots of the charge distribution are used to help explain the differences between the model compounds.

Notes: The arginine and lysine residues of calf thymus histone H1 were modified with large molar excesses of 2,3-butanedione and O-methylisourea, respectively. Kinetic study of the modification reaction of the arginine residue revealed that the reaction is divided into the two pseudo-first-order processes. About a third (1 Arg) of the total arginine residues of the H1 molecule was rapidly modified without causing any detectable structural change of the molecule, and the slow modification of the remaining arginine residues (2 Arg) led to a loss of the folded structure of H1. In the case of lysine residue modification, 93% (56 Lys) of the total lysine residues of the H1 was modified with the same rate constant, while 7% (4 Lys) of lysine residue remained unmodified. When the reaction was performed in the presence of 6M guanidine-HCl, all of lysine residues were modified. It is concluded that the 2 arginine and 4 lysine residues resistant to modification are buried in interior regions of the H1 molecule and play an important role in the formation of the H1 globular structure, while the other 1 arginine and 56 lysine residues are exposed to solvent.

Notes: The molar Kerr constants mK, molar refractions mR, and dipole moments μ are reported for the N-methylacetamides CX3CONHCH3 (X = H, CH3, F. CI, Br) and acetamides CX3CONH2 (X = H, F, Cl, Br). The components of the polarizability tensor α are deduced for N-methylacetamide and acetamide on the basis of the bond additivity approximation. This α is found to be considerably more anisotropic than was indicated in previous determinations by other methods. The data for N-methylacetamide were used to calculate mK, μ, and γ2 (anisotropy squared) of N-acetyl-N′-methylglycine amide and N-acetyl-N′-methyl-alanine amide as functions of the torsional angles (φ,Ψ). The statistical mechanical averages of mK, μ, and γ2 were calculated from conformational energies obtained by the methods of Scheraga.

Notes: The physical properties of single, 5-8-μm diameter, water-swollen elastin fibers have been investigated on a microtest apparatus attached to a polarizing microscope. Analysis of the mechanical and optical properties at extensions below 100% indicate that the elastic modulus (G) has a value of 4.1 × 105 N m-2, the average molecular weight of chains between crosslinks is in the range of 6000-7100, and the stress optical coefficient (C′) is 1 × 10-9 m2 N-1 at 24°C. Analysis of the temperature dependence of the stress optical coefficient indicates that the polarizability of the random link decreases with increasing temperature, with an apparent activation energy for this process of the order of 1.6 kcal/mol. Analysis of the non-Gaussian mechanical and optical properties at extensions above about 100% suggest that the chains between crosslinks contain approximately 10 “effective” random links, with each link consisting of 7-8 amino acid residues. These parameters for the random chains in the elastin network have been used to predict the dimensions of other random proteins. The close correlation of these predictions with published values for the dimensions of a series of proteins in solution in 6M guanidinium hydrochloride provides an independent test of the appropriateness of our analysis.

Notes: Heavy meromyosin binding to F-actin saturated with tropomyosin is studied theoretically. The problem is formulated as a special case of n-mer adsorption to a one-dimensional Ising lattice which is divided into m-site-long blocks.

Notes: A Monte Carlo analysis is presented which establishes a relationship between the rotational diffusion coefficients and the flexibility (persistence length, P) of short, wormlike chains. The results of this analysis are presented in terms of experimentally observable quantities; namely, the rotational relaxation times for the field-free decay of optical anisotropy. The pertinent theoretical quantity is R, defined as the ratio of the longest rotational relaxation time of a wormlike chain to the transverse rotational relaxation time of a rigid cylinder having the same axial length (L) and segmental volume. R, so defined, is essentially independent of the axial ratio of the cylinder for any value of L/P within the range of validity of the present analysis (axial ratio 〉 20; 0.1 〈 L/P 〈 5). It is pointed out that P can be determined with reasonable accuracy even in the absence of a precise knowledge of the local hydrodynamic radius of the chain.

Notes: Solvents which accelerate DNA renaturation rates have been investigated. Addition of NaCl or LiCl to DNA in 2.4M Et4NCl initially increases renaturation rates at 45°C and then leads to a loss of second-order behavior. The greatest accelerations are seen with LiCl and dilute DNA. Volume exclusion by dextran sulfate is the most effective method of accelerating DNA renaturation with concentrated DNA. Addition of dextran sulfate beyond 10-12% in 2.4M Et4NCl fails to increase the acceleration beyond approximately 10-fold. Accelerations of 100-fold may be achieved with 35-40% dextran sulfate in 1M NaCl at 70°C. No other mixed solvent system was found to be more effective, although acceleration may be achieved in solvents containing formamide or other denaturants. The acceleration in 2M NaCl occurs without loss of the normal concentration and temperature dependence of DNA renaturation and is also independent of dextran sulfate concentration if sufficient dextran sulfate is used. Dextran sulfate may be selectively precipitated by use of 1M CsCl.

Notes: In the preceding article, a Monte Carlo analysis was presented which provides a quantitative numerical relationship between the rotational diffusion coefficients, as measured by the decay of optical anisotropy following an electric field pulse, and the flexibility (persistence length) of short, wormlike chains. In the present article, the results of the foregoing analysis are applied to the observed rates of decay of birefringence for a series of sequenced DNA fragments ranging in size from 104 to 910 base pairs. Under the conditions used in this study, the DNA fragments exist as native, duplex molecules. Furthermore, conditions are defined in which the observed relaxation times are not dependent on DNA concentration, field strength, or the duration of the pulse. It is pointed out that the ionic atmosphere associated with a wormlike polyion does not exert any significant (direct) influence on the rotational diffusion of the polyion and, therefore, that the rotational relaxation times are a true measure of the configurations of the DNA molecules in solution. Moreover, excluded-volume effects are shown not to be significant for the moderately short molecules employed in this study. The major conclusion of this study is that there is no strong ionic strength dependence of the persistence length for ionic strengths above 1 mM and that the persistence length, under conditions where electrostatic contributions are negligible, is approximately 500 Å. For ionic strengths significantly lower than 1 mM, electrostatic contributions to the stiffness of DNA become significant.

Notes: The aqueous and hydroalcoholic solutions of the heptenic macrolide amphotericin B display strong and variable signals in CD and absorption spectroscopies in the range of the π* ← π transition. An interpretation of the spectroscopic changes is proposed based on the equilibrium between two forms of the intermolecular organization: the aggregated one (A) with strong excitonic interaction and the nonaggregative one (B) whose spectra are like those of linear conjugated polyenes in true solution with a well-developed vibrational structure. The intermediate spectra are fitted by linear combination of the A- and B-form spectra. A two-level organization of the aggregates is proposed for the A-form: (1) a close packing of few molecules, which is the origin of the absorption maxima hypsochromic shift; and (2) interaction between the preceding small units inside the aggregates, which is spectroscopically expressed by the intense CD couplet.

Notes: Polypeptides of dicarboxylic amino acids having the monomer units linked in α- and ω-peptide bonds contain two kinds of carboxyls of different acidity. How well potentiometric titration can distinguish these two carboxyls and so characterize the nature of the peptide bonds is evaluated critically. An analysis of the equation describing the dependence of pH on the degree of neutralization based on neglecting the polymer effect and a discussion of the dissociation behavior of polyanions show that the method of evaluating experimental data found in the literature is incorrect. Nevertheless, if a conformational transition does not interfere, some useful and reliable information may be gained by this method; namely, an indication of the presence of two different peptide bonds, their mole ratio, and an approximate pK value for the carboxyl of the amino acid linked in the ω-peptide bond. The presence of two types of carboxyls complicates the evaluation of the titration curves in the conformation studies.

Notes: Conformational analysis of antiparallel double-helical polynucleotides with Watson-Crick base pairing was reduced to a four-dimensional problem using original mathematical methods. In the four-dimensional conformational space the family of structures, characterized by the base-pair stacking with the most stable conformations in water solution as well as in the solid state, was localized. For the C′2-endo sugar pucker, both right-handed and left-handed structures were found; right-handed structures only, however, seem to be allowed for the C′3-endo pucker, the only possible one for ribonucleotides with base stacking.