ALBERT

Notes: An increase in temperature causes an increase in the amplitudes of intramolecular and intermolecular motions and mainfests itself as an increase in the effective radii of the various atoms, especially hydrogen atoms. If one uses a Lennard-Jones 6-12 potential function to account for nonbonded interactions, this increase in the radius of the hydrogen atom is equivalent to a modification of the coefficient of the repulsive part of the Lennard-Jones potential. Accordingly, this effect of increasing temperature on the helix sense of several polyamino acids is computed by allowing the effective radius of the hydrogen atom to increase. It is found that the preferred sense of several polyamino acid helices changes from right- to left-handed as the temperature is increased. This accounts for recent experimental observations of changes in helix sense with temperature.

Notes: Potentiometric titrations and some complementary optical rotation data are presented for solutions of poly(L- glutamic acid) (PGA) in several H2O-ethanol mixtures. The data allow the determination of the intrinsic pK (pK0), slope of the apparent. pK (pKapp), versus degree of ionization curves and of the enthalpy of ionization as a function of ethanol concentration. The variation of the degree of ionization at which the helix-coil transformation occurs with ethanol and temperature is also determined. Finally free energy, enthalpy, and intropy changes associated with the helix-coil transformation for the uncharged conformers are determined from the titration curves. The effect of the ethanol is to increase the stability of the helical conformation of PGA for both the charged and the uncharged forms of the polymer. The stabilization of the uncharged helix is essentially an entropic effect.

Notes: Infrared spectra of polypeptides were measured in the region of 1800-400 cm-1. For the α-helical form, disordered form, and antiparallel-chain β-form, amide V band- arising from N-H out-of-plane bending models were observed at 610-620, around 650, and 700-705 cm-1, respectively, and amide V′ bands arising from N-D out-of-plane bending modes were observed at 455-465, around 510, and a 515-530 cm-1, respectively. These correlations are useful for conformation diagnoses, particularly for copolyamino-acids or proteins which are not oriented. The nature of low-frequency amide bands are discussed with reference to potential energy distributions calculated for the α-helical form and β form.

Notes: Some experimental data are given on the infrared spectra between 3300 and 3500 cm-1 of dilute solutions in carbon tetrachloride of three types of model compounds: CH3-CONH-CH(R1)-CONH(R2), (I); CH3-CON(CH3)-CH(R1)-CONH(R2), (II) and CH3-CONH-CH(R1)-CON(R2)2, (III). In studying the N-H stretching bands, it was found that there are two types of intramolecular hydrogen bonds in these molecules; these result in two different cyclized conformations, C5 and C7, which contain respectively, five and seven atoms in the ring. By using model substances I, II, and III, in which the nitrogen atoms are unequally substituted, it is possible to identify the N-H stretching bands which are to be ascribed to the N-H oscillators included in the two different chelated conformations. It is found also that the stretching frequency of a free N-H oscillator depends upon the substituent on the nitrogen atom. Thus, it is possible to observe, with some of the model compounds I, four different absorption bands located at 3340, 3420, 3440, and 3460 cm-1. The first two are ascribed to the N-H oscillators included in the H—bonds which lock the C7 and C5 conformations; the last two correspond to free N-H which differ with the substituent on the nitrogen atom.

Notes: Molecular orbital calculations of the extended Hückel type have been used to study the conformations of glycyl and alanyl residues in ground and excited states. The ground-state surfaces show features similar to those obtained with the standard calculational methods in which the total energy is partitioned into components such as torsions, nonbonded and electrostatic interactions. The molecular orbital calculations provide the first independent theoretical check on such calculations. The excited-state surfaces, Uniquely available from the molecular orbital calculations, exhibit a better definition and sharpening of potential minima in the sterically allowed regions.

Notes: We derive equations for calculating the number-, weight-, and z-average lengths between single-strand breaks in DNA molecules. Provision is made for the existence of preformed breaks under genetic control: these are assumed to be fixed in number in each molecule, but may be variable in position. Breaks due to random degradation may be superimposed on the pre-existing breaks. An example is given for illustrative purposes.

Notes: The conformation in solution of fractionated 30 S and 19 S ribosomal RNA from rabbit reticulocytes has been studied by optical rotatory dispersion, analysis of thermal melting profiles and their derivatives, and spectrophotometric acid-base titration. From a consideration of the limitations of these methods, it has been possible to set limiting values on the degree of base-pairing and the lengths of the double helices: between 60 and 80% of the bases in 19 S and 30 S RNA are estimated to be paired. The paired segments are not shorter than 4 base pairs, and evidence from other sources is available which indicates that they are not longer than 8-16 base pairs. The spread of helix lengths is greater in the 30 S than in 19 S RNA; and other differences are noted. Several distinct populations of double helices, differing in their thermal stability, are present. Estimates are presented from spectrophotometric and titration data for the base compositions of the paired and unpaired regions.

Notes: A small perturbation analysis is carried out to determine the stability of a fluid containing two layers of diffusing solutes in a common solvent and acted upon by a uniform gravitational field. It is found that instability can arise even though the unperturbed diffusion does not lead to the formation of a density inversion within the fluid.

Notes: The kinetics of denaturation of DNA have been studied by relaxation techniques. Examination of the terminal relaxation times for a variety of DNA's under a variety of conditions has shown that DNA denaturation is principally a hydrodynamically limited process. Measurements within the helix-coil transition have demonstrated that the experimentally measured terminal relaxation times are a function of the following: (1) position in the helix-coil transition; (2) ionic strength of the solvent; (3) solvent viscosity; (4) DNA concentration; (5) molecular weight; (6) number and position of single-strand breaks. The dependence of the terminal relaxation time on the above mentioned factors can be attributed to hydrodynamic effects. Thus a hydrodynamic model for DNA unwinding is required. The model which best fits the data involves the assumption of a rotational frictional coefficient independent of molecular weight. This assumption is suggested by the fact that the relaxation time is proportional to the first power of the molecular weight.

Notes: Sodium counterion association with partially neutralized poly(D-glutamic acid) or poly(DL-glutamic acid) was measured by use of Wall's transference method with radioactive sodium. In the region where both polyacids are in completely random coil form, fractions of association were considerably less than that with poly(acrylic acid) in the same region of degree of neutralization. Even in the region where poly (D-glutamic acid) is in the helical form, the fraction of association was less than that with poly(acrylic acid) in the same region. No pronounced characteristics attributable to counterion association corresponding to the helix-coil transition could be found. The association phenomena were discussed on the basis of a rodlike model of polyelectrolyte.

Notes: The isomerization of poly-L-proline in different solvents has been studied by NMR spectroscopy. Different resonance signals for the CHα protons have been obtained for the two different helical conformations of thus compound, namely form I and form II.

Notes: Poly-β-N-diphenylmethyl-L-asparagine and poly-γ-N-diphenylmethyl-L-glutamine were prepared from the corresponding N-carboxyanhydrides. Poly-L-aspuragine and poly-L-glutamine were obtained by removal of the diphenylmethyl protecting groups with liquid anhydrous hydrofluoric acid.

Notes: The Hydrogen-tritium exchange character of poly-D,L-alanine was studied in detail as a model for the hydrogen exchange behavior of the unhindered, polymeric peptide group. The random chain nature of poly-D,L-alanine was evident in the uniformity of exchange rate of all its hydrogens and in the similarity between this rate and that of random chain poly-D,L-lysine and other known, unhindered secondary amide groups. An equilibrium isotope effect favoring the binding of tritium over protium to the extent of 21% was measured. Specific acid and base catalysis of the exchange and the absence of detectable general catalysis were demonstrated. Apparent energy of activation is 17 kcal/mole for deprotonation, largely due to dependence of Kw on temperature, and 15 kcal/mole for protonation, which correlates with the extreme apparent pK. The hydrogen -tritium exchange half-time rate; of poly-D,L-alamine at any pH and temperature (T: °C) is given by the equation: \documentclass{article}\pagestyle{empty}\begin{document}$$ \[t_{\frac{1}{2}} \,(\min )\, = \,200\,\, \times \,10^{0.05{\rm T}} /\,[10^{{\rm pH} - 3} \, + \,10^{3 - {\rm pH}} ]\] $$\end{document}

Notes: The specific rotation of starch components, corrected for refractive index variation, exhibits a discontinuity in the region of the water-dimethyl sulfoxide (H2O-DMSO) system that corresponds to the composition of the complex 2H2O-DMSO. This discontinuity is a property dependent upon the presence of a number of consecutively linked α-1,4 glucose units and, therefore, must reflect a change in symmetry of a segment, of polymer chain. The optical rotation of amylose between 26.5 and 92.5°C. does not change in DMSO and is only slightly lowered in water at the higher temperature. The behavior of amylose in both DMSO and H2O is like that of a random coil, as indicated by viscosity and sedimentation measurements. These results may be interpreted either as being compatible with models of amylose in solution in which the polymer backbone has helical twist, or as indicating removal of strong interactions between polymer chain segments by a good solvent.

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 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: 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 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: 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: 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: Dies ist Teil I einer zweiteiligen Aufsatzfolge. Teil II wird „Neuentwicklungen der Düngemittelforschung“ behandeln und im nächsten Heft erscheinen.

Notes: The osmotic pressure equation for nonideal, associating systems of the type nA +mB ⇄ AnBm, has been derived, by using the assumption yAnBm/yAnyBm = 1. This treatment can also be applied to related associations such as nA + mB ⇄ AB + AB2 + A2B + …. From osmotic pressure experiments on the pure reactants it is possible to obtain the molecular weights (MA and MB) of the reactants and also the virial coefficients (BAA and BBB) of the reactants. The osmotic pressure of a nonreacting mixture of A and B can be calculated from these measurements. It can be used along with osmotic pressure measurements on equilibrium mixtures of A and B to obtain expressions containing the equilibrium constant (or constants) and the cross-virial coefficients (BAB and BBA). Several procedures are described for the evaluation of the equilibrium constant (or constants) and the BAB or BBA terms. It appears that this procedure is a general one which is applicable to associations of the type nA + mB ⇄ AB + A2B + AB2 + …. By correcting for nonideal behavior, one should then be able to apply it to any method available for analyzing ideal associations of the types considered here. In addition it is possible, subject to certain restrictions, to analyze associations of the type 3A + B ⇄ A2 + AB.

Notes: A method has been developed for characterizing at, submicrogram levels the heterogeneity of histones from purified nuclei. The histones are eluted with a smooth concentration gradient from nuclei trapped in polyacrylamide-gel threads and are collected in a micro fraction collector suitable for volumes in the 10-100 μl range. The gradient and fraction collection systems are governed by cam driven syringes. Samples obtained are subjected to electrophoresis in a starch-gel system and the gels are stained with a highly sensitive stain specific for guanidinium groups. Seven major and a similar number of minor components are demonstrated in the histones. The method of differential elution of trapped macromolecules is suitable for use with systems other than nuclei and histones.

Notes: A method is presented for the treatment of distribution functions obtained by ultra-centrifugal sedimentation velocity experiments on the plasma lipoproteins, so that adequate corrections can he made for the effects of diffusion and concentration dependence of the sedimentation coefficient. The method involves the use of modified equations derived from those presented in 1952 by Gosting for the transform from g* (S) to g(S) distributions. The evaluation of the molecular parameters of lipoprotein density and size, from a set of g(Si°) distribution functions obtained in solvents of varying density, is also outlined. Preparations of the β-lipoprotein of human plasma are found to follow a bivariate normal distribution that requires evaluation of five parameters, x̄1, σ1, x̄2, σ2and p. The first two represent the mean and standard deviation of the lipoprotein density, the third and fourth the same quantities for the size, expressed as ft “logarithmic diameter parameter”, and the final value is a measure of the correlation between these two variables.

Notes: High-resolution nuclear magnetic resonance spectra at 100 MHz and 220 MHz have been obtained on two samples of poly-L-alanine of differing molecular weights (2500 and 42 500) in the chloroform-trifluoroacetic acid system under various conditions of solvent composition, temperature, and polypeptide concentration. Separate helix and random coil peaks are observed for the α-CH and peptide NH backbone proton resonances, thereby permitting the determination of helix content. This observation of separate peaks demonstrates that the lifetimes of the helix and random coil portions of poly-L-alanine have lower limits of about 10-1 sec. It is suggested that solvent-peptide versus peptide-peptide hydrogen bond competition, coupled with a destabilizing effect of the trifluoroacetic acid on the helix, is responsible for the helix-random coil transformation.

Notes: The combination of an agarose gel (Bio-Gel A) and a dioxane-water (1:1) solvent system allowed the fractionation, on a preparative scale, of a very polydisperse, non-derivatized lignin preparation (enzymatically liberated lignin prepared from sweetgum sapwood with Lenzites trabea). Three fractions differing markedly in molecular weight were obtained. A gel of crosslinked alkylated dextran (Sephadex LH-20) with the same solvent system allowed division of the lowest molecular weight fraction into two fractions. These materials were characterized by measurements of intrinsic viscosity and number-average molecular weights in dimethylformamide and dioxane-water. It was established that the two highest molecular weight fractions were associated in an average trimeric form in dioxane-water (1:1) as compared to the form (considered to be molecular) that occurred in dimethylformamide. Molecular size distributions and eluant volumes of the fractions were determined with a Sephadex G-100-formamide system, the latter being one of the most powerful nonaqueous solvents for lignin. Adsorption effects were known to be absent in this case, and the lignin molecules were considered to be unassociated in formamide. The four fractions were distinguishable with the formamide-G-100 system, thus indicating that the original fractionation was based on molecular size. The enzymatically liberated lignin contained molecules that comprised a continuum of molecular weights from approximately monomeric to molecules that were at the limit of the solvating power of dioxane-water (1:1) and dimethylformamide. Limited physicochemical data were consistent with a compact, approximately spherically symmetric shape of the lignin in solution.

Notes: A comparison has been made of the intrachain potential energy of an infinite straight α-helix of poly-L-alanine with that of the distorted form adopted in a coiled coil conformation. The energy terms included were the van der Waal's, electrostatic, hydrogen-bond, and the rotational potential terms. The results indicate that the potential energies of the structures investigated are almost the same, and so a transformation from one state to another may occur without significant changes in potential energy. Particular care has been taken to ensure that the electrostatic and van der Waal's interaction terms are fully convergent. The values obtained for the α-helix were compared with those already published, and some significant differences were found.

Notes: Ultraviolet optical rotatory dispersion curves of mucopolysaccharides exhibit particular Cotton effects in the spectral region of the n-π and π-π amide transitions. Two general patterns emerge: (1) enhancement of negative rotation and of the first negative Cotton effect (troughs 217-220 mμ) and (2) relative dominance of the positive Cotton effect in the π-π transition region (peak ∼198 mμ). Groups (1) and (2) can be correlated with a structural difference in the linkages of the amino sugars: (1) occurs with polymers containing 3-1-linked glycosamino sugars and (2) with glycosamino moieties linked 4-1 by either α- or β-glycosidie bonds. Measurements of the circular dichroic absorption bands support the qualitative conclusions from optical rotation. All mucopolysaccharides exhibit a first, negative band centered at 208-211 mμ, while only those in group (2) show, in addition, a positive band centered at 189-192 mμ. A suggested unifying model considers that difference in kind and/or degree of preferred geometry of the amide groups obtains from two forms of secondary order: (1) having a linear hydrogen bond from the N (acceptor) to the (C2)O - H of the preceding uronic acid and (2) having a linear hydrogen bond from the N (acceptor) to the (C2 or 3) O - H of the following sugar. The hydrogen bonds would have similar strength but opposite directions in two systems towards the nonreducing end (1) or towards the reducing end (2)], closing eight-membered rings

Notes: The myosin molecule was extracted from the smooth muscle parts of horse esophagus and purified by ammonium sulfate fractionation. The schlieren pattern of the sedimentation velocity run showed a very sharp single peak of.5.9. S (s20,w). Molecular weight of the protein was measured by means of the Archibald and sedimentation equilibrium methods, both in 0.5M KCI buffered by 1/150 M phosphate at pH 7.5 and at 5°C. The values obtained were 6.25 × 105 and 5.81 × 105respectively, for the two methods. The second virial coefficients were 1.1 × 104 and 1.2 × 10-4 ml/g. Denatured smooth muscle myosin was prepared in a solution of 5M guanidine HC1 containing 0.4 M KC1 and 0.2 M β-mercaptoet hanol buffered at pH 8.0. The weight-average molecular weight of the denatured smooth muscle myosin was 2.24 × 105 and the second virial coefficient was 7.6 × 10-4 ml/g. The values described above are in good agreement with those reported for rabbit skeletal myosin with ammonium sulfate fractionation. The molecular dimension of the molecule is estimated as the value for an axial ratio of 100, assuming a rigid rod molecular model for this molecule, both the thermodynamical and hydrodynamical treatment being in a good agreement with this estimation.

Notes: No. Abstract

Notes: The enthalpy change accompanying the double helix-coil transition of polyriboadenylic acid (poly A) in aqueous solution has been measured optically and calorimetrically in the pH range 5.7-4.5. The course of this cooperative transition was followed optically by measuring changes in ultraviolet absorption as a function of temperature at different pH values, and calorimetrically by determining the heat capacity of the solution through the transition region. From the latter measurements, the enthalpy of transition was calculated. It is shown, that ΔH is dependent on pH as it is expected from the influence of protonation of the double helix of poly A.

Notes: The interaction between ionizable carboxyl groups and the conformation of poly-(glutamic acid) (PGA) in aqueous solution were investigated by the mechanical method. The dynamic rigidity of the PGA solution has a maximum value at the pH corresponding to about 50% neutralization point. This may be due to establishing of a maximum attractive force by proton/charge fluctuation between ionizable carboxyl groups at that pH. The dynamic viscosity has a sharp change in the region of pH 5.5-6.5. It is suggested that this behavior is due to the helix-coil transition.

Notes: By use of D2O we found that the shortening of the longitudinal proton relaxation time which occurs in the investigated aqueous yeast DNA solutions (≦ 2.4% with 2% protein) was not based on a hydration effect, but was caused by magnetic impurities only. An estimate shows that the mobility of the hydrated water molecules is reduced by less than two orders of magnitude in comparison with the free water molecules.

Notes: A general approach to the determination of relative stability of any pair of con-formational states of biological macromolecules or their complexes (in particular, to the determination of relative stability of native and disordered states of the macromolecule) has been suggested. For determining the free energy difference of the two states under the conditions when one of them is considerably more advantageous than the other, it is necessary for the macromolecule to be influenced by the transforming agent which levels free energies of both the conformational states, and to determine the external parameter derivative of the free energy difference in the region of the conformational transition induced by the change in this parameter. If the character of the dependence of this derivative on the external parameter (temperature, solvent composition, etc.) is known, then this allows the determination of the free energy difference of the two states under the conditions considered, even including conditions far from the transition region. The value of the derivative of the free energy difference in the transition region in many cases can be measured directly (in particular, when using calorimetry), while in cases when a direct measurement of the derivative is impossible, it can often he estimated experimentally from the steepness of the conformational transition. The methods of this estimation and also a possible character of the change of the considered derivative during variation of the external parameter are considered for the case when the transforming agent is one of the components of the solvent and, consequently, the derivative of the free energy difference is equal to the difference of number of molecules of this component hound with the macromolecule in two conformational states.

Notes: Wool-water systems, at water contents above a threshold value of 22.7 g of water per 100 g of wool keratin, yielded curves of specific heat against temperature showing peaks that are ascribed to the fusion of the absorbed water. Integral and incremental heats of fusion were obtained. Integral heats increase as the square of the water content above the threshold, and incremental heats follow a linear relationship. The incremental heat reaches a value of 55 cal/absorbed water, at the saturation water content, at 0°C of 33.9 g of water per 100 g of keratin. The state of the absorbed water is discussed. Clustering of water molecules takes place only above the threshold water content.

Notes: The laser-excited Raman spectra have been obtained of poly-L-proline in the solid and aqueous solution. In the solid state, the Raman spectra are of the form II which is the left-handed helical form with the peptide bonds in the trans configuration. Dissolution of this form in water produced only minor changes in the Raman spectra. The differences in frequencies produced by dissolving the polymer are associated with vibrational modes of the pyrrolidine ring as indicated by comparison with the Raman spectrum of L-proline. The similarity of the Raman spectra dictates that, the conformations of poly-L-proline chain are similar in the solid and aqueous solution.

Notes: A comparative study on sonicated calf thymus DNA was made by using electron microscopy and sedimentation methods. The length distribution, established by electron microscopy by using Kleinschmidt's technique, is related to the sedimentation curve obtained with the same solution. The relation permits a determination of the sedimentation constant corresponding to a given length up to 8000 Å and vice versa.

Notes: The π-electron polarizability of conjugated base molecules such as uracil and adenine are calculated semiempirically by using molecular orbital theories. The Hüekel method and the self-consistent-field (SCF) theory are used in the present calculation. The theoretical values obtained by the Hüekel method are consistently larger than the experimental results. The values obtained by the SCF theory are in good agreement with observed values. It is clearly demonstrated that the SCF method is better than the Hükel method for the calculation of polarizability. The polarizability of the adenine-uracil pair is calculated as the sum of the polarizabilities of adenine and uracil and also calculated by solving the eigenvalue problem for the A-U pair as an entity. The second method gives considerably larger polarizability than the first method. Discussions are presented and it is demonstrated that the second method is the correct way to calculate the polarizability of the A-U pair.

Notes: The reversibility of the thermal denaturation of a low-sulfur fraction of solubilized wool keratin (SCMKA) has been studied under a variety of conditions of time, protein concentration, and pH. Two types of irreversibility for the transition have been encountered. One of these is associated with an aggregation of the protein on denaturation to give a product which may contain elements of a β conformation. This type of irreversibility is favored by high protein concentration, and the original conformation may in fact be regained if the aggregated structure is broken down by a solvent such as 8M urea and the urea subsequently removed by dialysis. The other type of irreversibility appears to be due to racemization of the protein. It does not seem to be dependent on protein concentration and is apparent only at temperatures beyond the actual transition range (∼40-65°C) at pH values below 11, At pH 12, however, racemization appears to proceed slowly even at 4°C. The thermal transition at pH 9 and pH 10 has been shown to be multistage in nature. Over the pH range 9-12 there is a progressive decrease in thermal stability with increase of pH. Addition of NaCl at pH 10 leads to an increase in thermal stability of the molecule.

Notes: We report studies of the optical properties of the proflavine-DNA complex, using absorbance and circular dichroism spectroscopy. From comparison of the absorption spectra of proflavine complexed with calf thymus and T2 DNA, we conclude that stacking of the dyes external to the double helix is comparatively much weaker with T2 DXA, probably because of its glucosylation. Several sources are found for the circular dichroism induced in proflavine when it is complexed with DNA. There is a relatively weak circular dichroism induced when the dye is infinitely dilute on the DNA lattice; this presumably arises from the environmental asymmetry of the binding site. Stronger circular dichroism effects are induced by interaction of intercalated and stacked dyes; studies with T2 DNA, for which stacking seems to be blocked, permit a tentative resolution of effects due to the two modes of binding. One recurring theme of these studies is the observation that the optical properties are quite dependent on environment. The most dramatic example is a strong variation with salt concentration of the amplitude of the circular dichroism induced in the isolated (intercalated) monomer by the surrounding DNA. This suggests that the structure of the intercalated complex is quite sensitive to external conditions.

Notes: The binding of n-butanol, benzyl alcohol, and trifluoroethanol to polyproline I (all peptide bonds in cis configuration) and II (all peptide bonds in trans configuration) has been demonstrated. It was found by infrared spectroscopy that hydrogen bonds are formed between the CO groups of the polymer and the OH groups of the alcohols. Their strength increases from n-butanol via benzyl alcohol to trifluoroethanol. The binding of benzyl alcohol could also be followed by optical rotatory dispersion due to the fact that the electronic transitions of the benzyl ring are rendered optically active by the binding to the polymer.

Notes: The statistical mechanics of the effect of a small number of randomly occurring defects on the melting of a polypeptide chain is developed by using a perturbation of Boltzmann factors technique. The method is illustrated by an example where a second-component impurity contributes more to the energy of the hydrogen-bonded superstructure of the polypeptide chain than does the dominant amino acid residue, resulting in the overall destabilization of the helical conformation.