ALBERT

Notes: Abstract Partial molar volumes at 15, 25, and 45°C and partial molar heat capacities and expansivities at 25°C for ethylammonium nitrate + water mixtures are reported. The results are compared with those for other aqueous cosolvents, particularly hydrazine and ammonium nitrate.

Notes: Exchange-rate probability-density functions (pdf) have been calculated for lysozyme hydrogen-exchange data by numerical Laplace inversion over the temperature range 5-45°C. The smoothest numerical solutions show three broad overlapping peaks. Analysis of the temperature dependence of the cumulative exchange-rate distributions provides the model-independent probability-density function for the activation energies. For the most rapidly exchanging protons, the activation energies are low, consistent with hydroxyl-ion catalysis in the protein-water interface. The second peak of the exchange-rate pdf's contains those protons located in regions of lower motility we call “matrices,” for which exchange rates are limited by gated-diffusion of the hydroxyl-ion catalyst. The most slowly exchanging protons are located on groups forming strong, dense “knot” structures, identified by neutron-diffraction and nmr data as clustered segments of β-sheet with well-organized hydrogen bonding and sections of the internal faces of α-helices. Exchange from knot structures occurs through local disordering with little loss of strength or stability to expose one or more protons at a time for exchange. Knots appear to be responsible for the two-state character of thermal unfolding that occurs by cooperative disruption of the dominant structures of this type. Below about 55°C, all protons exchange from folded states. Contributions to exchange from unfolding processes occur only at temperatures above 55°C.There is a qualitative difference between the two types of structures indicated by the appearance of two and only two enthalpy-entropy compensation patterns. The compensation temperature, Tc, for the matrices is about 470 K; that for the knots, about 360 K. The preservation of rank-order with temperature change is shown to be a consequence of the fact that all exchange rates in the slow and very slow peaks of the pdf lie on one or the other of the two compensation lines. Although the same electrostatic factors are present in all parts of the protein, we have been forced to conclude that given certain necessary geometric possibilities, these factors cooperate to produce the knots. The knots appear to be the most significant structural element in globular proteins responsible for the structural form of the matrix regions and the dynamic behavior of the protein interior. The knots have high density and low permeability to water, hydroxyl ion, etc., and are probably the explanation for the very low compressibilities, the matrices being nearly mechanically transparent. The knots must make some contribution to folded stability, but it is not clear that this contribution is large. Their major thermodynamic function is to establish kinetic stability; that is, to make the activation free energy for unfolding high. The most important factor in the existence of knots appears to be the ease with which hydrogen bonds adapt in length, angle, and strength to local electrostatic conditions. In proteins, as in water, adaptation is cooperative in local groups of hydrogen bonds, and as in water, this cooperation is enhanced by contact with aromatic and aliphatic groups.

Notes: This article presents evidence for the existence of a specific linear relationship between the entropy change and the enthalpy change in a variety of processes of small solutes in water solution. The processes include solvation of ions and nonelectrolytes, hydrolysis, oxidation-reduction, ionization of weak electrolytes, and quenching of indole fluorescence among others. The values of the proportionality constant, called the compensation temperature, lie in a relatively narrow range, from about 250 to 315 °K, for all these processes. Such behavior can be a consequence of experimental errors but for a number of the processes the precision of the data is sufficient to show that the enthalpy-entropy compensation pattern is real. It is tentatively concluded that the pattern is real, very common and a consequence of the properties of liquid water as a solvent regardless of the solutes and the solute processes studied. As such the phenomenon requires that theoretical treatments of solute processes in water be expanded by inclusion of a specific treatment of the characteristic of water responsible for compensation behavior. The possible bases of the effect are proposed to be temperature-independent heat-capacity changes and/or shifts in concentrations of the two phenomenologically significant species of water. The relationship of these alternatives to the two-state process of water suggested by spectroscopic and relaxation studies is examined. The existence of a similar and probably identical relationship between enthalpy and entropy change in a variety of protein reactions suggests that liquid water plays a direct role in many protein processes and may be a common participant in the physiological function of proteins. It is proposed that the linear enthalpy-entropy relationship be used as a diagnostic test for the participation of water in protein processes. On this basis the catalytic processes of chymotrypsin and acetylcholinesterase are dominated by the properties of bulk water. The binding of oxygen by hemoglobin may fall in the same category. Similarities and differences in the behavior of small-solute and protein processes are examined to show how they may be related. No positive conclusions are established, but it is possible that protein processes are coupled to water via expansions and contractions of the protein and that in general the special pattern of enthalpy-entropy compensation is a consequent of the properties of water which require that expansions and contractions of solutes effect changes in the free volume of the nearby liquid water. It is shown that proteins can be expected to respond to changes in nearby water and interfacial free energy by expansions and contractions. Such responses may explain a variety of currently unexplained characteristics of protein solutions. More generally, the enthalpy-entropy compensation pattern appears to be the thermodynamic manifestation of “structure making” and “structure breaking,” operationally defined terms much used in discussions of water solutions. If so, the compensation pattern is ubiquitous and requires re-examination of a large body of molecular interpretations derived from quantitative studies of processes in water. Theories of processes in water may have to be expanded to accommodate this aspect of water behavior.

Notes: The theory, character, and properties of cooperative transitions are developed with special reference to the abrupt changes of state which occur in protein solutions. Comparisons of helix-coil processes and protein conformational reactions show that though cooperation dominates both of these processes, there are important differences. Tests of two types for the validity of the two-state approximation are presented with specific applications to proteins. Available experimental evidence demonstrates that the thermally induced reversible transitions of ribonuclease, α-chymotrypsin, and chymotrypsinogen A under conditions thus far examined are two-state processes.