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Abstract

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.