Thermodynamic functions of biopolymer hydration. II. Enthalpy–entropy compensation in hydrophilic hydration processes

The thermodynamic functions of biopolymer hydration were investigated by multitemperature vapor pressure studies. Desorption measurements were performed that allowed determination of reversible isotherms in the hydration range of 0.1 to 0.3–0.5 g H₂O/g dry polymer. These isotherms are accessible to thermodynamic interpretation and are relevant to the interaction of water with biopolymers in their solution conformation. The results obtained on a series of different biopolymers (lysozyme, α-chymotrypsin, apo-lactoferrin, and desoxyribonucleic acid), show the following common features of interest: (1) The differential excess enthalpies (ΔH^e ) and entropies (ΔS^e ) are negative, and exhibit pronounced anomalies in a well-defined low-humidity range (approx. 0.1 g H₂O/g dry polymer). These initial extrema are interpretable by structural changes, induced in the native biopolymer structures by water removal below a critical degree of hydration. (2) The ΔH^e and ΔS^e terms exhibit statistically significant linear enthalpy–entropy compensation effects in all biopolymer–water systems investigated. The compensation temperatures \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta = \overline {\Delta H} ^e /\overline {\Delta S} ^e $\end{document} are approximately identical for all biopolymers, ranging from 360 to 500 K. The compensation effects are attributable to phase transitions of water molecules between the bulk liquid and the inner-sphere hydration shell of native biopolymers. (3) The negative excess free energies (ΔG^e ) decrease monotonically with increasing water content and are close to zero at 0.3 to 0.5 g H₂O/g polymer. This result indicates that only transitions between the bulk liquid and the inner-sphere hydration shell are associated with significant net free energy effects. The outer-sphere hydration water is thermodynamically comparable to bulk water. The importance of the proportionality factor \documentclass{article}\pagestyle{empty}\begin{document}$ \hat \beta $\end{document} in the control of the free energy term is discussed.