Does the release of hydration water come with a Gibbs energy contribution?

Abstract

Hydration is a major force driving molecular recognition events involving biopolymers. Such events are generally accompanied by release or uptake of water molecules from the hydration shells of participating biomolecular solutes. The exchange of water molecules between the hydration and bulk populations is known to proceed with changes in volume, enthalpy, and entropy. It is less clear if the water exchange also contributes to free energy. The thermodynamic analysis performed in this study suggests that the release or uptake of individual water molecules may contribute not only to enthalpy and entropy but also to free energy. While water molecules solvating a particular solute or an atomic group may exhibit a near exact enthalpy–entropy compensation resulting in a zero contribution to free energy, this tendency is not universal to all solutes. Another level of complexity stems from the modulation of Gibbs energy contributions of released water molecules by pressure and temperature. We emphasize the need for further studies to quantify the thermodynamics of hydration and understand how pressure and temperature modulate the energetics of water release from the hydration shells of various atomic groups and, hence, to elucidate the contribution of hydration as a driving force in biomolecular events as a function of temperature and pressure.