Link between the hydration enthalpy of lysozyme and the density of its hydration water: Electrostriction

Hydration shells around proteins in solution are on average denser than bulk water. Variations in enthalpy are observed during hydration/dehydration of proteins. To explain consistently those phenomena, a common mechanism-electrostriction-underlying the mechanical and contributing to thermal effects is proposed. The mean mass density of the hydration shell of lysozyme derived from the neutron and X-ray scattering is explained as following the compression of water in the fields of the order of 10 9 V m(-1) due to the charged sites at the boundary of the protein. The mean enthalpy of mixing Delta H-mean of lysozyme in water calculated on the basis of the measured mean mass density falls in the middle of the values of the enthalpy of mixing Delta H-mix observed in sorption experiments. This testifies that Delta H-mix is due in part to the work done by the electrostriction pressure in hydration shell regions situated in high electric fields. The dependence of the sorption enthalpy of exemplary proteins on the number of adsorbed H2O molecules is also described in terms of electrostriction.