Resolving the ambiguity of the dynamics of water and clarifying its role in hydrated proteins

Abstract

The dynamics of water in aqueous mixtures with various hydrophilic solutes can be probed over practically unrestricted temperature and frequency ranges, in contrast to bulk water where crystallization preempts such study. The characteristics of the dynamics of water and their trends observed in aqueous mixtures on varying the solutes and concentration of water, in conjunction with that of water confined in spaces of nanometer size, lead us to infer the fundamental traits of the dynamics of water. These include the universal secondary relaxation, here called the ν-relaxation, the low degree of intermolecular coupling/cooperativity, and the ‘strong’ character of the structural primary relaxation. The dynamics of hydration water in hydrated proteins at sufficiently high hydration levels are similar in every respect to that in aqueous mixtures. In particular, the ν-relaxation of hydration water has a relaxation time nearly the same as that of the ν-relaxation of aqueous mixtures above and below the glass transition temperature. This can explain the dynamics transition observed by Mössbauer spectroscopy and neutron scattering. The fact that it is coupled to atomic motions of the hydrated protein, like similar situation in aqueous mixtures, explains why the dynamic transition is observed by neutron scattering at the same temperature whether the hydration water is H2O or D2O. The possibility that the ν-relaxation of the solvent is instrumental for biological function of hydrated biomolecules is suggested by the comparable temperature dependences of the ligand escape rate and the reciprocal of the ν-relaxation time.