Nature of the water specific relaxation in hydrated proteins and aqueous mixtures

The dynamic transition found by Mossbauer spectroscopy and neutron scattering in hydrated and solvated proteins has been an active research area for the past three decades. By now a consensus among some researchers has been reached that it originates exclusively from relaxation of the hydration water (HW) coupled to the protein. The dynamic transition temperature T-d depends on energy resolution of the spectrometer and is higher than the glass transition temperature T-g. Recently demonstrated is the presence of yet another transition at T-g, which is independent of the resolution of the spectrometer and coexists with the dynamic transition at a higher temperature T-d. The transition at T-g is similar to that found in various kinds of glass-formers by neutron and dynamic light scattering at short times when molecules are mutually caged via the intermolecular potential. Like in the case of conventional glass-formers, the transition at T-g of hydrated proteins has been explained by the sensitivity of the extent of the caged dynamics to change of specific volume and entropy on crossing T-g. The caged dynamics are terminated by the onset of relaxation of HW, which in turn gives rise to the dynamic transition at T-d > T-g. Despite these important roles played by the caged dynamics and the HW relaxation in the observed dual transitions of the hydrated proteins, their exact nature is still unclear. In this paper we clarify their nature in hydrated proteins by use of various experimental data, with the assist of the results from studies of mixtures of water with hydrophilic solutes, taking advantage of the fact that the properties are similar in both systems. (C) 2013 Elsevier B. V. All rights reserved.