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

The dynamic transition found by Mössbauer 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.