Mössbauer spectroscopy and neutron scattering measurements on proteins embedded in solvents including water and aqueous mixtures have emphasized the observation of the distinctive temperature dependence of the atomic mean square displacements, , commonly referred to as the dynamic transition at some temperature T(d). At low temperatures, increases slowly, but it assumes stronger temperature dependence after crossing T(d), which depends on the time/frequency resolution of the spectrometer. Various authors have made connection of the dynamics of solvated proteins, including the dynamic transition to that of glass-forming substances. Notwithstanding, no connection is made to the similar change of temperature dependence of obtained by quasielastic neutron scattering when crossing the glass transition temperature T(g), generally observed in inorganic, organic, and polymeric glass-formers. Evidences are presented here to show that such a change of the temperature dependence of from neutron scattering at T(g) is present in hydrated or solvated proteins, as well as in the solvent used, unsurprisingly since the latter is just another organic glass-former. If unaware of the existence of such a crossover of at T(g), and if present, it can be mistaken as the dynamic transition at T(d) with the ill consequences of underestimating T(d) by the lower value T(g) and confusing the identification of the origin of the dynamic transition. The obtained by neutron scattering at not so low temperatures has contributions from the dissipation of molecules while caged by the anharmonic intermolecular potential at times before dissolution of cages by the onset of the Johari-Goldstein β-relaxation or of the merged α-β relaxation. The universal change of at T(g) of glass-formers, independent of the spectrometer resolution, had been rationalized by sensitivity to change in volume and entropy of the dissipation of the caged molecules and its contribution to . The same rationalization applies to hydrated and solvated proteins for the observed change of at T(g).

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 nu-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 nu-relaxation of hydration water has a relaxation time nearly the same as that of the nu-relaxation of aqueous mixtures above and below the glass transition temperature. This can explain the dynamics transition observed by Mossbauer 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 H(2)O or D(2)O. The possibility that the nu-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 nu-relaxation time

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Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature Tg near 136 K, not much known are the spectral dispersion of the structural α-relaxation and the temperature dependence of its relaxation time τ α,bulk (T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (JG) β-relaxation is a question that has not been answered. By studying the structural α-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures Tg close to 136 K, we deduce the properties of the α-relaxation and the temperature dependence of τ α,bulk (T) of bulk water. The frequency dispersion of the α-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of τ α,bulk (T) at low temperatures as well as at high temperatures from neutron scattering and GHz-THz dielectric relaxation, and hence, there is no fragile to strong transition. The Tg-scaled VFT temperature dependence of τ α,bulk (T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG β-relaxation in bulk water is supported by its equivalent relaxation observed in water confined in spaces with lengths of nanometer scale and having Arrhenius T-dependence of its relaxation times τ conf (T). The equivalence is justified by the drastic reduction of cooperativity of the α-relaxation in nanoconfinement and rendering it to become the JG β-relaxation. Thus, the τ conf (T) from experiments can be taken as τ β,bulk (T), the JG β-relaxation time of bulk water. The ratio τ α,bulk (Tg)/τ β,bulk (Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch α-correlation function, exp[−(t/τ α,bulk ) 1−n ], having (1−n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with Tg higher than that of water show the presence of a secondary ν-relaxation from the water component. The ν-relaxation is strongly connected to the α-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time τν(T) is longer than τ β,bulk (T), but τν(T) becomes about the same as τ β,bulk (T) at sufficiently high water content. However, τν(T) does not become shorter than τ β,bulk (T). Thus, τ β,bulk (T) is the lower bound of τν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τ β,bulk (T) but not τα(T) that is responsible for the dynamic transition of hydrated globular proteins.

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