The JG alpha-relaxation in water and impact on the dynamics of aqueous mixtures and hydrated biomolecules

Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature T near 136 K, not much known are the spectral dispersion of the structural alpha-relaxation and the temperature dependence of its relaxation time tau_alpha-bulk(T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (JG) beta-relaxation is a question that has not been answered. By studying the structural alpha-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures T close to 136 K, we deduce the properties of the alpha-relaxation and the temperature dependence of tau_alpha(T) of bulk water. The frequency dispersion of the alpha-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of tau_alpha(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 T-scaled VFT temperature dependence of tau_alpha(T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG beta-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 tau_conf(T). The equivalence is justified by the drastic reduction of cooperativity of the alpha-relaxation in nanoconfinement and rendering it to become the JG beta-relaxation. Thus, the tau_conf(T) from experiments can be taken as tauJG_bulk(T), the JG beta-relaxation time of bulk water. The ratio tau_alphaBulk(Tg)/tau_betaBulk(Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch alpha-correlation function, exp[-(t/tau_alphaBulk)^(1-n)], having (1-n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with T higher than that of water show the presence of a secondary nu-relaxation from the water component. The nu-relaxation is strongly connected to the alpha-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time tau_nu(T) is longer than tau_betaBulk(T), but tau_nu(T) becomes about the same as tau_betaBulk(T) at sufficiently high water content. However, tau_nu(T) does not become shorter than tau_betaBulk(T). Thus, tau_betaBulk(T) is the lower bound of tau_nu(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is tau_betaBulk(T) but not tau_alpha(T) that is responsible for the dynamic transition of hydrated globular proteins.