The JG β-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_g 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 T_g 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 T_g-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(T_g)/τ_β,bulk(T_g) 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 T_g 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 τ_ν(T) is longer than τ_β,bulk(T), but τ_ν(T) becomes about the same as τ_β,bulk(T) at sufficiently high water content. However, τ_ν(T) does not become shorter than τ_β,bulk(T). Thus, τ_β,bulk(T) is the lower bound of τ_ν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τ_β,bulk(T) but not τ_α(T) that is responsible for the dynamic transition of hydrated globular proteins.