The Dynamics of Hydrated Proteins Are the Same as Those of Highly Asymmetric Mixtures of Two Glass-Formers

Customarily, the studies of dynamics of hydrated proteins are focused on the fast hydration water nu-relaxation, the slow structural alpha-relaxation responsible for a single glass transition, and the protein dynamic transition (PDT). Guided by the analogy with the dynamics of highly asymmetric mixtures of molecular glass-formers, we explore the possibility that the dynamics of hydrated proteins are richer than presently known. By providing neutron scattering, dielectric relaxation, calorimetry, and deuteron NMR data in two hydrated globular proteins, myoglobin and BSA, and the fibrous elastin, we show the presence of two structural alpha-relaxations, alpha 1 and alpha 2, and the hydration water nu-relaxation, all coupled together with interconnecting properties. There are two glass transition temperatures T-g(alpha 1) and T-g(alpha 2) corresponding to vitrification of the alpha 1 and alpha 2 processes. Relaxation time tau(alpha 2)(T) of the alpha 2-relaxation changes its Arrhenius temperature dependence to super-Arrhenius on crossing T-g(alpha 1) from below. The nu-relaxation responds to the two vitrifications by changing the T-dependence of its relaxation time tau(nu)(T) on crossing consecutively T-g(alpha 2) and T-g(alpha 1). It generates the PDT at T-d where tau(nu)(T-d) matches about five times the experimental instrument timescale T-exp, provided that T-d > This condition is satisfied by the hydrated globular proteins considered in this paper, and the nu-relaxation is in the liquid state with tau(nu)(T) having the super-Arrhenius temperature dependence. However, if T-d < T-g(alpha 1), the nu-relaxation fails to generate the PDT because it is in the glassy state and tau(nu)(T) has Arrhenius T-dependence, as in the case of hydrated elastin. Overall, the dynamics of hydrated proteins are the same as the dynamics of highly asymmetric mixtures of glass-formers. The results from this study have expanded the knowledge of the dynamic processes and their properties in hydrated proteins, and impact on research in this area is expected.