Dynamics of hydrated proteins and bio-protectants: Caged dynamics, β-relaxation, and α-relaxation

Background: The properties of the three dynamic processes, alpha-relaxation, nu-relaxation, and caged dynamics in aqueous mixtures and hydrated proteins are analogous to corresponding processes found in van der Waals and polymeric glass-formers apart from minor differences. Methods: Collection of various experimental data enables us to characterize the structural alpha-relaxation of the protein coupled to hydration water (HW), the secondary or nu-relaxation of HW, and the caged HW process. Results: From the T-dependence of the nu-relaxation time of hydrated myoglobin, lysozyme, and bovine serum albumin, we obtain Ton at which it enters the experimental time windows of MOssbauer and neutron scattering spectroscopies, coinciding with protein dynamical transition (PDT) temperature Td. However, for all systems considered, the alpha-relaxation time at Ton or Td is many orders of magnitude longer. The other step change of the mean-square-displacement (MSD) at Tg_alpha originates from the coupling of the nearly constant loss (NCL) of caged HW to density. The coupling of the NCL to density is further demonstrated by another step change at the secondary glass temperature Tg_beta in two bio-protectants, trehalose and sucrose. Conclusions: The structural alpha-relaxation plays no role in PDT. Since PDT is simply due to the v-relaxation of HW, the term PDT is a misnomer. NCL of caged dynamics is coupled to density and show transitions at lower temperature, Tg_beta and Tg_alpha. General significance: The so-called protein dynamical transition (PDT) of hydrated proteins is not caused by the structural alpha-relaxation of the protein but by the secondary nu-relaxation of hydration water