The protein-solvent glass transition

The protein dynamical transition and its connection with the liquid-glass transition (GT) of hydration water and aqueous solvents are reviewed. The protein solvation shell exhibits a regular glass transition, characterized by steps in the specific heat and the thermal expansion coefficient at the calorimetric glass temperature T-G approximate to 170 K. it implies that the time scale of the structural alpha-relaxation has reached the experimental time window of 1-100 s. The protein dynamical transition, identified from elastic neutron scattering experiments by enhanced amplitudes of molecular motions exceeding the vibrational level [1], probes the alpha-process on a shorter time scale. The corresponding liquid-glass transition occurs at higher temperatures, typically 240 K. The GT is generally associated with diverging viscosities, the freezing of long-range translational diffusion in the supercooled liquid. Due to mutual hydrogen bonding, both, protein-and solvent relaxational degrees of freedom slow down in paralled near the GT. However, the freezing of protein motions, where surface-coupled rotational and librational degrees of freedom are arrested, is better characterized as a rubber-glass transition. In contrast, internal protein modes such as the rotation of side chains are not affected. Moreover, ligand binding experiments with myoglobin in various glass-forming solvents show, that only ligand entry and exit rates depend on the local viscosity near the protein surface, but protein-internal ligand migration is not coupled to the solvent. The GT leads to structural arrest on a macroscopic scale due to the microscopic cage effect on the scale of the intermolecular distance. Mode coupling theory provides a theoretical framework to understand the microcopic nature of the GT even in complex systems. The role of the alpha- and beta-process in the dynamics of protein hydration water is evaluated. The protein-solvent GT is triggered by hydrogen bond fluctuations, which give rise to fast P-processes. High-frequency neutron scattering spectra indicate increasing hydrogen bond braking above T-G. (C) 2009 Elsevier B.V. All rights reserved.