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) approximately 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, 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 paralleled 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 microscopic 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 beta-processes. High-frequency neutron scattering spectra indicate increasing hydrogen bond braking above T(G).