Properties of hydration water and its role in protein dynamics

The low-temperature properties of confined water and the relation between protein and solvent dynamics have been studied by broadband dielectric spectroscopy with the aim to understand the role of hydration water for protein dynamics. At low temperatures ( below approximately 200 K) confined water generally exhibits two relaxation processes: one process that is due to the local beta-relaxation, and a faster and even more local process that is interpreted to arise from the motion of Bjerrum-type defects. These relaxation processes, showing Arrhenius temperature dependences, are also observed in glycerol water solvents of myoglobin containing >= 50 wt% water. In the temperature regime below 200 K, only a local protein process is observed. The activation energies of this protein process and the beta-relaxation in the solvent are similar, suggesting that this local protein process is determined by the beta-relaxation in the solvent. At about 200 K the nature of the dynamics changes dramatically and an onset of cooperative and large-scale dynamics is observed for both the water-rich solvent and the protein. We believe that the reason for this crossover is that the beta-relaxation in the solvent merges with the non-observable a-relaxation at this temperature, giving rise to a merged alpha-beta-relaxation in the solvent at higher temperatures. Also above this temperature the fastest observed protein processes seem to be determined by the solvent dynamics, as suggested for 'solvent-slaved' protein motions.