Role of solvent for the dynamics and the glass transition of proteins

For the first time, a systematic investigation of the glass transition and its related dynamics of myoglobin in water-glycerol solvent mixtures of different water contents is presented. By a combination of broadband dielectric spectroscopy and differential scanning calorimetry (DSC), we have studied the relation between the protein and solvent dynamics with the aim to better understand the calorimetric glass transition, T(g), of proteins and the role of solvent for protein dynamics. The results show that both the viscosity related α-relaxation in the solvent as well as several different protein relaxations are involved in the calorimetric glass transition, and that the broadness (ΔT(g)) of the transition depends strongly on the total amount of solvent. The main reason for this seems to be that the protein relaxation processes become more separated in time with decreasing solvent level. The results are compared to that of hydrated myoglobin where the hydration water does not give any direct contribution to the calorimetric T(g). However, the large-scale α-like relaxation in the hydration water is still responsible for the protein dynamics that freeze-in at T(g). Finally, the dielectric data show clearly that the protein relaxation processes exhibit similar temperature dependences as the α-relaxation in the solvent, as suggested for solvent-slaved protein motions.