The protein glass transition as measured by dielectric spectroscopy and differential scanning calorimetry

The glass transition and its related dynamics of myoglobin in water and in a water-glycerol mixture have been investigated by dielectric spectroscopy and differential scanning calorimetry (DSC). For all samples, the DSC measurements display a glass transition that extends over a large temperature range. Both the temperature of the transition and its broadness decrease rapidly with increasing amount of solvent in the system. The dielectric measurements show several dynamical processes, due to both protein and solvent relaxations, and in the case of pure water as solvent the main protein process (which most likely is due to conformational changes of the protein structure) exhibits a dynamic glass transition (i.e. reaches a relaxation time of 100 s) at about the same temperature as the calorimetric glass transition temperature Tg is found. This glass transition is most likely caused by the dynamic crossover and the associated vanishing of the α-relaxation of the main water relaxation, although it does not contribute to the calorimetric Tg. This is in contrast to myoglobin in water-glycerol, where the main solvent relaxation makes the strongest contribution to the calorimetric glass transition. For all samples it is clear that several proteins processes are involved in the calorimetric glass transition and the broadness of the transition depends on how much these different relaxations are separated in time.