Inertial suppression of protein dynamics in a binary glycerol-trehalose glass.

The traditional approach used to predict the ability of a glassy matrix to maximally preserve the activity of a protein solute is the glass transition temperature (T(g)) of the glass. Recently it has been shown that the addition of a low T(g) diluent (glycerol) can rigidify the structure of a high T(g) glassy matrix in binary glycerol-trehalose glasses. The optimal density of glycerol in trehalose minimizes the average mean square displacements of non-exchangeable protons in the glass samples. The amount of glycerol added to a trehalose glass coincides with the maximal recovery of biological activity in a separate study using similar binary glass samples. In this study, we use molecular dynamics (MD) simulations to investigate the dynamics of a hydrated protein encased in glycerol, unary trehalose and binary glycerol-trehalose glasses. We have found that we are able to reproduce the rigidification of the glycerol-trehalose glassy matrix and that there is a direct correlation between bulk glass dynamics and the extent of atomic fluctuation of protein atoms. The detailed microscopic picture that emerges is that protein dynamics are suppressed mainly by inertia of the bulk glass and to a lesser extent specific interactions at the protein-solvent interface. Thus, the inertia of the glassy matrix may be an influential factor in the determination of pharmaceutically relevant formulations.