Dynamics of amorphous and partially crystallized proline solutions

Dynamics of water in the hydration shells of proteins in the no man's land temperature region is a currently debated topic. Previously [1], we proposed the use of lysine aqueous solutions as a model system to study the dynamics of water in a genuine biological solution at any temperature. We found that lysine can be completely dissolved in water without crystallization, making it possible to access the dynamics of amorphous water in a biological environment. By contrast, in this work we studied proline aqueous solutions, which partially crystallize upon cooling. We found similar results in the two systems: the presence of three water relaxations where the two slowest processes (processes 2 and 3) resemble the behavior of normal glasses. Moreover, we observed the change of a cooperative (α-like) water relaxation to a more local β-like water relaxation with decreasing temperature whereas a faster water relaxation process is also present. In addition, we studied amorphous samples in a restricted temperature interval, and also found three different dynamics (processes 3, 4 and 5). As in the case of the hydrated protein powders, the two faster dynamics (3 and 4) are coupled or slaved.