Time scales of water dynamics at biological interfaces: peptides, proteins and cells

Water H-2 and O-17 spin relaxation is used to study water dynamics in the hydration layers of two small peptides, two globular proteins and in living cells of two microorganisms. The dynamical heterogeneity of hydration water is characterized by performing relaxation measurements over a wide temperature range, extending deeply into the suspecooled regime, or by covering a wide frequency range. Protein hydration lavers can be described by a power-law distribution of rotational correlation times with an exponent close to 2. This distribution comprises a small fraction of protein-specific hydration sites, where water rotation is strongly retarded, and a dominant fraction of generic hydration sites, where water rotation is as fast as in the hydration shells of small peptides. The generic dynamic perturbation factor is less than 2 at room temperature and exhibits a maximum near 260 K. The dynamic perturbation is induced by H-bond constraints that interfare with the cooperative mechanism that facilitates rotation in bulk water. Because therse constraints are temperature-independence of bulk water. There is no significant difference in cell water dynamics between mesophilic and halophilic organisms, despite the high K+ and Na+ concentrations in the latter.