The terahertz dance of water with the proteins: the effect of protein flexibility on the dynamical hydration shell of ubiquitin.

The role of water in the functioning of proteins has been a hot topic over the years. We use terahertz (THz) spectroscopy as an experimental tool to probe the protein-induced fast solvation dynamics of ubiquitin. In order to investigate the effect of protein flexibility on the changes in the solvation dynamics, we have measured the concentration-dependent THz absorption of several site-specific ubiquitin mutants. The observed non-linear dependence of absorption on concentration is a signature of a long-range hydration shell with properties distinct from bulk water. We determined a dynamical hydration shell of a thickness of at least 18 A on the protein surface. This exceeds the static hydration layer as it is typically observed by scattering methods (3 A) by far. We also conclude that any increase in flexibility obtained by side-chain truncations that decrease the structural rigidity of the protein results in more bulk-like behaviour of the dynamical hydration shell. Furthermore, our THz measurements show that a single phenylalanine-to-tryptophan substitution to introduce a fluorescent marker leads to measurable changes in the solvation dynamics.