Resolving anisotropic distributions of correlated vibrational motion in protein hydration water

In this study, we analyze correlations of vibrational motion on the surface of a small globular protein and in its hydration shell. In contrast to single particle hydration water dynamics, which are perturbed by interactions with the protein solute only in the first few hydration layers, we find that correlated, collective motions extend into the surrounding solvent on a 10 Å length scale, specifically at far-infrared frequencies below 100 cm^-1. As a function of frequency, we analyze the distribution of correlated longitudinal motions in the three-dimensional environment of the protein solute, as well as in the vicinity of different protein-water interfaces. An anisotropic distribution of these correlations is observed, which is related to specific protein-water vibrations and interactions at the interfaces, as well as flexibilities of solvent exposed sites. Our results show that coupling of protein and water dynamics leaves a three-dimensional imprint in the collective dynamics of its hydration shell, and we discuss potential implications for biomolecular function, e.g., molecular recognition and binding, and the dynamical coupling of proteins to their native solvation environment.