Protein Crowding Effects on Hydration Water Dynamics

We propose a time-resolved optical Kerr effect study of the structural and vibrational dynamics of the hydration water surrounding the lysozyme on a very fast time scale. Measurements as a function of lysozyme concentration make it possible to distinguish the hydration water contribution from that of both the bulk water and the protein. Our results provide experimental evidence of the existence of two structural dynamics of hydration water, associated with a hydrogen bond exchange relaxation process and with the reorganization of water molecules induced by protein structural fluctuations. Likewise, we evaluated the vibrational dynamics of the water hydration layer at subpicosecond time scales. Our measurements of hydration water properties reveal the presence of a crossover point at a specific protein concentration. This crossover marks the transition between two clustering regimes with distinct hydration characteristics and establishes a possible threshold for protein crowding.