The Molecular Origin of Ultrafast Water-Protein Coupled Interactions.

The fluctuations of hydration water and the protein are coupled together at the protein surface and often such water-protein dynamic interactions are controlled presumably by hydration water motions. However, direct evidence is scarce and it requires measuring the dynamics of hydration water and protein sidechain simultaneously. Here, we use a unique protein with a single tryptophan to directly probe interfacial water and related sidechain relaxations with temperature dependence. With systematic mutations to change local chemical identity and structural flexibility, we found that the sidechain relaxations are always slower than hydration water motions and the two dynamic processes are linearly correlated with the same energy barriers, indicating the same origin of both relaxations. The charge mutations change the rates of hydration water relaxations but not the relaxation barriers. These results convincingly show that the water-protein relaxations are strongly coupled and the hydration water molecules govern such fluctuations on the picosecond timescales.