Yang J, Wang Y, Wang L et al.
Journal of the American Chemical Society. Mar 2017.
Protein surface hydration is fundamental to its structure, flexibility, dynamics and function, but it has been challenging to disentangle their ultimate relationships. Here, we report our systematic characterization of hydration dynamics around a β-barrel protein, rat liver fatty acid-binding protein (rLFABP), to reveal the effect of different protein secondary structures on hydration water. We employed a tryptophan scan to the protein surface one at a time and examined total 17 different sites. We observed three types of hydration water relaxations with distinct time scales, from hundreds of femtoseconds to a hundred of picoseconds. We also examined the anisotropy dynamics of the corresponding tryptophan sidechains and observed two distinct relaxations from tens to hundreds of picoseconds. Integrating our previous findings on α-helical proteins, we conclude that (1) the hydration dynamics is highly heterogeneous around the protein surface of both α-helical and β-sheet proteins. The outer-layers water of the hydration shell behaves bulk-type and relaxes in hundreds of femtoseconds. The inner-layers water collectively relaxes in two time scales, reorientation motions in a few picoseconds and network restructuring in tens-to-a hundred of picoseconds; (2) the hydration dynamics are always faster than local protein relaxations and in fact drive the protein fluctuations on the picosecond time scale; (3) the hydration dynamics in general are more retarded around β-sheet structures than α-helical motifs. A thicker hydration shell and a more rigid interfacial hydration network are observed in the β-sheet protein. Overall, these findings elucidate the intimate relationship of water-protein interactions and dynamics on the ultrafast timescale.