Hydrogen‐Bond Free Energy of Local Biological Water

Abstract

Here,w ep ropose an experimental methodology based on femtosecond-resolved fluorescence spectroscopyt o measure the hydrogen (H)-bond free energy of water at protein surfaces under isothermal conditions.Ademonstration was conducted by installing anon-canonical isostere of tryptophan (7-azatryptophan) at the surface of ac oiled-coil protein to exploit the photoinduced proton transfer of its chromophoric moiety,7-azaindole.The H-bond free energy of this biological water was evaluated by comparing the rates of proton transfer, sensitive to the hydration environment, at the protein surface and in bulk water,a nd it was found to be higher than that of bulk water by 0.4 kcal mol À1 .T he free-energy difference is dominated by the entropic cost in the H-bond network among water molecules at the hydrophilic and charged protein surface. Our study opens ad oor to accessing the energetics and dynamics of local biological water to give insight into its roles in protein structure and function.