| Affiliation: | 1. Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and Technology, Ulsan, 44919 Republic of Korea;2. Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan, 44919 Republic of Korea;3. Department of Molecular Science and Technology, Ajou University, Suwon, 16499 Republic of Korea;4. Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and Technology, Ulsan, 44919 Republic of Korea Center for Soft and Living Matter, Institute for Basic Science (IBS), Ulsan, 44919 Republic of Korea |
| Abstract: | Here, we propose an experimental methodology based on femtosecond-resolved fluorescence spectroscopy to measure the hydrogen (H)-bond free energy of water at protein surfaces under isothermal conditions. A demonstration was conducted by installing a non-canonical isostere of tryptophan (7-azatryptophan) at the surface of a coiled-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, and it was found to be higher than that of bulk water by 0.4 kcal mol−1. The 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 a door to accessing the energetics and dynamics of local biological water to give insight into its roles in protein structure and function. |
