Ultrafast energy relaxation dynamics of amide I vibrations coupled with protein-bound water molecules

The influence of hydration water on the vibrational energy relaxation in a protein holds the key to understand ultrafast protein dynamics, but its detection is a major challenge. Here, we report measurements on the ultrafast vibrational dynamics of amide I vibrations of proteins at the lipid membrane/H2O interface using femtosecond time-resolved sum frequency generation vibrational spectroscopy. We find that the relaxation time of the amide I mode shows a very strong dependence on the H2O exposure, but not on the D2O exposure. This observation indicates that the exposure of amide I bond to H2O opens up a resonant relaxation channel and facilitates direct resonant vibrational energy transfer from the amide I mode to the H2O bending mode. The protein backbone motions can thus be energetically coupled with protein-bound water molecules. Our findings highlight the influence of H2O on the ultrafast structure dynamics of proteins.