Mapping photoisomerization dynamics on a three-state model potential energy surface in bacteriorhodopsin using femtosecond stimulated Raman spectroscopy.

The process of proton translocation in , triggered by light, is powered by the photoisomerization of all--retinal in bacteriorhodopsin (bR). The primary events in bR involving rapid structural changes upon light absorption occur within subpicoseconds to picoseconds. While the three-state model has received extensive support in describing the primary events between the H and K states, precise characterization of each excited state in the three-state model during photoisomerization remains elusive. In this study, we investigate the ultrafast structural dynamics of all--retinal in bR using femtosecond stimulated Raman spectroscopy. We report Raman modes at 1820 cm which arise from C[double bond, length as m-dash]C stretch vibronic coupling and provide direct experimental evidence for the involvement of the I and J states with 2A symmetric character in the three-state model. The detection of the C[double bond, length as m-dash]C vibronic coupling mode, C[double bond, length as m-dash]N stretching mode (1700 cm), and hydrogen out-of-plane (HOOP) mode (954 cm) further supports the three-state model that elucidates the initial charge translocation along the conjugated chain accompanied by -to- photoisomerization dynamics through H(1B ) → I(2A ) → J(2A ) → K(13- ground state) transitions in all--retinal in bR.