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

The process of proton translocation in Halobacterium salinarum, triggered by light, is powered by the photoisomerization of all-trans-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-trans-retinal in bR using femtosecond stimulated Raman spectroscopy. We report Raman modes at 1820 cm^-1 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^- _g 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^-1), and hydrogen out-of-plane (HOOP) mode (954 cm^-1) further supports the three-state model that elucidates the initial charge translocation along the conjugated chain accompanied by trans-to-cis photoisomerization dynamics through H(1B⁺ _u) → I(2A^- _g) → J(2A^- _g) → K(13-cis ground state) transitions in all-trans-retinal in bR.