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 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 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 C vibronic coupling mode, C 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.