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

Abstract

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 CC 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 CC vibronic coupling mode, CN 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.