Ultrafast isomerization dynamics of retinal in bacteriorhodopsin as revealed by femtosecond absorption spectroscopy

A femtosecond (fs) broad-band absorption apparatus was used to measure the early photoisomerization process of bacteriorhodopsin’s (BR) photocycle to reveal the character of the important intermediate of J₆₂₅ and to obtain a deeper understanding of the role of photoisomerization in BR photocycle. Two time constants of 0.5 ps (95%) and 2.0 ps (5%) were brought out by global fitting on thirty curves in the near-infrared region. We suggest that the first time constant results from the decay of I₄₆₀ intermediate, and the longer component might be associated with BR isomer. The global analysis over 450, 540, 630, 710 and 870 nm traces identified two time constants, ∼0.5 and ∼3 ps. The slower component can be extracted from the processes of both J₆₂₅→BR₅₆₈ (540 nm) and J₆₂₅→K₅₉₀ (630 nm), suggesting J-intermediate takes a partial cis configuration. The obvious negative feature in early delay time of 700–780 nm regions was attributed to the radiative transition (stimulated emission) from the Franck-Condon active configuration along the isomerization potential surface of all-trans-retinal. Contributed equally to this work Supported by the National Natural Science Foundation of China (Grant Nos. 20673144 and 20433010)