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 photoisomerlzation process of bacteriorhodopain＇s （BR） photocycle to reveal the character of the important Intermediate of J625 and to obtain a deeper understanding of the role of photoisomerizaUon in BR photocycle. Two time constants of 0.5 pa （95%） and 2.0 pa （5%） were brought out by global fitting on thirty curves In the nser-lnfrared reglon. We suggest that the first time constant results from the decay of I480 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 pa. 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- 760 nm reglona was attributed to the radiative transition （stimulated emission） from the Franck-Condon actlve conflguratlon along the isomerization potential surface of all-trans-retinal.