Vibrational relaxation during the retinal isomerization in Bacteriorhodopsin

Femtosecond time-resolved optical pump–infrared probe spectroscopy was employed to characterize the ethylenic (CC)-stretch vibrational dynamics (1498–1542 cm−1) of the retinal chromophore in Bacteriorhodopsin (BR) during the initial, all-trans to 13-cis isomerization. The early branching reaction was observed, i.e. formation of the ground-state 13-cis product, K, and partial recovery of the all-trans educt state BR570. The BR570 recovery occurs within 2 ps and thus is faster than K-formation (3–4 ps). The IR transients are described in terms of a kinetic model involving vibrational precursors for K and for the recovering BR570, respectively.