Reversible Photochromic Reactions of Bacteriorhodopsin from at Femto- and Picosecond Times.

The operation of bacteriorhodopsin () from the archaeon is based on the photochromic reaction of isomerization of the chromophore group (the retinal protonated Schiff base, RPSB) from the all- to the 13- form. The ultrafast dynamics of the reverse 13- → all- photoreaction was studied using femtosecond transient absorption spectroscopy in comparison with the forward photoreaction. The forward photoreaction was initiated by photoexcitation of by pulse I (540 nm). The reverse photoreaction was initiated by photoexcitation of the product at an early stage of its formation (5 ps) by pulse II (660 nm). The conversion of the excited to the ground state proceeds at times of 0.19, 1.1, and 16 ps with the relative contributions of ~20/60/20, respectively. All these decay channels lead to the formation of the initial state of as a product with a quantum yield of ~1. This state is preceded by vibrationally excited intermediates, the relaxation of which occurs in the 16 ps time range. Likely, the heterogeneity of the excited state of is determined by the heterogeneity of its chromophore center. The forward photoreaction includes two components-0.52 and 3.5 ps, with the relative contributions of 91/9, respectively. The reverse photoreaction initiated from proceeds more efficiently in the conical intersection (CI) region but on the whole at a lower rate compared to the forward photoreaction, due to significant heterogeneity of the potential energy surface.