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

The operation of bacteriorhodopsin (<i>BR</i>) from the archaeon <i>Halobacterium salinarum</i> is based on the photochromic reaction of isomerization of the chromophore group (the retinal protonated Schiff base, RPSB) from the all-<i>trans</i> to the 13-<i>cis</i> form. The ultrafast dynamics of the reverse 13-<i>cis</i> → all-<i>trans</i> photoreaction was studied using femtosecond transient absorption spectroscopy in comparison with the forward photoreaction. The forward photoreaction was initiated by photoexcitation of <i>BR</i> by pulse I (540 nm). The reverse photoreaction was initiated by photoexcitation of the product <i>K</i>₅₉₀ at an early stage of its formation (5 ps) by pulse II (660 nm). The conversion of the excited <i>K</i>₅₉₀ 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 <i>BR</i> 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 <i>K</i>₅₉₀ 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 <i>K</i>₅₉₀ 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.