Ultrafast Photochemical Reaction of Exiguobacterium sibiricum Rhodopsin (ESR) at Alkaline pH

Rhodopsin from the eubacterium Exiguobacterium sibiricum (ESR) performs the function of light-dependent proton transport. The operation of ESR is based on the ultrafast photochemical reaction of isomerization of the retinal chromophore, which triggers dark processes closed in the photocycle. Many parameters of the photocycle are determined by the degree of protonation of Asp85 – the primary counterion of the chromophore group and the proton acceptor. ESR in detergent micelles pumps protons most efficiently at pH 9, when Asp85 is almost completely deprotonated. In this work, the photochemical reaction of ESR at pH 9.5 was studied by femtosecond laser absorption spectroscopy. It was shown that photoisomerization of the chromophore group occurs in 0.51 ps, and the contribution of the reactive excited state is about 80%. A comparison with the data we obtained at pH 7.4 showed that at pH 9.5 the reaction proceeds much faster and more efficiently. The data obtained confirm the important role of the chromophore group counterion in the photoactivated processes of rhodopsins.