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

Objective: Rhodopsin from the eubacterium Exiguobacterium sibiricum (ESR) performs the lightdependent proton pumping function. The operation of ESR is based on the ultrafast photochemical reaction of isomerization of the retinal chromophore, which triggers dark processes closed in a photocycle. Many parameters of the photocycle are determined by the presence of a hydrogen bond between the primary counterion Asp85 and the chromophore. ESR in detergent micelles pumps protons most efficiently at pH > 9, when such a bond is most probable. Methods: In the present study, the photochemical reaction of ESR at pH 9.5 was investigated by femtosecond laser absorption spectroscopy. Results and Discussion: It was shown that photoisomerization of the chromophore group occurs in 0.51 ps, with the contribution from the reactive excited state being ca. 80%. A comparison with the earlier obtained data for pH 7.4 showed that, at pH 9.5, the reaction proceeds much faster and more efficiently. Conclusions: The present results confirm the important role of the chromophore group counterion in the photoactivated processes of rhodopsins.