Interaction of the Fluorophenyl Analog of Retinal with Proteorhodopsin from Exiguobacterium sibiricum

Abstract

—We have developed an alternative method for the synthesis of an analog of natural retinal, which contains the p-fluorophenyl fragment instead of the trimethylcyclohexene ring. The proposed scheme for the synthesis of the target all-E-isomer of the target retinoid consists of using C5-phosphonate that contains the terminal nitrile group under Horner–Emmons reaction conditions. It has been shown that this scheme is more efficient and provides a higher total yield of the target product than the previously described variant. The procedure has been developed for the preparation of an analog of microbial proteorhodopsin ESRh from Exiguobacterium sibiricum, which contains a modified chromophore. It has been found that, as in the case of bacterioopsin from Halobacterium salinarum, the replacement of the trimethylcyclohexene ring in the natural chromophore by the p-fluorophenyl fragment does not prevent the formation of the artificial pigment F-Phe-ESRh from proteorhodopsin ESRh, which preserves the cycle of photochemical reactions. Certain differences have been found between the properties of native recombinant ESRh and its analog F-Phe-ESRh including a shift in the absorption maximum to the short-wavelength region, the formation of M intermediate at lower pH values, the presence of “long-lived M,” and a general slowdown in the photocycle. The reduced stability of the resulting proteorhodopsin analog F-Phe-ESRh to prolonged exposure to visible light has been also demonstrated.