Bacteriorhodopsin analogue from anthryl chromophores

Abstract

Preparation and properties of the bacteriorhodopsin (bR) analogue having the 3,7-dimethyl-9-(9-anthryl)-2E,4E,6E,8E-nonatetraenal (12) chromophore is described. Synthesis of the chromophore has been achieved by successive introduction of C₅ units to 9-anthraldehyde (3) via the Horner reaction. The all-trans chromophore has been characterized by its ultraviolet-visible and ¹H nuclear magnetic resonance spectra. Incubation of 12 with bacterioopsin suspension (prepared by photobleaching of bR isolated from Halobacteriumhalobium) at ambient temperature in the dark gave the new bR analogue 15, which showed an absorption band at 545 nm, and an opsin shift of 5575 cm⁻¹. The new pigment is stable to hydroxylamine in the dark. It showed light-dark adaptation with the light-adapted form absorbing at a slightly red-shifted value of 550 nm. All-trans-retinal did not replace the anthryl chromophore in competitive bindings. Photolysis of the bR analogue 15, followed by difference spectrophotometric analysis, indicated formation of a photoproduct with an absorption band near 400 nm. The results are discussed in terms of the stereoelectronic requirements of the bR reaction centre.