Synthetic pigment analogues of the purple membrane protein

Nonphysiological analogues of retinal have been shown to form pigments in reactions with the apoprotein of the purple membrane of Halobacterium halobium. Both the all-trans and 13-cis isomers of a retinal analogue, having an elongated chain with an extra double bond, formed pigments. Unlike the native all-trans and 13-cis retinal1-based pigments, the new pigments were not interconvertible with each other and were unstable against hydroxylamine. When incorporated into phospholipid vesicles, they showed no proton pumping activity upon illumination. The ability of the extended-length retinal to form pigments contrasts with its nonreactivity with opsin (apoprotein of rhodopsin), suggesting a less stringent binding site for the purple membrane chromophore. All-trans retinal2 also combined with bleached purple membrane to form a blue pigment absorbing at ca. 590 nm. Like the native purple membrane, the blu membrane showed proton pumping activity upon illumination in phospholipid vesicles.