8,16‐ And 8,18‐methanobacteriorhodopsin. Synthesis and spectroscopy of 8,16‐ and 8,18‐methanoretinal and their interaction with bacterioopsin

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Abstract
8,16‐ And 8,18‐methanoretinal with a locked 6‐s‐trans and a 6‐s‐cis conformation have been synthesized and characterized. Their 9‐Z and 13‐Z isomers could also be prepared, in contrast to the 11‐Z. 8,16‐Methanoretinal easily binds with bacterioopsin (bO) to form a bacteriorhodopsin analogue which closely resembles natural bacteriorhodopsin (bR) (λmax, light‐dark adaptation and proton pump action). The interaction of 8,18‐methanoretinal with bO is complex. These findings give additional support to the fact that in bR the chromophore occurs in the planar 6‐s‐trans conformation. Approximately 1200 cm−1 of the opsin shift in bR (5100 cm−1) arises because the chromophore changes upon binding to the protein from a 40° twisted 6‐s‐cis to a planar 6‐s‐trans conformation. Both 8,16‐ and 8,18‐methano bR have a ∼3900 cm−1 opsin shift, since the C6‐C7 conformation does not allow protein‐induced changes. This is consistent with the idea that the opsin shift is mainly due to the perturbation of the Schiff base region.