Opsin pigments formed with acyclic retinal analogues

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Abstract
Three acyclic derivatives (3,7‐dimethyl‐10‐ethyl‐2,4,6,8‐dodecatetraenal, 3,7,10‐trimethyl‐2,4,6,8‐dodecatetraenal, and 3,7,10‐trimethyl‐2,4,6,8‐undecatetraenal) have been synthesized and the 2‐cis, 6‐cis, 2,6‐dicis and all‐trans isomers of each isolated. Unlike the undecatetraenal compound, the dodecatetraenal derivatives, have methyl groups which can mimic the C‐1 and/or C‐5 methyl group of retinal. The 6‐cis and 2,6‐dicis isomers of the dodecatetraenal compounds form photosensitive pigments with bovine opsin. No pigment is obtained with the undecatetraenal derivative. Therefore, although the retinal cyclohexyl ring is not essential for pigment formation, at least one methyl group corresponding to the C‐1 or C‐5 retinal methyl may be required for obtaining stable opsin pigments.