Contribution of Methyls in Retinal Side Chain to Regioselective Photoisomerization of Retinochromes
All-trans-9-demethyl-11-methylretinal was synthesized to elucidate the chromophoric interaction between hydrophobic part in retinochrome and 9-methyl substituent of retinal. Photoirradiation of its pigment efficiently gave the 11-cis-isomer in 90% regioselectivity, whereas that of 9-demethylretinochrome provided the 11-cis isomer in 49% selectivity. Structural contribution to regiospecific photoisomerization was proved, in comparison with the other retinochrome analogues, that 9- or 11-methyl substituent is a requisite for highly regioselective photoisomerization, but 13-methyl group is not.