Chiral discrimination of 5,6‐epoxy‐3‐dehydroretinal by aporetinochrome and cattle opsin

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Abstract
The combination of a racemic all-trans 5,6-epoxy-3-dehydroretinal (EDR) with aporetinochrome formed a mixture of 2 diastereomeric pigments. One of the diastereomeric pigments which contained the all-trans EDR with a negative circular dichroic (CD) band, hereafter called EDR(-)-chrome, has its visible absorption maximum around 438 nm, while the other pigment, called EDR(+)-chrome, has its maximum at 464 nm. These were substantiated by measuring the optical activities of the 2 EDR isomers which were extracted from a mixture of racemic all-trans EDR and a smaller amount of aporetinochrome following exposure to orange light (> 530 nm) that irradiates EDR(+)-chrome selectively. The extracted all-trans EDR had a negative CD band around 240 nm and the extracted 11-cis EDR had a positive band in that region with 2 negative bands on either side of the main band. In the case of both pigments, the effect of alkalinization on the increase of absorbance in the near-UV and the decrease of absorbance in the visible region was proportionate, qualitatively, to that on the positive CD intensities in both regions. The chromophore EDR in each pigment probably binds to the same binding site via a Schiff base. The EDR(+)-chrome exhibited properties similar to those of retinochrome, but EDR(-)-chrome showed some different properties, i.e., its formation rate was slower than that of the former one and its absorption band in the near-UV appeared even at neutrall pH. Moreover, by exposing EDR(-)-chrome to yellow light (> 480 nm), only a part of its prosthetic all-trans EDR was isomerized and resulted in the formation of 11-cis and 13-cis isomers. This variation in photoisomerizing activity was supposed to be due to the difference in conformation of the side chain between EDR(-) and EDR(-) in aporetinochrome. Combination of 11-cis EDR with cattle opsin also resulted in the formation of 2 diastereomeric pigments. The absorption maxima of the diastereomers containing 11-cis EDR(+) and EDR(-) were at about 446 and 474 nm, respectively.