Circular dichroism of cephalopod rhodopsin and its intermediates in the bleaching and photoregeneration process.

In the bleaching process of cephalopod rhodopsin, a new intermediate was found in the conversion process from lumirhodopsin to metarhodopsin. This intermediate of octopus has an absorption peak at about 475 nm and has been named as M475. The circular dichroism value of M475 is too small to be evaluated. On the other hand, lumirhodopsin shows a negative CD at 470 nm, a positive CD at 350 nm and a large positive CD band with three peaks at 280, 287 and 295 nm. Such a large CD band in the ultraviolet region is not observed in rhodopsin, M475 and metarhodopsin. This CD seems to be mainly due to tryptophan and tyrosine residues restricted in free rotation in the protein moiety of lumirhodopsin. The intermediate in the photoregeneration process of cephalopod rhodopsin, P380, has a positive CD band at the main peak, 380 nm, and also a large positive CD band in the ultraviolet region like lumirhodopsin.