Structure around C₆−C₇ Bond of the Chromophore in Bathorhodopsin:  Low-Temperature Spectroscopy of 6s-cis-Locked Bicyclic Rhodopsin Analogs

To elucidate the structural changes near the β-ionone ring region of the chromophore during the photobleaching process of rhodopsin, the photochemical and subsequent thermal reactions of rhodopsin analogs, whose retinylidene chromophores were fixed in a 6s-cis form with a five-membered ring (6,5-rhodopsin) and a seven-membered ring (6,7-rhodopsin), respectively, were investigated by low-temperature spectroscopy. Like rhodopsin, both the rhodopsin analogs convert to the respective batho-intermediates upon absorption of light at −190 °C. The extinction coefficient of batho-intermediate of 6,5-rhodopsin is similar to that of bathorhodopsin, while that of 6,7-rhodopsin is considerably smaller than that of bathorhodopsin. Like bathorhodopsin, the batho-intermediate of 6,5-rhodopsin directly converts to lumi-intermediate, while that of 6,7-rhodopsin first converts to a blue-shifted intermediate and then to lumi-intermediate. These results strongly suggest that the structure around the β-ionone ring region of the bathorhodopsin chromophore resembles 6,5-retinal rather than 6,7-retinal. From the comparison of the structural features among retinal, 6,5-retinal, and 6,7-retinal, a possible conformation around C6−C7 bond of the bathorhodopsin chromophore is discussed.