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 th...