Abstract
Progress in the studies of visual pigment rhodopsin using retinal anaiogues is reviewed. At first, the external point-charge model based on the hydrorhodopsin data was presented to explain the wavelength regulation in visual pigments. In order to clarify the primary photochemical event by chemical methods, two types of rhodopsin analogues [(3) and (4)] were synthesized. The former (3) was derived from 9-cis-retro-γ-retinal (5) possessing two separated chromophore systems. The latter (4) was prepared from 11-cis-locked-cyclopentatrienylidene retinal analogue (6). Photochemical studies of these analogues showed that an 11-cis to 11-trans isomerization is a pre-requisite for visual transduction. The same confirmation was also obtained by employing a cycloheptatrienylidene rhodopsin (27). Comparison of the CD data for (4), rhodopsin, and (27) demonstrated that the a-CD band of rhodopsin has its origin in the twisted 12s-trans-conformation in the chromophore. Subsequently, in order to investigate the origin of β-CD band of rhodopsin, 6s-cis-locked-and6s-trans-locked-bicyclic rhodopsins [(51) and (52)] were synthesized.
