PHOTOEXCITATION BY A HALF-CAROTENOID - SYMBIOSIS BETWEEN RETINAL AND THE VISUAL PROTEIN OPSIN

The visual process is initiated by photoactivation of a receptor prote in, e.g. the vertebrate rod visual pigment rhodopsin. Upon absorption of light its chromophore, 11-cis retinal, stereoisomerizes to the all- trans (AT) state. This conformational change triggers subsequent subtl e conformational changes in the protein (opsin), which pass through se veral distinct intermediate states (photocascade) and within milliseco nds culminate in the ''active'' intermediate (Meta II). The light-depe ndent trigger reaction basically is a simple 11-cis --> AT photoisomer ization of a retinoid. However, the : interaction of 11-cis retinal wi th opsin has bestowed some unique features upon this reaction: 1) it p roceeds with unsurpassed speed, and is complete within 200 fs, 2) it i s fully stereospecific and 3) it has an unusually high quantum yield ( (sic) = 0.7). How opsin has adapted to establish such a highly effecti ve symbiosis is still largely unclear. Modern spectroscopic techniques are slowly beginning to unveil some of the secrets of this mechanism. Here, we will address some features of 11-cis retinal which seem part icularly relevant: its side-chain methyl groups and torsional strain.