Photo-excitation by a half-carotenoid: Symbiosis between retinal and the visual protein opsin

Abstract

The visual process is initiated by photoactivation of a receptor protein, 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 subtle conformational changes in the protein (opsin), which pass through several distinct intermediate states (photocascade) and within milliseconds culminate in the "active" intermediate (Meta II). The light-dependent trigger reaction basically is a simple 11-cis → AT photoisomerization of a retinoid. However, the interaction of 11-cis retinal with opsin has bestowed some unique features upon this reaction: 1) it proceeds with unsurpassed speed, and is complete within 200 fs, 2) it is fully stereospecific and 3) it has an unusually high quantum yield (Φ = 0.7). How opsin has adapted to establish such a highly effective 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 particularly relevant: its side-chain methyl groups and torsional strain.