Introduction

Retinal is a biological chromophore ubiquitous in visual receptors of higher life forms, but serving also as an antenna in light energy transformation and phototaxis of bacteria. The chromophore arises in various retinal proteins, the best known two being the visual receptor rhodopsin and the light-driven proton pump bacteriorhodopsin. The ubiquitous nature of retinal in photobiology is most remarkable as the molecule shows an extremely wide adaptability of its spectral absorption characteristics and a precise selection of its photoproducts, both properties steered by retinal proteins.

Rhodopsin (Rh) is a membrane protein of the rod cells in the retina of animal eyes and contains a retinal molecule as a chromophore surrounded by the protein's seven transmembrane helices (Khorana, 1992), as shown in Figure 11.1. Rhodopsin serves as the receptor protein for monochromic vision, in particular, for vision in the dark. Analogous retinal proteins, called iodopsins, exist in the cone cells of the retina and serve as receptor proteins for colour vision in daylight (Nathans et al., 1986).

Retinal proteins also serve in certain bacteria as light-driven proton pumps that maintain the cell potential, as in case of bacteriorhodopsin (bR) (Schulten and Tavan, 1978), or as light sensors in bacterial phototaxis (Spudich and Jung, 2005).

All retinal proteins are structurally homologous, being composed of seven transmembrane helices with a retinal chromophore bound to a lysine side group. The photoactivation mechanisms of the proteins' retinal moieties are similar, but distinct from each other. The primary event of retinal photoactivation is a photoisomerization reaction (Birge, 1990).