Resonance Raman Studies of Visual Pigments

Visual pigments are composed of a small chromophore (absorption center) called retinal (the aldehyde of vitamin A) covalently linked to surrounding protein, called opsin. The pigments are situated in specialized membranes. The best studied, rhodopsin, is found in velebrate rods, the cells of the retina responsible for low light level vision as opposed to color vision. The purple membrane protein of a bacterial cell called halobacterium halobium also contains retinal as its chromophore surrounded by a colorless protein. The absorption of light by visual pigments causes, eventually, a neural response giving rise to vision. Light absorption by the purple membrane results in protons being pumped across the cell wall of the bacterium; the energy of this electral gradient is then used to produce available chemical energy for the cell in terms of high energy chemical bonds (formation of ATP).