Rhodopsins opsin related proteins

Akhtar et al. [74] proposed that, in rhodopsin, an acceptor group on the protein forms a charge-transfer complex with the unprotonated Schiff base of retinal furthermore, upon ll-cis to trans isomerization, separation of donor and acceptor moieties would occur and the Schiff base linkage would be exposed to hydrolysis. This model can now be discarded as unrealistic the resonance Raman experiments have shown that it is not an unprotonated Schiff base, but a protonated base which is bound to opsin. Further, this and related models were examined by Komatsu and Suzuki [223] using theoretical calculations, who found that charge-transfer type models cannot satisfactorily explain the red shifts seen in visual pigments. 

Chemical processes that are triggered by photons are also understood using the kinetics concepts mentioned in this chapter. The chemistry of vision is one example. The currently accepted mechanism for vision involves a compound called rhodopsin, which is composed of a protein molecule (opsin) attached to a colored polyene molecule called c/s-retinal. (C/s-retinal is chemically related to a class of molecules called carotenes, which are highly colored compounds responsible for the colors of carrots and tomatoes. Eating carrots does help vision, specifically in low light.) The vision process begins when the c/s-retinal absorbs a photon and is isomerized about one of its double bonds to make frans-retinal ... 

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