19:2/ Molecular Basis of Vision 



353 



physiological pYL the peak is shifted several m/z. This similarity is 

 taken as evidence that the purified rhodopsin is essentially the same 

 as the initial rhodopsin. If the extraction is performed in a bright 

 light, no rhodopsin is present in the final suspension; only the protein, 

 opsin, remains. 



Rhodopsin is a protein complex with a molecular weight of approxi- 

 mately 40,000. It is a conjugated complex of a particular protein, 

 opsin, with a much smaller hydrocarbon group, called neoretinene b or 

 retinene. Retinene! is the aldehyde 1 of vitamin A x . Vitamin A 1 has 

 the structure shown in Figure 1. The carbon atoms in the ring and 

 along the chain are numbered for convenient reference. Compounds 

 with a ring structure of this type, plus a chain of about nine additional 

 carbon atoms, are referred to as carotenoids because many of them are 

 found in carrots. Vitamin A x is one of these carotenoids. 



For every double bond along the vitamin A x chain, there are two 

 spatial isomers. If one of the atoms on each carbon is hydrogen, one 

 can represent the cis isomer as 



HC— a 



HC— j8 



and similarly the trans isomer as 



a- 



-CH 



HC— j8 



In many cases, it is possible to distinguish between the cis and trans 

 compounds either by chemical or by X-ray techniques. In more 

 complex cases, this distinction is difficult to demonstrate. In the case 



H 



I 

 1 An aldehyde is a compound containing the structure — C=0. Neoretinene 

 (and all other retinenes) have this structure on the fifteenth (terminal) carbon 

 atom. Vitamin \ x has an alcohol structure on this atom, as is shown in Figure 1. 

 The aldehyde form is an oxidized form, whereas the alcohol is a reduced form. 



