CHAPTER FIVE 



Isomerism and the Visual Pigments 



In 1951 HUBBARD and wald found that rhodopsin (visual purple) 

 could be synthesized by placing in darkness a solution containing 

 four components. The four components were opsin — the protein 

 moiety of the rhodopsin molecule, vitamin A^ — the precursor of the 

 chromophore, and liver alcohol dehydrogenase and cozymase — the 

 enzyme and coenzyme, respectively, which oxidize vitamin A^ to 

 retinenci. 



The synthesis was originally carried out using a fish Hver oil con- 

 centrate as the source of vitamin A^. When, later, an attempt was 

 made to repeat the experiment with crystalHne vitamin A^, hardly any 

 rhodopsin was formed. 



The difference in behaviour between the two samples of vitamin A^ 

 suggested to hubbard and wald that a particular /orw of vitamin A^ 

 was required for the synthesis. Liver oils are known to contain a 

 mixture of different varieties or isomers of vitamin A^ — all of which 

 have the same formula and differ only in respect of their spatial 

 conformation. Crystalline vitamin A^, on the other hand is a single 

 isomer and one which, as the experiment showed, is unsuitable for 

 the synthesis. 



These important observations by wald and his collaborators, 

 though recent, have already led to a considerable extension of our 

 knowledge of the visual pigments. In order to appreciate the results 

 obtained it is necessary first to consider some aspects of isomerism. 



CIS-TRANS ISOMERISM 



When two atoms are united by a single covalent bond, either atom 

 is capable of independent rotation about the bond. When, however, 

 they are Hnked by a double covalent bond, the mutual aspect of the 

 atoms — and hence of any other groups which are attached to them — 

 is fixed. In other words free rotation about a double bond cannot 



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