ISOMERISM AND THE VISUAL PIGMENTS 



chemical treatment. On these grounds it would seem unlikely that 

 /5-retinenes could pass into a-retinenes merely as the result of 

 isomerization by light. 



Yet another possibility, which so far has not been considered, is 

 worth examining. The configuration of ^-ionone itself has been the 

 subject of some speculation (braude et al, 1949, henbest and 

 WOODS, 1952). ^-ionone can exist in two stereoisomeric forms. 

 The spatial configuration of these is shown in Fig. 5.5, the large, 

 medium and small circles representing, respectively, the spheres of 

 influence of methyl, oxygen and hydrogen groups. Steric hindrance 

 between the hydrogen atom and the gem dimethyl group interferes 

 with rotation about the single bond marked s in Fig. 5.5, with the 

 resuh that s-trans and s-cis forms of /5-ionone are possible (see p. 129). 

 Assuming that a similar situation occurs in the retinene molecule, the 

 exceptional properties of the /50-retinenes could be explained by sup- 

 posing that they are s-cis forms of ^-retinene rather than that they 

 are hypothetical a-retinenes. 



wo-retinenci a 

 Amax =376 mji 

 (s-cis, /S-retinene) 



CHO 



wo-retinenci b 



Amax = 369 mfi 



{s-cis, 5-cis, or s-cis, 3-c/>j5-retinene) 



Alternative possible structure for the /50-retineneSi (compare p. 142). 



It is doubtful, however, whether all the complexities possible with 

 the retinenci molecule are yet known. The steric possibihties of even 

 the cjc/ohexane ring are still under discussion (see, for example, 

 KLYNE, 1954). The structures of the five known isomers of retinene^ 

 must, therefore, remain uncertain for the present. It is possible that 

 further isomers will be discovered and these may help to unravel the 

 problem. 



143 



