ISOMERISM AND THE VISUAL PIGMENTS 



sterically hindered cis linkages can be prepared. But the absorption 

 spectra of such compounds are much altered (low intensities of 

 absorption and Amax displaced 20-40 mfx). None of the presumably 

 mono-cis isomers of retinene^ show such degradation of absorption. 



The /50-retineneSi a and b show certain peculiarities in their absorp- 

 tion spectra in comparison with the remaining retinenes. Below 

 240 m/^ (Fig. 5.4) their absorption curves rise to cross the other 

 spectra while /^o-retinenci a has a unique plateau of absorption in the 

 range 280-320 m//. hubbard, gregerman and wald (1953) also 

 observed that the behaviour of the /^o-retinenes on subjecting them to 

 isomerizing treatments set them apart from the others. Thus while 

 wo-retinenci b readily isomerizes to wo-retinenci a the conversion of 

 wo-retinenci a to the remaining retinenes is difficult. 



In Une with these differences, woodward (cited by hubbard, 

 GREGERMAN and WALD, 1953) Suggested that one or more of the 

 retinenes might have an a-ionone rather than a ^-ionone ring struc- 

 ture : 



a-ionone 



The change from a ^^-ionone to an a-ionone ring results in much the 

 same sort of Amax displacement as would the introduction of a cis- 

 link. According to this idea, the two /50-retinenes are to be regarded 

 as a-retinenes and the three other retinenes as /5-retinenes as follows: 



all trans retinene^ 



Amax = 383 mw 



(all transy 5-retinene) 



neo-retinenei a 

 Amax = 377 m/^ 

 (5 cis, ^-retinene) 



141 



nQO-retinenei b 

 Amax = 377-5 m^ 

 (3 cis, /3-retinene) 



