THE VISUAL PIGMENTS 



\so-retinenei a 



Amax = 376 m^ 



(all transy a-retinene) 



iso-retinenej b 



Amax = 369 m^ 



(5 cis or 3 cis, a-retinene) 



Possible structures for the five known isomers of retinenej. The Amax 



are for alcohol solutions. 



{Hubbard, Gregerman and Wald, 1953) 



As indicated above A^eo-retinenci b is written as the symmetrical 

 3-cis isomer because it has the more intense cis peak and the less 

 intense main band (see Fig. 5.4 and compare p. 1 34)— leaving «eo- 

 retinenci a as the 5-cis isomer (robeson and Baxter, 1947). 



According to these ideas 3,5 di-cis, /^-retinenci, another mono-c/5, 

 a-retinenci and 3,5 di-c/5, a-retinenej remain to be discovered. 



It is not easy to assess the likelihood of an a-retinene structure for 

 the /50-retinenes. The conversion of /5- into a-carotene (which differ 

 in the same sense as a- and /5-retinenes) requires very energetic 



lA 



S-trans S-c/s 



Fig. 5.5. s-trans and s-cis forms of ^-ionone. 

 142 



