THE VISUAL PIGMENTS 



All-trans retinenci and weo-retinenci a were prepared from all-trans 

 vitamin Aj and /leo-vitamin A-^a by oxidation with manganese dioxide. 

 Other stereoisomers of retinene^ were obtained by irradiating a solu- 

 tion of sdl-trans retinenei and then chromatographing the mixture of 

 isomers. Operations subsequent to the isomerization by hght were 

 carried out in dim hght to avoid further changes to which the retinene 

 isomers were very susceptible. By these means three isomers in 

 addition to the all-trans form (ball, goodwin and morton, 1948) 



£40 280 320 360 400 



440 



460 



S20 



Fig. 5.4. Density spectra (in ethyl alcohol) of five retinenCi isomers. 

 (Hubbard, Greyer man and Wald, 1953) 



andneo-retineneiflr(DALViand morton, 1952) were prepared. These 

 were called weo-retinenci b, isolated by hubbard, gregerman and 

 WALD (1953) and /50-retineneSi a and b isolated by the Organic 

 Research Laboratory of Distillation Products Industries (see hub- 

 hard and WALD, 1952; hubbard, gregerman and wald, 

 1953). The melting points and absorption characteristics of these 

 isomers, some of which were obtained crystaUine, are shown in 

 the following table. The full absorption spectra in ethyl alcohol of 

 the five isomers are shown in Fig. 5.4. 



hubbard, gregerman and wald (op. cit.) found that all five 

 isomers gave a blue colour when mixed with antimony trichloride in 

 chloroform. The absorption spectra of the blue solutions were 

 identical in each case (Amax = 660 m//, £'max (1 per cent, 1 cm) = 



138 



