76 



VITAMINS A AND CAROTENES 



isomerization is possible only about the double bonds adjacent to methyl 

 side groups. Anywhere else in the molecule the cis configuration is hindered 

 sterically. Hence vitamin A and retinene, which possess two such double 

 bonds, should exist in four stereoisomeric modifications: a\\-trans, S-cis, 

 5-cis, and 3,5-di-as. These are shown in Fig. 21. 



Ordinary crystalline vitamin A, as well as the bulk of commercial syn- 

 thetic vitamin A, is the all-^rans isomer. Fish liver oils, however, are known 

 to contain mixtures of all the stereoisomers of vitamin A. The observation 



1 1 



3— \/itaminAj+- 

 cozymase v- 

 opsin 



b- same + liver al- 

 cohol dehydrogenase 



400 4SO SOO SSO 600 



Wave/ength'-m/j 



Fig. 20. Sj^nthesis of rhodopsin by a solution of known components. The upper 

 curve shows the difference spectrum — the difference in absorption spectrum before 

 and after bleaching — of rhodopsin synthesized by incubating together vitamin A 

 from fish liver oils, crystalline horse liver alcohol deh3^drogenase, yeast cozymase, 

 and frog opsin. The lower curve shows the rhodopsin formed in an identical mixture 

 lacking only the alcohol dehydrogenase. (From R. Hubbard and G. Wald.''*) 



described above means, therefore, that rhodopsin cannot be synthesized 

 in vitro from aW-trans vitamin A, but requires instead a cis isomer present 

 in liver oils. 



One of the cis isomers of vitamin A, called neovitamin A, was isolated by 

 Robeson and Baxter. ^^ For reasons which will appear below we shall refer 

 to tiiis hereafter as neovitamin Aa. This substance also, on incubation in 

 our four-component system, yields no rhodopsin. 



It is well known from Zechmeister's work that any single geometrical 



li" R. Hubbard and G. Wald, Science 114, GO (1952). 



" C. D. Robeson and J. G. Baxter, /. Am. Chem. Soc. 69, 136 (1947). 



