690 Vn. VITAMINS A 



binations in the retina are now recognized as the compounds rhodopsin and 

 porphyropsin, although it is beheved that vitamin A aldehyde, rather than 

 the alcohol, is the component concerned. 



3. Structure of the Vitamins A 



(/) Vitamin A (A,) 



Although carotene possesses vitamin A activity, it has already been in- 

 dicated that the provitamins A and vitamins A are different substances. 

 In the first place, all evidence points to the fact that a marked divergency in 

 molecular weight obtains between these compounds. Bruins, Overhoff, 

 and Wolff^^^ found a molecular weight of 536 for carotene compared with 

 one of 330-333 for vitamin A. These workers made a direct comparison of 

 the diffusion constants of these products prepared from sheep liver fat. A 

 number of other workers have confirmed the fact that the molecular weight 

 of vitamin A approximates 300. Karrer, Morf, and Schopp^^ obtained 

 values varying between 300 and 320 by the liast method, while Heilbron 

 and collaborators^' reported a mean value of 327 for a series of determina- 

 tions in which the cryoscopic method was emplo.V^ed. These same authors, ^^ 

 using the Smith and Young^-^ modification of the Rast micromethod, found 

 the apparent molecular weight of vitamin A to be 312. 



A second obvious difference between carotene and \ntamin A is the fact 

 that the latter is an alcohol \\hile the former is a hydrocarbon. Drummond, 

 Channon, and C/owai'd'^-^ suspected that vitamin A might be an alcohol, 

 although evidence was inconclusive; this hypothesis was confirmed })y 

 Karrer, Morf, and Schopp.^' The latter workers were able to prepaie an 

 acetate ester as well as a p-nitrobenzoate from a concentrate made from 

 the liver oil of the Atlantic saury (Scomberesox saurus) ; the free vitamin A 

 alcohol could be regenerated from these products. The most probable 

 molecular formula for vitamin A alcohol was determined as C20H30O or 

 C22H32O, on the basis of analyses of the two above-mentioned vitamin A 

 esters and of the alcohol regenerated from the acetate. We now know the 

 first empirical formula to be correct; this would be in accordance with a 

 molecular weight of 286. 



The further structure of vitamin A was elucidated by degradation reac- 

 tions similar to those used for establishing the structure of the carotenoids. 

 The presence of a jS-ionone ring was established by the isolation of geronic 

 acid after treatment with ozone. '^*' The presence of the methyl groups in 

 the side chain and in the wliole molecule was shown to be equivalent to 



123 H. R. Bruins, J. Overhoff, and L. K. Woltf, Biochem. J ., 25, 430-438 (1931). 

 '24 J. H. C. Smitli and W. G. Young, J. Biol Chem., 75, 289-298 (1927). 



125 J. C. Drummond, H. J. Channon, and K. H. Coward, Biochem. J., 1.9, 1047-1067 

 (1925). 



126 p. Karrer, R. Morf, and K. Sch()pp, Helv. Chim. Ada, I4, 1036-1040 (1931). 



