III. BIOCHEMICAL SYSTEMS 63 



powtler to a solution of vitamin A in petroleum ether, and to leave this in 

 the refrigerator for several days. At the end of this period the vitamin A 

 had been converted almost entirely to retinene.^ 



TABLE XVI 

 Propkrties of the Vitamins A a.\d Retinenes 

 The properties shown arc those of the crystalline substances, except in the case 

 of vitamin A2 , not yet crystallized. Each of these substances has been shown to have 

 identical properties, whether isolated from natural sources or prepared by total 

 synthesis. The properties given for the crj-stalline substances refer primarily to the 

 prevalent all-</ons stereoisomer. Xmax is the wavelength of ma.ximum absorption; 

 ■^1 on ^^ ^^^^ extinction of a 1% solution, weight by volume, measured in a layer 1 cm. 

 in depth. 



On re-examining this reaction, we found it to take the following course. 

 Vitamin A is adsorbed very strongly on manganese dioxide and is oxidized 

 in the adsorbed state to retinene. Retinene, however, is much less strongly 

 adsorbed and so is displaced from the manganese dioxide surface by the 

 remaining vitamin A as fast as it is formed. In this way all the vitamin A 

 passes over the surface of the adsorbent and is replaced by retinene in the 

 solution. ^^ 



Once this mechanism was understood, the process was modified accord- 

 ingly. All that is needed is to pack a small amount of manganese dioxide 



8 J. G. Baxter and C. D. Robeson, J. Am. Chem. Soc. 64, 2411 (1942); J. D. Cawley, 



C. D. Robeson, L. Weisler, E. M. Shantz, N. D. Embree, and J. G. Baxter, Science 



107,346 (1948). 

 " R. Hubbard, R. I. Gregerman, and G. Wald, /. Gen. Phijsiol. 36, 415 (1952-1953); 



G. Wald and P. K. Brown, /. Gen. Phxjsiol. 37, 189 (1953-1954). 

 "> E. M. Shantz, Science 108, 417 (1948). 



'I E. M. Shantz and J. H. Brinkman, J. Biol. Chem. 183, 467 (1950). 

 '2 K. R. Farrar, .J. C. Hamlet, H. B. Henbest, and E. R. H. Jones, /. Chem. Soc. 



1952, 2657. 

 '^ K. R. Farrar, J. C. Hamlet, H. B. Henbest, and E. R. H. Jones, Chemistry & 



Industry 1952, 49. 

 '* H. R. Cama, P. D. Dalvi, R. A. Morton, M. K. Salah, G. R. Steinberg, and A. L. 



Stubbs, Biochem. J. 52, 535 (1952). 

 1" G. Wald, J. Gen. Physiol. 31, 489 (1947-1948). 



