VOL. 4 (1950) 



RETINENES AND VITAMINS A 



217 



THE OXIDATION OF VITAMIN Aj TO RETINENEi 



In their simplest procedure for oxidizing vitamin A, to retinenej, Ball et al. (1946) 

 added a little manganese dioxide powder to a solution of vitamin Aj in petroleum ether, 

 and placed this mixture in a refrigerator. After 3-4 days they found that retinene, had 

 replaced vitamin Aj in the supernatant solution. 



On examining this process we found its mechanism to be as follows. Vitamin Aj 

 is strongly adsorbed on manganese dioxide, and is oxidized to retinene ^ in the adsorbed 

 condition. Retinene, is much less strongly adsorbed and so is displaced from the manga- 

 nese dioxide by new vitamin A^ as fast as it is formed. In this way all the vitamin A, 

 passes over the manganese dioxide surface, and is replaced by retinene j in the super- 

 natant solution. At the close of the process, the final charge of vitamin Aj on the 

 adsorbent is oxidized to retinenci, and then, wdth no vitamin A^ remaining to displace 

 it, is oxidized further to what I have called the 545 m/^-chromogen. This can be recoveied 

 from the manganese dioxide by elution with a polar organic solvent such as ethanol. 



300 



uoo 



600 700 



Wavelength-ma 



Fig. I. Comparison of natural and synthetic retinene^. Absorption spectra of cattle retinene^ in 

 chloroform and of the blue product which squid retinene^ yields with antimony chloride, compared 

 with similar preparations of synthetic retinencj. The absorption is plotted as extinction or optical 

 density, log I^/I, in which I^ is the incident and I the transmitted intensity (From Wald, 1947-48). 



For this reason the proportions of vitamin Aj and manganese dioxide used in the pro- 

 cedure are important. If too much manganese dioxide is used, it adsorbs all the vitamin 

 Al at once, and oxidizes all of it to the 545 m^u-chromogen (Wald, 1947-48). 



Once the nature of this reaction was appreciated, we recast it in more convenient 

 form. The manganese dioxide powder is packed into a short column such as is used in 

 chromatography. To oxidize 10 mg of vitamin A^, about 0.6 g of manganese dioxide 

 is employed. A solution of crystaUine vitamin A^ in peti oleum ether is poured in at the 

 top of the column, and a solution of nearly pure retinene^ is drawn off under light suction 

 in the filtrate. 



On washing through the column for a time with more petroleum ether, a high yield 

 of retinene^ is obtained. This can be freed of traces of contaminating substances by 

 chromatographic adsorption on a column of calcium carbonate. It is adsorbed as a 

 diffuse yellow zone, which travels slowly down the column on washing with petroleum 

 ether, and is collected as an isolated fraction of high purity in the filtrate. The properties 

 of this product are virtually identical with those of purified natural retinene^ (Fig. i). 

 References p. 228. 



