738 VII. VITAMINS A 



anhydrovitamin A, but is incapable of shifting the double bonds to their 

 normal position. Thus, only a partial return to the normal vitamin A 

 structure is possible, and merely a small degree of vitamin A activity re- 

 sults. 



Rehydrovitamin A has two definite absorption maxima, at 351 and at 

 369 m/i, and an inflection at 331 mix. A comparison of the absorption 

 curves of anhydrovitamin A, rehydrovitamin A, and vitamin A is given in 

 Figure 9. 



c. Isoanhydrovitamin A. When anhydrovitamin A is allowed to stand 

 for prolonged periods with alcoholic hydrochloric acid, the characteristic 

 bands of anhydrovitamin A disappear and are replaced by new ones having 

 maxima at 330, 350, and 370 myu.'*^--^' Although the absorption curve of 

 isoanhydrovitamin A in the ultraviolet and that of rehydrovitamin A are 

 quite similar, Shantz^-^' believes that the differences in other physical prop- 

 erties preclude the possibility that the two compounds are identical. 



d. Sub vitamin A. Subvitamin A'**'* is another vitamin derivative 

 which is responsible for the distortion of the vitamin A absorption curve 

 on the short wave length side of 328 m/x. Embree and Shantz'^* suggested 

 that subvitamin A is probably an oxygenated derivative of vitamin A, 

 while Hawkins and Hunter^'*^ claim that it is vitamin A in which the double 

 bond in the ^S-ionone ring has been oxidized. It possesses no biological 

 activity. 



