694 VI r. VITAMINS a 



Ai, under similar circumstances, gives a ketone with 23 carbons. This 

 would seem to be almost absolute proof that the C'22 formula (I) is the cor- 

 rect one. The ketone of vitamin A so prepared has an absorption maxi- 

 mum of 401 mpL (in alcohol), while the ketone prepared from vitamin A2 

 has a maximum absorption at 41 1 m^u. 



However, several reactions on the part of vitamin Ao are difficult to ex- 

 plain if one accepts the Gillam formula (I). The main difficulty lies in ex- 

 plaining the proximity of the elimination maxima of vitamins Ai and A2 

 when these are subjected to molecular distillation. Gray,^-^^ as well as 

 Lovern and collaborators, ^^^ found that the elimination maximum of vita- 

 min A2 is only 3°C. higher than that of vitamin Ai; this would indicate 

 that the two vitamins have the same number of carbons but that vitamin 

 A2 has an additional unsaturated linkage. This conviction was further sub- 

 stantiated in later work of Gray and Cawley,^^'^ who showed that the elimi- 

 nation maximum was lowered 2°C. by each increase in a non-conjugated 

 double bond, and to the extent of 3°C. by each additional conjugated 

 double bond, while it was raised 5°C. by each added CH2 group. Karrer, 

 Riiegger, and Geiger^^** synthesized a material which presumably had the 

 stiiicture given in Formula I, except for an additional methyl group. 

 This substance failed to show the color reactions for vitamin A2, although it 

 had previously been demonstrated that alkyl groups located in such posi- 

 tions would not interfere in the color reactions of the carotenoids. Last, 

 the spectral absorption characteristics predicated for jS-apo-5-carotenol (C22) 

 by Karrer e( al.^^^'^^^ differ from those actually found for vitamin A2, and so 

 it is not believed that the C22 formula applies to vitamin A->. Embree and 

 Shantz^*'' have demonstrated that the products formed on cyclization of 

 vitamins Ai and A2 with alcoholic hydrochloric acid have very similar ab- 

 sorption spectra. This may be considered as support of a C20 rather than a 

 C22 formula. 



In 1941 Karrer and his collaborators^^^ proposed the open-chain formula 

 (II), largely because of the production of acetone when vitamin A2 was 

 treated with ozone. Another finding which supported the hypothetical 

 structure was the fact that the absorption maximum was at 345 myn. The 

 lycopene structure now appears less probable, inasmuch as Karrer and 

 Bretscher^^i were later unable to prove the presence of acetone on the basis 

 of the formation of its p-nitrophenylhydrazone, although the iodoform reac- 

 tion indicated the presence of acetone. However, these authors mention 

 the possibility that an unidentified substance other than acetone may be 



136 E. Le B. Gniv, J. Biol. Client., ISl, 317-326 (1939). 



136 J. A. Lovern, T. H. Mead, and R. A. Morton, Biochem. J., 33, 338-343 (1939). 



1" E. Le B. Gray and J. D. Cawley, /. Blot. Chem., 134, 397-401 (1940). 



138 P. Karrer, A. Riiegger, and A. Geiger, Helv. Chim. Acta, 21, 1171-1174 (1938). 



139 H. V. Euler, P. Karrer, and U. Solmssen, Helv. Chim. Acta, 21, 211-222 (1938). 

 iw N. D. Embree and E. M. Shantz, J. Biol. Chem., 132, 619-626 (1949). 



1" P. Karrer and E. Bretscher, Helv. Chim.. Acta, 25, 1650-1653 (1942). 



