II. CHEMISTRY 



413 



vitamin B12 forms a "vitamin Bi2-cyanide complex" with excess cyanide 

 ion, its absorption spectrum breaks into a doublet with a "notch" at 

 X = 2885 A., characteristic of the benzimidazole absorption spectrum.^' This 

 is interpreted in one of two representations as indicated below and explained 

 by assuming that the combination of the excess cyanide ion with the cobalt 



n 3 



•N. 



N 



CN 

 1 



Co = 

 T 

 CN 



-1 e 



leads to an increased electronegativity of the cobalt atom and a correspond- 

 ing decrease in the electronic contribution of the N^-benzimidazole nitrogen 

 to the cobalt electronic cloud. ^^ This effect is then said to allow for the 

 more "normal" contribution of the 5 , 6-dimethylbenzimidazole grouping 

 to the absorption spectrum. 



The position of the Dg-l-amino-2-propanol in the representation of the 

 cyanocobalamin molecule (X) has not been determined. However, some 

 light has been shed on the problem by the determination of the order of 

 hydrolytic liberation of the various degradation products,^^ Acid hydrolysis 

 of cyanocobalamin at room temperature for 5 hours liberated 0.7 mole of 

 ribazole phosphate. Further contact with 6 N hydrochloric acid for not 

 less than 18 hours was necessary to liberate the Dg-l-amino-2-propanol.3'* 

 Therefore, it seemed that, since the two moieties are not liberated simul- 

 taneously, the phosphate cannot be esterified wdth the propanolamine. Some 

 other location must be found for this 3-carbon portion. 



The graphic representation of cyanocobalamin^^ described above is not 

 without some unsettled features. It has been found that cyanocobalamin 

 reversibly binds more than one cyanide ion.^^ However, there is still strong 

 evidence for the hypothesis that the benzimidazole chromophore is co- 

 ordinated to the cobalt atom.^^^' The differential absorption spectra of 

 vitamin B12 versus vitamin B12 "red fragment" (resulting from hydrolysis 

 which is known to liberate the a-ribazole phosphate) have been plotted at 

 pH 4, at pH 10, and at pH 10 plus potassium cyanide. The resulting curves 

 then are the absorption spectra of the portion of the vitamin B12 molecule 

 absent in the red fragment. The differential curve measured in the presence 

 of potassium cyanide is very similar to that of the a-ribazole phosphate 

 and strongly indicates the presence of the latter in the postulated position. 



"J. B. Conn, S. L. Norman, and T. G. Wartman, Science 113, 658 (1951). 

 "» G. H. Beaven and E. R. Holiday, /. Pharm. Pharmacol. 4, 342 (1952). 



