ESMOND E. SNELL 



delayed growth in children, in stimulating growth of farm 

 animals fed rations based chiefly or exclusively on plant products, 

 and, from the biochemical side, because of its relationship to 

 nucleic acid synthesis and other biosynthetic reactions, it has 

 stimulated unusual interest, and many review articles dealing 

 with it are available {cf. 3,15). 



The structure of vitamin B12 is not yet known in detail, but 

 it contains an atypical porphyrin skeleton (4a) the nitrogen atoms 

 of which occupy four of the six coordination positions of cobalt 

 (formula XIV) . * The fifth is occupied by a nitrogen of a nucleo- 



atypical porphyrin 



CHo ^ 

 I 

 HOCHCH2NH2 



HCCHOHCHCHCHoOH , 



0PO3H2-'' 



( XIV) Vitamin 6^2 (partial formula) 



tide, the nature of which can vary in vitamin B12 isolated from 

 different source materials. Coordination position six is occupied 

 typically (but not exclusively) by an anionic grouping — cyanide 

 hydroxide, oxalate, sulfate, nitrite, etc. — so that for any one 

 parent structure containing a single nucleotide, a family of 

 closely related compounds of slightly different properties can be 

 obtained. Recent results (15a) show that by incorporating any 

 of ten or so different nitrogen bases in the medium, Escherichia 

 coli 113-3 will produce vitamin Bi2-like substances containing the 

 corresponding nucleotides in the molecule — a finding that 



* Since this was written, the probable complete structure of vitamin B12 

 has been clarified by a combination of chemical and x-ray diffraction 

 methods (D. C. Hodgkin, et al., Nature, 176, 325 (1955) and R. Bonnett, 

 etal., Nature, 176, 328 (1955)). 



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