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Vitamin B12-factor A (2-Methyladeninyl-cyanocobamide), Vitamin B12, X-Ray Crystal Structure Analysis The crystal and molecular structure of 2-methyladeninyl-cyanocobamide (factor A) has been determined. This compound crystallizes in space group P21212, with a = 2630.6 (15), b = 2210.6 (13) and c = 1592.1 (9) pm. The structure has been solved by the heavy-atom method and refined by least-squares methods on the basis of 3682 X-ray counter data to R = 0.166 and R w = 0.148. As far as we know, this is the first X-ray-investigation of a purine-corrinoid, which differs from cyanocobalamin (vitamin B12) by containing a purine base instead of 5,6-dimethylbenz-imidazole. The structure analysis of 2-methyladeninyl-cyanocobamide shows unambiguously, that the purine base coordinates with cobalt via N B9. The choice of N B9 as coordinative atom can be ascribed partly (or mainly?) to steric influences, since coordination via N B3, which was also discussed, would presumably lead to severe distortion of the nucleotide loop.