text: Vitamin B, CHCoNOP, is the most complex of all known vitamins. Its chemical structure had been determined by x-ray crystal structure analysis in 1956 by the research group of Dorothy Hodgkin (Oxford University) in collaboration with Kenneth N. Trueblood at UCLA and John G. White at Princeton University.
  Core of the molecule is the corrin structure, a nitrogenous tetradentate ligand system. This is biogenetically related to porphyrins and chlorophylls, yet differs from them in important respects: the carbon skeleton lacks one of the four meso carbons between the five-membered rings, two rings (A and D, fig. 1) being directly connected by a carbon-carbon single bond. The corrin chromophore system is thus non-cyclic and expands over three meso positions only, incorporating three vinylogous amidine units. Lined up at the periphery of the macrocyclic ring are eight methyl groups and four propionic and three acetic acid side chains. Nine carbon atoms on the corrin periphery are chirogenic centers. The tetradentate, monobasic corrin ligand is equatorially coordinated with a trivalent cobalt ion which bears two additional axial ligands.
  Several natural variants of the B structure exist that differ in these axial ligands. In the vitamin itself, the cobalt bears a cyano group on the top side of the corrin plane (cyanocobalamin), and a nucleotide loop on the other. This loop is connected on its other end to the peripheral propionic amide group at ring D and consists of structural elements derived from aminopropanol, phosphate, ribose, and 5,6-dimethylbenzimidazole. One of the nitrogen atoms of the imidazole ring is axially coordinated to the cobalt, the nucleotide loop thus forming a nineteen-membered ring. All side chain carboxyl groups are amides.
  Cobyric acid, one of the natural derivatives of vitamin B, lacks the nucleotide loop; depending on the nature of the two axial ligands, it displays instead its propionic acid function at ring D as carboxylate (as shown in fig. 1), or carboxylic acid (with two cyanide ligands at cobalt).