The Structure of Vitamin B$_{12}$. IX. The Crystal Structure of Cobyric Acid, Factor V 1 a

The structure of cobyric acid (factor V 1a), the aquocyanide of the natural nucleus of the B$\_{12}$ vitamins, has been determined by X-ray crystallographic analysis. The crystals are monoclinic, space group, P2$\_{1}$; the asymmetric unit has the contents, C$\_{46}$H$\_{66}$O$\_{9}$N$\_{11}$Co.11H$_{2}$O. The intensities of the X-ray spectra recorded with CuK$\alpha $ radiation were measured visually; they showed marked intensity differences between hkl and $h\overline{k}l$ reflexions due to anomalous dispersion, which permitted the calculation of phase angles for 1994 reflexions; the ambiguity in phase angle determination from the differences was resolved by choosing the one nearer to the phase of the cobalt atom. The first electron density map obtained revealed the structure almost completely. It has been refined by successive structure factor and difference electron density maps and block diagonal least squares calculations; in the later stages adjustments were made to the individual observed F values, 6188 reflexions in all. Many details of the geometry of the cobyric acid nucleus and of the attached side chains are well defined; one acetamide group, that on ring B, is rotated from its usual position to make a hydrogen bonded contact with a water molecule attached to the cobalt atom.