Crystal chemistry and binding of NO2, SCN and SeCN to Co in cobalamins.

Results of the accurate crystal structure determination of NO(2)Cbl.2LiCl (1), NO(2)Cbl.NaCl (2), NCSCbl (3) and NCSeCbl (4), based on synchrotron diffraction data collected at 100 K, are described. The nitro group in (1) was found to be disordered with two orientations that differ by a rotation of approximately 60 degrees about the Co-NO(2) bond, whereas in (2) the nitro group has only one orientation. The first X-ray structural determination of a cobalamin with a Co-Se bond is reported. Comparison of the axial distances indicates that SeCN has a bond length of 2.384 (3) A and that the trans influence on the Co-N bond is only slightly greater than that of SCN. The crystals of the thiocyanate cobalamin contain both the S- and N-bonded coordination isomers in a 3:2 ratio. The structural features of the Co-S bond in cobalamins are discussed. The crystal chemistry of cobalamins is discussed in terms of packing of roughly spherical molecules. The unit-cell parameters can be used to group the cobalamins' crystal structures in different arrays intermediate between distorted hexagonal close packing and primitive hexagonal arrangements. The structural features of cobalamins, and of cobaloximes that have the same axial fragment as the cobalamins, are reviewed and discussed in terms of the cis influence of the equatorial ligand.