Crystal Chemistry of Cobalamins. Structural Characterization of the Co−S Bond in Cobalamins

The first accurate structural characterization of the axial fragment in cobalamins, containing axial S ligand, is
described and discussed. The preparations and the crystal structure determinations of two cobalamins, (SO3Cbl)(NH4)·nH2O (1) and [(NH2)2CSCbl]Cl·nH2O (2), based on synchrotron data collected at 100 K, are reported. The
Co−S distances in 1 (2.231 (1) Å) and 2 (2.216(7) Å) are very close, whereas the trans Co−NB3 distance of
2.134(7) Å in 1 is significantly longer than that of 2.01(1) Å in 2. This suggests that sulfite exerts a strong trans
influence, close to that of a methyl group, whereas thiourea has a fairly weak trans influence, close to that of
azide. An evaluation of the Co−NB3 and Co−SR distances, not yet experimentally determined, is given and
allows us to suggest that the SR trans influence is greater than that of thiourea but significantly smaller than that
of sulfite. These findings support the previous suggestion, based on structural and spectroscopic results for
cobaloximes, that the thiolate ligation by homocysteine in MeCbl is unlikely to be an important step in the processes
involving methionine synthase. Some features of the crystal chemistry, such as the crystal packing and the H-bond
scheme among the crystallization water molecules, in 1 and 2, are also discussed in relation to those of other
cobalamins.