Electron Densities of Three B12 Vitamins

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Abstract
The electron densities of the three natural B12-vitamins, two of them being essential cofactors for animal life, were determined in a procedure combining high-order X-ray data collection at low to very low temperatures with high-level density functional calculations. In a series of extensive experimental attempts, a high-order data set of adenosylcobalamin (AdoCbl) could be collected to a resolution of sin θ/λ = 1.00 Å−1 at 25 K. This modification contains only minor disorder at the solvent bulk. For methylcobalamin (MeCbl), only a severely disordered modification was found (sin θ/λ = 1.00 Å−1, 100 K, measured with synchrotron radiation). The already published data set of cyanocobalamin (CNCbl) (sin θ/λ = 1.25 Å−1, 100 K) was reintegrated to guarantee similar treatment of the three compounds and cut to sin θ/λ = 1.11 Å−1 to obtain a higher degree of completeness and redundancy. On the basis of these accurate experimental geometries of AdoCbl, MeCbl, and CNCbl, state-of-the-art density functional calculations, single-point calculations, and geometry optimizations were performed on model compounds at the BP86/TZVP level of theory to evaluate the electronic differences of the three compounds. AdoCbl and MeCbl are known to undergo different reaction paths in the body. Thus, the focus was directed toward the characterization of the dative Co−Cax and Co−Nax bonds, which were quantifed by topological parameters, including energy densities; the source function including local source; and the electron localizability indicator (ELI-D), respectively. The source function reveals the existence of delocalized interactions between the corrin macrocycle and the axial ligands. The ELI-D indicates unsaturated Co−Cax bonding basins for the two biochemically active cofactors, but not for CNCbl, where a population of 2.2e is found. This may be related to significant π-backbonding, which is supported by the delocalization index, δ, of 0.15 between the Co atom and the N atom of the cyano ligand. Considering all results, the inherent electronic differences between AdoCbl and MeCbl are found to be small thus, supporting earlier findings that the interaction with the protein site mainly controls the type of Co−Cax bond cleavage.

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