A molecular mechanics force field for alkylcobaloximes, a model of vitamin B12 coenzyme -: Implications of steric and electronic factors in the Co-C bond cleavage

Molecular mechanics force field constants have been derived for alkylcobaloximes, (alkyl)Co(DH)(2)L (DH = monoanion of dimethylglyoxime and L = planar N-donor ligand), implementing the AMBER force field. Atomic charges have been calculated by the semiempirical ZINDO/1 method. Stretching and bending constants have been calculated by the Badger and Halgren equations, with the introduction of simple functions for the description of the electronic influence of the axial ligands on the coordination geometry. 26 parameters have been optimized, by the Simplex method, to fit 4523 bond lengths and angles of 52 alkylcobaloxime accurate crystal structures. In spite of the oversimplification of the adopted method for the description of the electronic effects, this approach provides a good description of the metal coordination geometries, The root-mean-square deviations of the calculated bond lengths and angles from the experimental values average to 0.023 Angstrom and 1.4 degrees, respectively. The molecular mechanics results are discussed in terms of the steric properties of the axial ligands and correlated with their calculated cone angles. The force field has been used to analyse some conformational features of these compounds, such as the influence of the rotation of the axial ligands on their coordination geometry. The calculated energy for the Co-N rotational barrier agrees well with the experimetal ones derived from dynamic and saturation H-1-NMR spectroscopy in (alkyl)Co(DH)(2)(2-NH2-py). In order to have more experimental data for this analysis, the crystal structures of two new cobaloximes, (CH2Cl)Co(DH)(2)L with L = py, 1, and 1-Me-Im, 2) have been determined. The results of a conformational analysis on ribosyl-imidazole derivatives, taken as a suitable vitamin B-12 coenzyme model, suggest that the 1-Me-imidazole-like ligands have a significantly greater rotation freedom with respect to the benzimidazole-like ones, but cause similar stretchings of the Co-C bond, and a significantly less stretching of the Co-N one. Implications for the recent findings on the binding mode of the coenzyme B-12 in the enzyme active site are discussed.