NMR Coupling versus NMR Chemical Shift Information in Metallobiochemistry. High-Resolution One-Bond (1)H-(13)C Coupling Constants Obtained by a Sensitive Reverse Detection Method

NMR observations of 13C-enriched coenzyme B12 bound to the ribonucleotide reductase from Lactobacillus leichmannii

The 13C NMR resonance and one-bond 1H-13C coupling constants of coenzyme B12 enriched in 13C in the cobalt-bound carbon have been observed in the complex of the coenzyme with the B12-dependent ribonucleotide reductase from Lactobacillus leichmannii. Neither the 13C NMR chemical shift nor the 1H-13C coupling constants are significantly altered by binding of the coenzyme to the enzyme. The results suggest that ground-state Co-C bond distortion is not utilized by this enzyme to activate coenzyme B12 for C-Co bond homolysis.

Strong Trans Influence Methoxymethyl Ligand in B(12) Cobaloxime and Imine/Oxime Model Complexes: Structural, Spectroscopic, and Molecular Mechanics Investigations

The CH(2)OCH(3) ligand has a large trans influence comparable to that of bulky alkyl groups but lacks the complication of marked steric effects. However, crystals of model complexes with this ligand have proved difficult to obtain. The crystal structures of the cobaloximes reported here, Me(3)BzmCo(DH)(2)CH(2)OCH(3).0.6CH(3)OH (1) and 4-MepyCo(DH)(2)CH(2)OCH(3) (2) [DH = monoanion of dimethylglyoxime, Me(3)Bzm = 1,5,6-trimethylbenzimidazole, and 4-Mepy = 4-methylpyridine], triple the number of cobaloxime structures with CH(2)OCH(3). Also, these are the first structures in this class of models with an N-donor planar heterocyclic axial donor ligand, L. The planes of the Me(3)Bzm and 4-Mepy ligands are almost perpendicular to the respective planes of the four equatorial DH nitrogen donors and bisect the (O(-))N-Co-N(OH) angles. The Co-N(L) bond distances average 2.096(2) Å, confirming the strong trans influence of CH(2)OCH(3). Geometry optimization via molecular mechanics using MacroModel 5.0 and an AMBER-type force field was applied to both cobaloxime-type and imine/oxime-type B(12) models. In our initial work with some imine/oxime models, the calculated structures did not compare well to the solid-state structures. Therefore, adjustments to the force field were evaluated. The major adjustment that improved the fit of the computed and experimental structures was an approximately 10% reduction of the van der Waals (vdw) parameters for both the N donors and the C(sp(2)) atoms linked to the N donors; this adjustment may reflect the electron-withdrawing effect of the metal center. Analysis of (1)H-(13)C coupling constants of Me(3)Bzm lends support to the concept that the atoms in the ligands were modified, but only slightly, by the metal center. To reproduce the dependence of geometric features on the trans influence, different force field parameters for L-Co bonds were used for compounds with weak (Cl) and those with strong (CH(2)OCH(3)) trans influence ligands. These small modifications allowed us to model the structural features of both classes of models well.