Spectroscopic and computational studies on [Ni(tmc)CH3]OTf: implications for Ni-methyl bonding in the A cluster of acetyl-CoA synthase.

The five-coordinate high-spin (S = 1) Ni(2+) complex Ni(tmc)CH(3) (1) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) serves as a model for a viable reaction intermediate of the A cluster of acetyl-CoA synthase (ACS) in which the distal nickel center is methylated. Spectroscopic and density functional theory (DFT) computational studies afford a quantitative bonding description for 1 that reveals a highly covalent Ni-CH(3) bond. From a normal coordinate analysis of resonance Raman data obtained for 1, a value of k(Ni-C) = 1.44 mdyn/Angstroms is obtained for the Ni-C stretch force constant of this species. This value is smaller than k(Co)(-C) = 1.85 mdyn/Angstroms, which is reported for the Co-C stretch in the methylcobinamide cofactor (5) that serves as the methyl donor to the A cluster in the ACS catalytic cycle. Experimentally calibrated DFT computations on viable methylated A cluster models reveal that the methyl group binds to the proximal (Ni(p)) rather than the distal (Ni(d)) nickel center and afford a simple electronic argument for this preference. By correlating the experimental force constants with the computed bond orders of the M-C bonds in 1 and 5, the Ni(p)(2+)-CH(3) bond strength for an A cluster model with a square-planar Ni(p) conformation, which is the most probable structure of the methylated A cluster on the basis of steric and energetic considerations, is predicted to be similar to the Co(3+)-CH(3) bond strength in CH(3)-CoFeSP. This similarity could be a crucial thermodynamic prerequisite for the reversibility of the enzymatic transmethylation reaction.