title: Spectroscopic and computational studies on [Ni(tmc)CH3]OTf: implications for Ni-methyl bonding in the A cluster of acetyl-CoA synthase.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, 53706, USA.
authors.forename: Ralph
authors.initials: R
authors.lastname: Schenker
authors.forename: Michael T
authors.initials: MT
authors.lastname: Mock
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Kieber-Emmons
authors.forename: Charles G
authors.initials: CG
authors.lastname: Riordan
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Brunold
mesh_terms: D000106:Acetate-CoA Ligase / Q000737:chemistry*; D008745:Methylation; D008958:Models, Molecular; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013057:Spectrum Analysis; D013816:Thermodynamics
chemical_list: D009942:Organometallic Compounds; D009532:Nickel; D000106:Acetate-CoA Ligase