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.

Spectroscopic and Computational Studies of a Series of High-Spin Ni(II) Thiolate Complexes

“…The experimental Abs spectrum of 4 is dominated by a broad feature at 17,840 cm −1 that was initially assigned as a LMCT transition; this feature is well-reproduced in the TD-DFT calculated spectrum (note that the higher energy of the calculated band relative to its experimental counterpart reflects the tendency of TD-DFT to overestimate the energies of electronic transitions48,49). An analysis of the electron density difference map (EDDM) associated with this transition reveals that electron density is transferred primarily from the SR π(op) HOMO to the Ni 3d(x 2 −y 2 )-derived LUMO (Figure 7, panels b and c).…”

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

“…Recent computational studies performed in our laboratory established that the proposed Ni-CH 3 and Co-CH 3 bonds are similarly strong (with M-CH 3 stretching force constants of 1.76 and 1.84 mdyn/Å, respectively), an intuitive requirement for the reversibility of the methylation. 84 This computational study also indicated that the conformational flexibility of the Ni site in ACS likely plays a significant role in promoting formation of the Ni-CH 3 bond. Thus, while the structural features of the A-cluster, which facilitate Ni-CH 3 bond formation, are becoming increasingly well understood, the mechanism by which CFeSP tunes the reactivity of its Me-Co 3+ corrinoid cofactor toward methyl-group transfer remained largely unknown.…”

Identification of an “End-on” Nickel−Superoxo Adduct, [Ni(tmc)(O2)]+

“…3 Recent advances in the area of Ni-O x complexes 4,5 have benefited from the accessibility of isolable Ni + precursors, sufficiently electron-rich to reduce dioxygen as demonstrated through spectroscopic studies of the resulting products. Specifically, efforts from these laboratories have led to the elucidation of Ni(η 2 -O 2 ), 6 Ni 2 (µ-1,2-O 2 ), 7,8 and Ni 2 (µ-O) 2 motifs. 9 The identity of the supporting ligand is critical both for the success in preparing the Ni + complexes and for directing the reactions toward variable product structures.…”