Chemical Plausibility of Cu(III) with Biological Ligation in pMMO

Abstract

The mechanisms of dioxygen activation and methane C-H oxidation in particulate methane monooxygenase (pMMO) are currently unknown. Recent studies support a binuclear copper site as the catalytic center. We report the low-temperature assembly of a high-valent dicopper(III) bis(μ-oxide) complex bearing marked structural fidelity to the proposed active site of pMMO. This unprecedented dioxygen-bonded Cu(III) species with exclusive biological ligation directly informs on the chemical plausibility and thermodynamic stability of the bis(μ-oxide) structure in such dicopper sites and foretells unusual optical signatures of an oxygenation product in pMMO. Though the ultimate pMMO active oxidant is still debated, C-H oxidation of exogenous substrates is observed with the reported Cu(III) complexes. The assembly of a high valent species both narrows the search for relevant pMMO intermediates and provides evidence to substantiate the role of Cu(III) in biological redox processes.

Copper–Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

“…Direct oxygenation of Cu(I) complexes of primary amines failed to yield isolable products, so the “ core capture ” method was used as described in section 3.1.1. 295,296 Thus, the bis( μ -oxo)dicopper complex ( 51 ) of L10a was prepared by reaction of a Cu(I) precursor with O 2 , and then this complex was treated with another ligand (2 equiv) at −125 °C to rapidly yield new bis( μ -oxo)dicopper products 61 – 63 (Figure 44). The shown thermodynamic stability order was determined through mixing experiments and DFT calculations.…”

A tale of two methane monooxygenases

“…5), suggesting that such a species could participate in methane oxidation by pMMO. Similarly, dicopper complexes assembled using either primary amines or histamine ligands to yield a planar bis(μ-oxide) ${\text{Cu}}_2^{\text{III}}$ coordination exhibit short Cu–Cu distances and strong Cu–N bonding, and are able to oxidize hydrocarbons with C–H bond strengths of ~76 kcal/mol [150, 151]. These compounds demonstrate that histidine imidazoles can stabilize Cu III ligation under the conditions employed, and may provide a chemical rationale for the presence of an N-terminal histidine ligand in pMMO (as well as LPMO).…”

High-valent copper in biomimetic and biological oxidations

“…Bidentate amine ligands ( e.g. , diamines, histamines, pyridylamines) almost exclusively form O species, which constitute the largest family of synthetic Cu-O 2 species to date [19,21,22]. Finally, the least sterically demanding bidentate ligands ( e.g.…”

“…Of course, the energies of the LMCT bands are sensitive to the nature of the ligand, but the optical features are generally consistent. Recent studies on O and T species with varied amine and imidazole ligation correlate more donating ligation to a systematic blue-shifting of the LMCT optical bands for complexes of the same structural type [21,22]. As many of these complexes are thermally sensitive and can exist as an equilibrium of isomers in solution ( vide infra ), resonance Raman (rR) is generally preferred over infrared spectroscopic characterization, as the former enables optical and vibrational correlations [26–28].…”