Richard A. Himes

Copper-Dioxygen Complex Mediated C-H Bond Oxygenation: Relevance for Particulate Methane Monooxygenase (pMMO)

Particulate methane monooxygenase (pMMO), an integral membrane protein found in methanotrophic bacteria, catalyzes the oxidation of methane to methanol. Expression and greater activity of the enzyme in the presence of copper ion suggest that pMMO is a cuprous metalloenzyme. Recent advances - especially the first crystal structures of pMMO - have energized the field, but the nature of the active site(s) and the mechanism of methane oxidation remain poorly understood-yet hotly contested. Herein the authors briefly review the current understanding of the pMMO metal sites and discuss advances in small molecule Cu-O(2) chemistry that may contribute to an understanding of copper-ion mediated hydrocarbon oxidation chemistry.

Structural Studies of Copper(I) Complexes of Amyloid-β Peptide Fragments: Formation of Two-Coordinate Bis(histidine) Complexes†

The beta bind: Copper(I) binds to amyloid {beta}-peptide fragments (see structure) as a stable bis(histidine), two-coordinate, near-linear complex, even in the presence of potential additional ligands. As has been proposed or assumed in other studies, the copper(I)-peptide complexes react with dioxygen to form the reactive oxygen species H{sub 2}O{sub 2}, without the need for a third histidine ligand to promote the chemistry.

Cupric superoxo-mediated intermolecular C-H activation chemistry.

The new cupric superoxo complex [LCu(II)(O(2)(•-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.

One is Lonely and Three is a Crowd: Two Coppers Are for Methane Oxidation

Mild activation of inert C H bonds attracts increasingly feverish research efforts as the global predicament of dwindling resources grows increasingly dire. The desirability of methane as a chemical feedstock presents itself in this context: Functionalization of this simplest hydrocarbon would unlock the world s reserves of natural gas for the synthesis of necessary chemicals from this abundant C1 building block. However, with a C H bond strength of 104 kcalmol , methane possesses the most difficult hydrocarbon bond to oxidize. Current industrial methods for converting methane into useful chemical products incur prohibitive energy costs and lack selectivity. Not so for nature s methane monoxygenases, the enzymes utilized in the metabolism of CH4 by bacteria that use methane as a primary energy and biosynthetic material source. Two enzyme classes, soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO), selectively oxidize CH4 to CH3OH at ambient temperature and pressure. The well-understood sMMO employs an activesite diiron cluster to bind and activate dioxygen for this twoelectron oxidation. Extensive bioinorganic research over several decades has successfully elucidated the probable mechanistic pathways for sMMO catalysis. By contrast, the transmembrane protein pMMO has been recalcitrant in yielding the secrets of its structure and reactivity. A copper ion was implicated as the key cofactor in this metalloenzyme, but clues to its role in pMMO had proven inscrutable. For example, measurements of pMMO copper loading and “per copper ion” oxidation activity varied. Three main hypotheses have arisen concerning the pMMO active site: 1) at least one, but possibly multiple, trinuclear copper (Cu3) clusters as loci of electron transfer and catalytic reactivity, 2) a diiron active site, and 3) a dicopper center. That last proposal rose to the forefront with the publication of the enzyme s first X-ray crystal structure (Figure 1). Although a major breakthrough, this fruit of Rosenzweig and

Spectroscopic and Computational Studies of an End-on Bound Superoxo-Cu(II) Complex: Geometric and Electronic Factors that Determine the Ground State

A variety of techniques including absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and resonance Raman (rR) spectroscopies are combined with density functional theory (DFT) calculations to elucidate the electronic structure of the end-on (η(1)) bound superoxo-Cu(II) complex [TMG(3)trenCuO(2)](+) (where TMG(3)tren is 1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine). The spectral features of [TMG(3)trenCuO(2)](+) are assigned, including the first definitive assignment of a superoxo intraligand transition in a metal-superoxo complex, and a detailed description of end-on superoxo-Cu(II) bonding is developed. The lack of overlap between the two magnetic orbitals of [TMG(3)trenCuO(2)](+) eliminates antiferromagnetic coupling between the copper(II) and the superoxide, while the significant superoxo π*(σ) character of the copper dz(2) orbital leads to its ferromagnetically coupled, triplet, ground state.

Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric−Superoxo Complex

To obtain mechanistic insights into the inherent reactivity patterns for copper(I)–O2 adducts, a new cupric–superoxo complex [(DMM-tmpa)CuII(O2•–)]+ (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation–oxygenation reactions. Compound 2 is characterized by UV–vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (kBT/e) ln k2 versus Eox of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, 18O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a CuII–OO–(ArO′) intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O–O cleavage) mainly occurs for p-R-DTBP which gives 18O-labeled DTBQ and elimination of the R group.

A new copper-oxo player in methane oxidation

In this issue of PNAS, Schoonheydt, Solomon, and coworkers (1) definitively characterize the structure of the Cu-ZSM-5 site that oxidizes methane to methanol. With the successful correlation of a specific copper-oxy structure to such reactivity, the importance of this work extends beyond hydrocarbon oxidation on zeolites. This breakthrough potentially has far-reaching implications in both the fundamental and applied sciences.

Copper-peptide complex structure and reactivity when found in conserved His-X(aa)-His sequences.

Oxygen-activating copper proteins may possess His-Xaa-His chelating sequences at their active sites and additionally exhibit imidiazole group δN vs εN tautomeric preferences. As shown here, such variations strongly affect copper ion’s coordination geometry, redox behavior, and oxidative reactivity. Copper(I) complexes bound to either δ-HGH or ε-HGH tripeptides were synthesized and characterized. Structural investigations using X-ray absorption spectroscopy, density functional theory calculations, and solution conductivity measurements reveal that δ-HGH forms the CuI dimer complex [{CuI(δ-HGH)}2]2+ (1) while ε-HGH binds CuI to give the monomeric complex [CuI(ε-HGH)]+ (2). Only 2 exhibits any reactivity, forming a strong CO adduct, [CuI(ε-HGH)(CO)]+, with properties closely matching those of the copper monooxygenase PHM. Also, 2 is reactive toward O2 or H2O2, giving a new type of O2-adduct or CuII–OOH complex, respectively.