No abstract

Strategies for O 2 activation by copper enzymes were recently expanded to include mononuclear Cu sites, with the discovery of the copper-dependent polysaccharide monooxygenases, also classified as auxiliary-activity enzymes 9-11 (AA9-11). These enzymes are finding considerable use in industrial biofuel production. Crystal structures of polysaccharide monooxygenases have emerged, but experimental studies are yet to determine the solution structure of the Cu site and how this relates to reactivity. From X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies, we observed a change from four-coordinate Cu(II) to three-coordinate Cu(I) of the active site in solution, where three protein-derived nitrogen ligands coordinate the Cu in both redox states, and a labile hydroxide ligand is lost upon reduction. The spectroscopic data allowed for density functional theory calculations of an enzyme active site model, where the optimized Cu(I) and (II) structures were consistent with the experimental data. The O 2 reactivity of the Cu(I) site was probed by EPR and stopped-flow absorption spectroscopies, and a rapid one-electron reduction of O 2 and regeneration of the resting Cu(II) enzyme were observed. This reactivity was evaluated computationally, and by calibration to Cu-superoxide model complexes, formation of an end-on Cu-AA9-superoxide species was found to be thermodynamically favored. We discuss how this thermodynamically difficult one-electron reduction of O 2 is enabled by the unique protein structure where two nitrogen ligands from His1 dictate formation of a T-shaped Cu(I) site, which provides an open coordination position for strong O 2 binding with very little reorganization energy.X-ray absorption spectroscopy | DFT | dioxygen activation | biofuels

We demonstrate a new approach to utilize copper(I) iodide coordination complexes as emissive layers in organic light-emitting diodes (OLEDs), by in situ codeposition of copper(I) iodide and 3,5-bis(carbazol-9-yl)pyridine (mCPy). With a simple three-layer device structure, pure green electroluminescence at 530 nm from a copper(I) complex was observed. Maximum luminance and external quantum efficiency (EQE) of 9700 cd/m2 and 4.4% have been achieved, respectively. The luminescent species has been identified as [CuI(mCPy)2]2 based on photophysical studies of model complexes and X-ray absorption spectroscopy (XAS).

The nature of the ligand is an important aspect of controlling structure and reactivity in coordination chemistry. In connection with our study of heme/copper/oxygen reactivity relevant to cytochrome c oxidase O 2 -reduction chemistry, we compare the molecular and electronic structure of two highspin heme-peroxo-copper [Fe III -O 2 2--Cu II ] + complexes containing N 4 -tetradentate (1) or N 3 -tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to DFT calculations we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish μ-(O 2 2-) sideon to the Fe III and end-on to Cu II (μ-η 2 :η 1 ) binding for the complex 1 but side-on/side-on (μ-η 2 :η 2 ) μ-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh 3 , acid and phenols. The comparison of a new X-ray structure of μ-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two μ-oxo complexes (∠Fe-O-Cu = 178.2° in 1a, ∠Fe-O-Cu = 149.5° in 2a). The reaction of 2 with one equivalent of exogenous N-donor axial base leads to the formation of a distinctive low-temperature stable, low-spin heme-O 2 -Cu complex (2b), but under the same conditions the addition of an axial base to 1 leads to the dissociation of the hemeperoxo-Cu assembly and the release of O 2 . 2b reacts with phenols performing hydrogen-atom (e -+ H + ) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [Fe IV =O] complex (2c). The nature of 2c was confirmed by comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the hydrogen-atom abstraction was either 1) directly detected by EPR spectroscopy using phenols that produce stable radicals or 2) indirectly by detection of the coupling product of two phenoxyl radicals.karlin@jhu.edu, edward.solomon@stanford.edu. Supporting Information Available. UV-visible spectra of the reaction of (2b) with 1 equiv. of 2,4-di-tertbutylphenol ( Figure S1), UVvis spectra of the reaction of [(F 8 )Fe III -(O 2 2-)-Cu II (TMPA)] + (1) with DMAP ( Figure S2), EPR spectra of (2b) reaction with 1 equiv.of 2,4-di-tert-butylphenol ( Figure S3), GC-MS trace of the oxidative coupling of 2,4-di-tert-butylphenol in presence of (2b) ( Figure S4), ORTEP diagram ( Figure S5) and crystal data and structure refinement for [(F 8 )Fe III -(O 2-)-Cu II (AN)] + (2a) X-ray structure (Table S1), XAS spectra and computational data.NIH Public Access

The multicopper oxidase Fet3p catalyzes the four-electron reduction of dioxygen to water, coupled to the one-electron oxidation of four equivalents of substrate. To carry out this process the enzyme utilizes four Cu atoms: a type 1, a type 2, and a coupled binuclear, type 3 site. Substrates are oxidized at the T1 Cu, which rapidly transfers electrons, 13 Å away, to a trinuclear copper cluster composed of the T2 and T3 sites where dioxygen is reduced to water in two sequential 2e − steps. This study focuses on two variants of Fet3p, H126Q and H483Q, that perturb the two T3 Cu's, T3α and T3β, respectively. The variants have been isolated in both holo and type 1 depleted (T1D) forms, T1DT3αQ and T1DT3βQ, and their trinuclear copper clusters have been characterized in their oxidized and reduced states. While the variants are only mildly perturbed relative to T1D in the resting oxidized state, in contrast to T1D they are both found to have lost a ligand in their reduced states. Importantly, T1DT3αQ reacts with O 2 but T1DT3βQ does not. Thus loss of a ligand at T3β, but not at T3α, turns off O 2 reactivity, indicating that T3β and T2 are required for the 2e − reduction of O 2 to form the peroxide intermediate (PI), whereas T3α remains reduced. This is supported by the spectroscopic features of PI in T1DT3αQ, which are identical to T1D PI. This selective redox activity of one edge of the trinuclear cluster demonstrates its asymmetry in O 2 reactivity. The structural origin of this asymmetry between the T3α and T3β is discussed as is its contribution to reactivity.