title: Dioxygen reactivity of copper and heme-copper complexes possessing an imidazole-phenol cross-link.
abstract: Recent spectroscopic, kinetics, and structural studies on cytochrome c oxidases (CcOs) suggest that the histidine-tyrosine cross-link at the heme a3-CuB binuclear active site plays a key role in the reductive O2-cleavage process. In this report, we describe dioxygen reactivity of copper and heme/Cu assemblies in which the imidazole-phenol moieties are employed as a part of copper ligand LN4OH (2-{4-[2-(bis-pyridin-2-ylmethyl-amino)-ethyl]-imidazol-1-yl}-4,6-di -tert-butyl-phenol). Stopped-flow kinetic studies reveal that low-temperature oxygenation of [CuI(LN4OH)]+ (1) leads to rapid formation of a copper-superoxo species [CuII(LN4OH)(O2-)]+ (1a), which further reacts with 1 to form the 2:1 Cu:O2 adduct, peroxo complex [{CuII(LN4OH)}2(O2(2-))]2+ (1b). Complex 1b is also short-lived, and a dimer Cu(II)-phenolate complex [CuII(LN4O-)]2(2+) (1c) eventually forms as a final product in the later stage of the oxygenation reaction. Dioxygen reactivities of 1 and its anisole analogue [CuI(LN4OMe)]+ (2) in the presence of a heme complex (F8)FeII (3) (F8 = tetrakis(2,6,-difluorotetraphenyl)-porphyrinate) are also described. Spectroscopic investigations including UV-vis, 1H and 2H NMR, EPR, and resonance Raman spectroscopies along with spectrophotometric titration reveal that low-temperature oxygenation of 1/3 leads to formation of a heme-peroxo-copper species [(F8)FeIII-(O2(2-))-CuII(LN4OH)]+ (4), nu(O-O) = 813 cm(-1). Complex 4 is an S = 2 spin system with strong antiferromagnetic coupling between high-spin iron(III) and copper(II) through a bridging peroxide ligand. A very similar complex [(F8)FeIII-(O2(2-))-CuII(LN4OMe)]+ (5) (nu(O-O) = 815 cm(-1)) can be generated by utilizing the anisole compound 2, which indicates that the cross-linked phenol moiety in 4 does not interact with the bridging peroxo group between heme and copper. This investigation thus reveals that a stable heme-peroxo-copper species can be generated even in the presence of an imidazole-phenol group (i.e., possible electron/proton donor source) in close proximity. Future studies are needed to probe key factors that can trigger the reductive O-O cleavage in CcO model compounds.
authors.affiliation: Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Eunsuk
authors.initials: E
authors.lastname: Kim
authors.forename: Kaliappan
authors.initials: K
authors.lastname: Kamaraj
authors.forename: Benedikt
authors.initials: B
authors.lastname: Galliker
authors.forename: Nick D
authors.initials: ND
authors.lastname: Rubie
authors.forename: Pierre
authors.initials: P
authors.lastname: Moënne-Loccoz
authors.forename: Susan
authors.initials: S
authors.lastname: Kaderli
authors.forename: Andreas D
authors.initials: AD
authors.lastname: Zuberbühler
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Karlin
mesh_terms: D003300:Copper / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D013057:Spectrum Analysis; D013816:Thermodynamics
chemical_list: D007093:Imidazoles; D010636:Phenols; D006418:Heme; D003300:Copper; D010100:Oxygen