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Superoxo, -Peroxo, and -Oxo Complexes from SPECIAL FEATURE Heme͞o2 and Heme-Cu͞o2 Reactivity: Copper Ligand Influences in Cytochrome C Oxidase Models

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Superoxo, -Peroxo, and -Oxo Complexes from SPECIAL FEATURE Heme͞o2 and Heme-Cu͞o2 Reactivity: Copper Ligand Influences in Cytochrome C Oxidase Models ␮ ␮ Superoxo, -peroxo, and -oxo complexes from SPECIAL FEATURE heme͞O2 and heme-Cu͞O2 reactivity: Copper ligand influences in cytochrome c oxidase models Eunsuk Kim*, Matthew E. Helton*†, Ian M. Wasser*, Kenneth D. Karlin*§, Shen Lu¶, Hong-wei Huang¶, Pierre Moe¨ nne-Loccoz¶, Christopher D. Incarvitoʈ, Arnold L. Rheingoldʈ, Marcus Honecker†, Susan Kaderli†, and Andreas D. Zuberbu¨ hler† *Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218; ¶Department of Biochemistry and Molecular Biology, OGI School of Science and Engineering at Oregon Health and Science University, Beaverton, OR 97006; ʈDepartment of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716; and †Department of Chemistry, University of Basel, 4056 Basel, Switzerland Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved February 13, 2003 (received for review November 25, 2002) II -tet ؍ The O2-reaction chemistry of 1:1 mixtures of (F8)Fe (1; F8 rakis(2,6-diflurorophenyl)porphyrinate) and [(LMe2N)CuI]؉ (2; LMe2N (N,N-bis{2-[2-(N؅,N؅-4-dimethylamino)pyridyl]ethyl}methylamine ؍ is described, to model aspects of the chemistry occurring in cyto- chrome c oxidase. Spectroscopic investigations, along with stopped-flow kinetics, reveal that low-temperature oxygenation of III 1͞2 leads to rapid formation of a heme-superoxo species (F8)Fe - ؊ (O2 ) (3), whether or not 2 is present. Complex 3 subsequently re- ؊ III 2 II Me2N ؉ acts with 2 to form [(F8)Fe –(O2 )–Cu (L )] (4), which thermally III II Me N ؉ converts to [(F8)Fe –(O)–Cu (L 2 )] (5), which has an unusually bent (Fe–O–Cu) bond moiety. Tridentate chelation, compared with CHEMISTRY tetradentate, is shown to dramatically lower the ␯(O–O) values ob- served in 4 and give rise to the novel structural features in 5. ioxygen binding to metal ion centers such as copper and iron is of great importance and interest, for fundamental Fig. 1. Structure of the fully reduced (FeIII⅐⅐⅐CuII) bovine cytochrome c oxidase D ⅐⅐⅐ ϭ and practical reasons. In nature, copper and iron proteins structure, Cu Fe 5.1 Å. The diagram was assembled by using PDB ID 1OCR coordinates and the program RASMOL. serve to process O2 in a variety of functions, such as in trans- port of dioxygen in blood, mono- or dioxygenation involving oxygen atom incorporation, and substrate oxidation (dehydro- ␮-␩2:␩2-peroxo-dicopper(II) species, which can be in equilibrium genation or removal of electrons) with concomitant reduction with bis-␮-oxo-dicopper(III) isomers (21, 23, 24). of O2 to hydrogen peroxide or water. Cytochrome c oxidases Thus, with the known tridentate chelation for CuB in cyto- mediate the four-electron four-proton reduction of O2 to wa- ter, coupling this exergonic reaction to membrane proton chrome c oxidases, we are intensifying efforts to study such situ- translocation, which drives ATP synthesis (1, 2). Protein x-ray ations in model systems, to see whether O2 intermediates form with FeII͞CuI precursors, and to understand the detailed role of structures reveal that O2 binding and reduction occur at a binuclear active site consisting of a high-spin heme group the copper ligand (tridentate versus tetradentate) in heme- ͞ (with proximal histidine), with a tris-histidine-ligated copper copper O2 chemistry. Here we describe the oxygenation chemis- II Me2N I ϩ ion (CuB) situated on the distal side (see Fig. 1) (3–6). Spec- try of 1:1 mixtures of (F8)Fe (1) and [(L )Cu ] (2). troscopic–mechanistic investigations suggest that after a possi- ble initial O2 interaction with CuB, a myoglobin-like Fe–O2 III Ϫ adduct forms (best described as a Fe –O2 superoxo species), subsequently leading to a FeIVAO (ferryl-oxo) species (7–10), III 2Ϫ II although a prior peroxo-bridged Fe –(O2 )-Cu intermedi- ate has not been ruled out. The tyrosine-crossed-linked CuB site (Fig. 1) is thought to be critical in electron transfer (from CuI and the phenol group) leading to reductive O–O cleavage and proton-translocation chemistry (11–13). Our research program is focused on elucidating the funda- mental aspects of O2 interactions with heme and copper centers. A very rich chemistry ensues (Scheme 1), wherein there is an Thus, we have reported several examples where reduced heme– initial rapid formation of a heme-superoxo complex II͞ I ␮ III 2Ϫ Fe Cu complexes react with O2, giving -peroxo Fe –(O2 )– CuII species (14–18). Most of these studies have used tris[2-pyri- dylmethyl]amine (TMPA) as the Cu ligand. Extensive studies on This paper was submitted directly (Track II) to the PNAS office. copper (only) dioxygen systems have shown that even subtle Abbreviations: TMPA, tris[2-pyridylmethyl]amine; F8, tetrakis(2,6-diflurorophenyl)porphy- ͞ rinate; LMe2N N,N-bis{2-[2-(NЈ,NЈ-4-dimethylamino)pyridyl]ethyl}methylamine; UV-Vis, changes in ligand structure denticity can dramatically alter the ultraviolet–visible; RR, resonance Raman; THF, tetrahydrofuran; EtCN, propionitrile. I͞ ϭ nature of the (ligand)Cu O2 product [giving Cun–O2 (n 1or ϭ Data deposition: The x-ray data have been deposited in the Cambridge Crystallographic 2) or Cun–(O)2, n 2 or 3] (19–21). Tetradentate TMPA in- Data Centre, www.ccdc,cam.ac.uk (CCDC 206505). II II duces formation of an end-on ligated (Cu –O–O–Cu ) peroxo- §To whom correspondence should be addressed at: Department of Chemistry, Johns I dicopper(II) structure upon O2 reaction with [Cu (TMPA)- Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. E-mail: ϩ (MeCN)] (22), whereas tridentate ligands generate side-on [email protected]. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0737180100 PNAS ͉ April 1, 2003 ͉ vol. 100 ͉ no. 7 ͉ 3623–3628 Downloaded by guest on September 26, 2021 II Me N I ϩ Fig. 2. UV-Vis spectra of the oxygenation reaction of (F8)Fe (1)͞[(L 2 )Cu ] (2) in CH2Cl2͞6% EtCN at 193 K. Shown are an equimolar mixture of 1 and 2 (black); Ϫ III 2 II Me2N ϩ [(F8)Fe –(O2 )–Cu (L )] (4) (red); a product mixture obtained after warming III II Me N ϩ III Scheme 1. 4, which includes [(F8)Fe –O–Cu (L 2 )] (5) and (F8)Fe –OH (green). Ϫ ϩ ϩ III Me2N I ϩ 3 Me2N II 2 (solvent)(F8)Fe –(O2 )(3) [referred to subsequently as 2[(L )Cu ] (2) O2 [{(L )Cu }2(O2)] [3] III Ϫ (F8)Fe -(O2 )(3)], as elucidated from stopped-flow kinetic 3 2, studies. Superoxo species subsequently reacts with form-Ϫ I͞ ␮ III 2 presence of a nitrile strongly inhibits Cu O2 reactivity. With ing the heterobinuclear -peroxo complex [(F8)Fe -(O )– ϩ 2 II Me2N 4 this background, we have designed a solvent system, Cu (L )] ( ), which has been characterized by ultraviolet– ͞ ͞ visible (UV-Vis), nuclear magnetic resonance (NMR), and CH2Cl2 6% (vol vol) EtCN, that should force a reaction of resonance Raman (RR) spectroscopies. Notably, the tridentate an initially formed iron–superoxide species with our copper(I) versus tetradentate Cu chelation leads to striking differences complex, in close analogy to one of the key steps of the O2- in the nature of the peroxo moiety (i.e., the O–O stretching reduction cycle in cytochrome c oxidase (see above). In III 2Ϫ ͞ frequency) formed in 4 versus that in [(F8)Fe –(O2 )– CH2Cl2 6% EtCN, the EtCN (i) serves as an axial base heme CuII(TMPA)]ϩ (6). The ␮-peroxo complex 4 thermally trans- ligand promoting iron–superoxide formation (Eq. 1) (26) and ϩ ϩ ␮ III II Me2N Me2N I ͞ forms to the -oxo complex [(F8)Fe -(O)–Cu (L )] (5), (ii) causes the [(L )Cu ] (2) O2 reaction to become insig- ͞ ͞ ͞ whose x-ray structure is described, and found to be dramatically nificant.** Yet, for the 1 2 O2 mixture in CH2Cl2 6% EtCN, III II ϩ different from that found in [(F8)Fe –(O)-Cu (TMPA)] (7). an iron–superoxo͞copper reaction occurs, leading to the ␮ III 2Ϫ -peroxo heterobinuclear species [(F8)Fe –(O2 )– Materials and Methods ϩ CuII(LMe2N)] (4). Such heme-peroxo-copper species were not II II Compounds (F8)Fe (1) (17, 25), (F8)Fe -d8 (1-d8) (17), and observed from the reactions of 3 with analogous CuI com- Me2N I [(L )Cu ]B(C6F5)4 (2) (25) have been previously described. R I ϩ ϭ ␮ III 2Ϫ plexes ([(L )Cu ] , where L N,N-bis[2-(2-pyridyl)ethyl]- The dioxygen adduct, -peroxo complex [(F8)Fe –(O )– ϩ 2 methylamine and R ϭ 4-pyridyl substituents Cl, H, or OMe); CuII(LMe2N)] (4), was generated by bubbling O through 2 the electron-donating dimethylamino groups in the LMe2N in- 178–193 K solutions of 1 and 2 that were prepared in an inert I 3 4 atmosphere glove box. The ␮-oxo complex 5 was isolated by crease reactivity of Cu with leading to formation of . ͞ Benchtop UV-Vis spectroscopic changes of the oxygena- generation of 4 in CH2Cl2 10% CH3CN with subsequent ther- II tion reaction of an equimolar mixture of (F8)Fe (1) and mal transformation (193 K to room temperature, 1 h) to the Me N I ϩ product. Precipitation with heptane gave microcrystalline [(L 2 )Cu ] (2) are shown in Fig. 2. The reduced mixture has III II Me2N ϩ ͞ [(F8)Fe –(O)–Cu (L )] (5) in 58% yield. See Experimen- absorptions at 415, 425, and 527 nm at 193 K in CH2Cl2 6% tal Methods in Supporting Text, which is published as support- EtCN solution. Bubbling with dioxygen produces a stable dioxy- ing information on the PNAS web site, www.pnas.org, for fur- gen adduct with new spectral features at 420, 540, and 567 nm.
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