Modeling studies of the iron/copper binuclear active site of bovine cytochrome c oxidase

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In attempts to model possible μ-imidazolato [1], μ-oxo [2, 3], and μ-mercapto [4] active site structures for resting (beef heart) cytochrome c oxidase where −J (Fe III −Cu II ) ⩾ 200 cm −1 [1], a number of [FE III(B)Cu II] heterobinuclear iron porphyrin complexes with B = imid −, O 2−, and RS − have been synthesized, isolated as analytically pure solids, and examined by magnetochemical, electrochemical, and various spectroscopic methodologies. The three idealized active site structures which have been targeted for synthesis and study are shown in Fig. 1. ▪ The μ-imidazolato model species have been prepared from either [Fe III hs(TPP)X] (X = C1 − or SO 3- CF 3 −) and [Cu II(imidH) 2DAP] 2+ [5], or from [Fe III hs-(UroTPP)C1] and [Cu II(acac) 2] where UroTPP 2− is the dianion of a new appendaged-tail porphyrin derived from urocanic acid chloride and mono-NH 2TPP [6]. Additionally, new mixed-metal binuclear complexes with [Co II(imid)Cu II] + and [Mn II(imid)-Cu II] + cores have also been prepared from [Co II-(TPP)] and [Mn II(TPP)] precursors. In most cases, −J in the μ-imidazolato species is ⩽ 15 cm −1; however, in two cases the imidazolate-bridged binuclear centers exhibit epr and full-temperature (4.2-300 K) magnetochemical behavior consistent with a ‘strong’ antiferromagnetic exchange interaction possibly as large as the 200 cm −1 value postulated for the [Fe III hs/Cu II] active site of the aerobically oxidized (resting) enzyme itself [1]. New magneto-chemical data for the cyanide and formate derivatized enzyme and for an anaerobically oxidized (‘resting’) enzyme from will also be reported and discussed in light of the μ-imidazolato (and μ-oxo and μ-mercapto) structural option. The lone [Fe IIIOCu II] + species, in hand, has been prepared by the low-temperature reaction of [O=Fe IV(TPP)] with [Cu I(imidH) 2DAP] + [7]. For this μ-oxo heterobimetallic compound the ground state appears most like S = 1 in nature, as reflected by its magnetic, 57Fe Mössbauer and epr (silent) behavior. Furthermore, results from a qualitative [FeO 18Cu] + + H 2O 16 ⇌ [FeO 16Cu] + + H 2O 18 isotopic exchange experiment indicate that the oxygen bridge of the μ-oxo model compound undergoes rapid scrambling with bulk water (CH 3CN/H 2O mixture). This finding is new support for the oxo-bridged postulation, since it adequately explains published Resonance Raman [8] and O 2-turnover [9] results which previously have been used to argue against the presence of an oxo-bridge originating from O 2. A complete catalytic cycle proposed for the enzymatic active site of oxidase (O 2 + 4H + + 4e− → 2H 2O and encompassing a μ-oxo structural model for the active site of the resting state is then shown in Fig. 2. ▪ ▪ Finally, our newest model compounds, μ-mercapto species, derived from [Fe III(TPP)(C1] and [Cu II-(imidR)(S)DAP] + (Fig. 3) or from [Cu I(imidH) 2- DAP] + [7] and a ‘thioferryl’ species of TPP 2− will also be discussed as they comment upon a recently proposed site structure suggested from the EXAFS studies of Chance and Power [4].