Protonation of the oxo-bridged heme/copper assemblies: Modeling the oxidized state of the cytochrome c oxidase active site.

In this study on model compounds for the resting oxidized state of the iron‑copper binuclear center in cytochrome c oxidase (CcO), we describe the synthesis of a new μ-oxo-heme/Cu complex, [(TPP)Fe-O-Cu(tmpa)][B(CF)] (2) {TPP: tetraphenyl porphyrinate(2-); TMPA: tris(2-pyridylmethylamine)}, as well as two protonation events for three μ-oxo-heme/Cu complexes with varying peripheral substituents on the heme site. The addition of increasing amounts of strong acid to these μ-oxo-heme/Cu systems successively led to the generation of the corresponding μ-hydroxo, μ-aquo, and the dissociated complexes. The heme/Cu assemblies bridged through a water ligand are reported here for the first time and the H NMR and F NMR spectral properties are consistent with antiferromagnetically coupled high-spin iron(III) and copper(II) centers. By titration using a series of protonated amines, the pK values for the corresponding μ-hydroxo-heme/Cu species (i.e., the first protonation event) have been reported and compared with the pK ranges previously estimated for related systems. These synthetic systems may represent structural models for the oxidized Fe-X-Cu resting state, or turnover intermediates and can be employed to clarify the nature of proton/electron transfer events in CcO. SYNOPSIS: The resting oxidized state of the cytochrome c oxidase active site contains an Fe-OH-Cu moiety. Here, we investigated two successive protonation events, for a series of μ-oxo-heme/Cu assemblies and reported the pK values for the first protonation event. The μ-aquo-heme/Cu complexes described here are the first examples of such systems.