Spectroscopic elucidation of a new heme/copper dioxygen structure type: implications for O···O bond rupture in cytochrome c oxidase.

Cytochrome c oxidase (CcO) catalyzes the four electron reduction of dioxygen to form water in the terminal step of the electron transport chain.[1] Dioxygen binds to a unique heme-copper bimetallic active site, wherein the copper is ligated ~5 A above the heme by three His residues.[2–3] One of these ligating His residues is covalently crosslinked to a nearby Tyr residue. The crosslinked Tyr is thought to participate in catalysis by providing the fourth electron needed to cleave the O-O bond in a net H• abstraction.[4] This hypothesis stems from the observation of an intermediate state (PM) in CcO that occurs after O-O bond cleavage, which is suggested to contain a tyrosyl radical based on chemical and spectral evidence.[5–8] The only observable enzymatic dioxygen intermediate before O-O bond rupture has been assigned as a ferric-superoxo species (A),[9–10] leading some to suggest this species is directly responsible for the net H• abstraction from the Tyr.[11–12] We and others favor an alternative mechanistic scenario, in which an unobserved peroxo intermediate functions as the active oxidant.[13–15] A putative peroxo moiety would take advantage of the His-Tyr crosslink and the copper ion as a pathway to access the fourth electron necessary for cleavage of its O-O bond. However, heme-peroxo-copper adducts are generally unreactive towards phenols, motivating efforts towards understanding factors required for O-O bond rupture by heme-copper sites. Recently, we reported preliminary evidence that the reaction of a heme-peroxo-copper adduct {[F8Fe]-O2-[CuAN]}+ (1, F8 = 5,10,15,20-tetrakis-(2,6-difluorophenyl)-porphyrinate, AN = bis(3-(dimethylamino)-propyl)-amine) with a coordinating base DCHIm (DCHIm = 1,5-dicyclohexylimidazole) results in formation of a discrete complex (2) that has enhanced reactivity towards phenols.[16] Herein we report the molecular and electronic structure of 2, which is an example of a heme-peroxo-copper complex in which the electronic state of the heme fragment is low-spin (LS).[17] Concomitant with the change in spin of the heme fragment from high-spin (HS) to LS upon conversion from 1 to 2, the Fe-O2-Cu core undergoes a change from µ-η2:η2 (“side-on”) in 1 to µ-1,2 (“end-on”) in 2 (Scheme 1). This novel bridging mode has not been observed previously in heme-copper model complexes, but it has been proposed in recent crystallographic studies on resting CcO.[18–20] However, comparison of the spectral features of resting CcO to those described herein for 2 reveal inconsistencies suggesting a reevaluation of the bridging mode of the peroxo group in resting CcO and providing insight into the electronic structure requirements for O-O bond cleavage.