Synthetic Models of the Active Site of Cytochrome c Oxidase;Influence of Tridentate or Tetradentate Copper Chelates Bearing a His-Tyr Linkage Mimic on Dioxygen Adduct Formation by Heme/Cu Complexes

摘要

Two synthetic models of the active site of cytochrome c oxidase- [(L~(N4-OH))Cu~I-Fe~(II)(TMP)]~+(1a) and [(L~(N3-OH))Cu~I-Fe~(II)(TMP)]~+(2a)-have been designed and synthesized.These models each contain a heme and a co-valently attached copper moiety supported either by a tetradentate N4-copper chelate or by a tridentate N3-copper chelate including a moiety that acts as a mimic of the crosslinked His-Tyr component of cytochrome c oxidase.Low-temperature oxygenation reactions of these models have been investigated by spectroscopic methods including UV/Vis,resonance Raman,ESI-MS,and EPR spectroscopy.Oxygenation of the tetradentate model la in MeCN and in other solvents produces a low-temperature-stable dioxygen-bridged peroxide [(L~(N4-OH))Cu~(II)-O2-Fe~(III)(TMP)]+{v_(O-O)=799 (~(16)O_2)/752 cm~(-1) (~(18)O_2)1,while a heme superoxide species [(TMP)Fe~(III)(O2-)…Cu~(II)L~(N3-0H)] {v_(Fe-O2);576 (~(16)O_2)/551 cm~(-1) (~(18)O_2)} is generated when the tridentate model 2 a is oxygenated in EtCN solution under similar experimental conditions.The coexistence of a heme superoxide species [(TMP)Fe~(III)(O_2~-)…Cu~(II)L~(N3-OH]) and a bridged peroxide [(L~(N3-OH))Cu~(II)-O2-Fe~(III)(TMP)]~+species in equal amounts is observed when the oxygenation reaction of 2 a is performed in CH_2Cl_2/7% EtCN,while the percentage of the peroxide (approx=70%) in relation to superoxide (approx=30%) increases further when the crosslinked phenol moiety in 2 a is deprotonated to produce the bridged peroxide [(L~(N3-OH))Cu~(II)-O_2-Fe~(III)(TMP)] ~+[v_(O-O);812 (~(16)O_2)/765 cm~(-1) (~(18)O_2)} as the main dioxygen intermediate.The weak re-ducibility and decreased O_2 reactivity of the tricoordinated Cu~I site in 2 a are responsible for the solvent-dependent formation of dioxygen adducts.The initial binding of dioxygen to the copper site en route to the formation of a bridged heme-O2-Cu intermediate by model 2 a is suggested and the deprotonated crosslinked His-Tyr moiety might contribute to enhancement of the O2 affinity of the Cu~I site at an early stage of the dioxygen-binding process.