An ab initio molecular orbital study of a binuclear dioxygen complex as a model of the binuclear active site in cytochrome c oxidase

Unrestricted Hartree–Fock calculations of a binuclear dioxygen complex as a model of the binuclear active site in cytochrome c oxidase are carried out in order to allocate the reaction intermediates in the early stage of the reduction of O2 to H2O. The heme a3 in the binuclear active site is replaced by Fe–4NH3 in our model. The following results are obtained: the geometries of the intermediates, [a32+, OO–CuB1+] and [a32+–OO, CuB1+], in the singlet state are estimated, the protonated state [a33+–OOH, CuB1+] is found to be 2.1 kcal/mol more stable than the [a32+, OO–CuB1+] state, and the early stage of the reduction of O2 proceeds cooperatively with the processes of O2 capture and protonation.