Observation of ligand transfer in ba 3 oxidase from thermus thermophilus: Simultaneous FTIR detection of photolabile heme a 3 2+-CN and transient CuB 2+-CN complexes

Abstract

FTIR and light-minus-dark FTIR spectroscopy have been employed to investigate the reaction of oxidized and fully reduced ba3 oxidase with cyanide. The characterization of the structures of the bound CN- in the binuclear heme Fe-CuB center is essential, given that a central issue in the function of ba3 oxidase is the extent to which the partially reduced substrates interact with the two metals. In the reaction of oxidized ba3 oxidase with cyanide the initially formed heme a 3 3+-C=N-CuB 2+ species with (CN) frequency at 2152 cm-1 was replaced by a photolabile complex with a frequency at 2075 cm-1 characteristic of heme a3 2+-CN-. Photolysis of the heme a3 2+-CN- adduct produced a band at 2146 cm-1 attributed to the formation of a transient CuB 2+-CN - complex. All forms are pH independent between pH 5.5-9.5 and at pD 7.5 indicating the absence of ionizable groups that influence the properties of the cyanide complexes. In contrast to previous reports, our results show that CN- does not bind simultaneously to both heme a3 2+ and CuB 2+ to form the mixed valence a 3 2+-CNA•CuB 2+CN species. The photolysis products of the heme a3 2+-CN -/CuB 2+ and heme a3 2+-CN-/CuB 1+ species are different suggesting that relaxation dynamics in the binuclear center following ligand photodissociation are dependent on the oxidation state of CuB. © 2012 American Chemical Society.