Mössbauer and Magnetization Studies of Heme-Copper-Bridged Assemblies Pertinent to Cytochrome c Oxidase

In order to elucidate the structural and electronic properties of the dioxygen-reducing site of cytochrome oxidases, we have studied heme-based molecular assemblies containing the bridge unit Fe(III)-X-Cu(II) with X = O(2)(-), OH(-), and HCO(2)(-). Here we describe the results of Mössbauer and magnetization studies of [(OEP)Fe-O-Cu(Me(6)tren)](1+) (3), [(OEP)Fe-(OH)-Cu(Me(5)dien)(OClO(3))](1+) (4), [(OEP)Fe-(O(2)CH)-Cu(Me(5)dien)(OClO(3))](1+) (5), and [(OEP)Fe{(O(2)CH)Cu(Me(6)tren)}(2)](3+) (6). On the basis of magnetization studies, the oxidized binuclear enzyme site has been reported to be strongly coupled, with an S = 2 ground state. For 3 and 4 the Mössbauer data were analyzed for strong antiferromagnetic coupling between a high-spin Fe(III) (S(1) = (5)/(2)) and Cu(II) (S(2) = (1)/(2)), resulting in an S = 2 ground state. The exchange coupling of hydroxo-bridged 4 is substantial, J = 170 cm(-)(1) (H(ex) = JS(1).S(2)), but is smaller than J >/= 200 cm(-)(1) reported for oxo-bridged 3. The collective evidence for synthetic complexes and the oxidized enzymes indicates that the Fe(III)-(OH)-Cu(II) bridge unit is probable for the latter in the resting state. The similarity of properties of formate-inhibited and "slow" cytochrome oxidase has suggested a carboxylate bridge in the binuclear site. Formate-bridged 5 maintains an anti-anti carboxylate bridge conformation and has J = +18 cm(-)(1). While the bridge conformation of 5 might not apply to a putative carboxylate bridge in the binuclear enzyme site, the order-of-magnitude difference between the J values of 3, 4, and 5 suggests that no stereochemically possible &mgr;-eta(1):eta(1) carboxylate bridge conformation is likely to approach the strong coupling (J >/= 200 cm(-)(1)) of the formate-inhibited and "slow" enzyme forms. If carboxylate does bridge, the &mgr;-eta(2) mode appears more likely. Complex 6, which is not biologically pertinent, displays weak ferromagnetic coupling between a central high-spin Fe(III) and two Cu(II) sites, -0.50 cm(-)(1) < J < -0.10 cm(-)(1).