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).
Kauffmann
Karl E.
KE
Departments of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, Harvard University, Cambridge, Massachusetts 02138, and the University of Minnesota, Minneapolis, Minnesota 55455.
Goddard
Christine A.
CA
Zang
Yan
Y
Holm
R. H.
RH
Münck
Eckard
E