Reversible Dioxygen Binding to Hemerythrin. 1. Electronic Structures of Deoxy- and Oxyhemerythrin
journal contributionposted on 31.08.1999, 00:00 by Thomas C. Brunold, Edward I. Solomon
Studies on the electronic structure of the two physiologically relevant forms of hemerythrin (Hr), i.e., deoxyHr (possessing a hydroxo-bridged diferrous site) and oxyHr (having an oxo-bridged diferric site with a terminal hydroperoxide), are presented and discussed. SQUID magnetic susceptibility data for deoxyHr confirm that the two ferrous ions are weakly antiferromagnetically coupled, J = −14(2) cm-1 (ℋ = −2JS1·S2), indicating that at 300 K the entire manifold of ground-state spin sublevels is available for the reaction with O2. From density functional calculations on deoxyHr, evaluated on the basis of experimental data, the redox active orbital on the five-coordinate iron (Fe2) is favorably oriented for a π-bonding interaction with O2 approaching along the open coordination site of that center, and reorientation of the redox active orbital on the six-coordinate iron (Fe1) for electron transfer to O2 through superexchange with Fe2 is energetically accessible. Analysis of existing spectroscopic data for oxyHr using Heller's time-dependent theory leads to the proposal that the peculiar behavior of the UV resonance Raman excitation profile for the symmetric Fe−O−Fe stretching mode ν(Fe−O) to peak with fairly minor absorption features arises from interference effects between oxo-to-Fe charge-transfer excited states. From density functional calculations on oxyHr and related structures the low frequency of the Fe−oxo stretch is ascribed to the hydrogen bond between the hydroperoxide and the bridging oxide, whereas the small |J| value appears to be due primarily to the strong hydroperoxide → Fe2 π-donor interaction that reduces Fe1 → Fe2 electron delocalization.