Nitric oxide adducts of the binuclear iron site of hemerythrin: spectroscopy and reactivity

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Abstract
Nitric oxide forms adducts with the binuclear iron site of hemerythrin (Hr) at [Fe(II), Fe(II)]deoxy and [Fe(II), Fe(III)]semimet oxidation levels. With deoxyHr our results establish that (i) NO binds reversibly, forming a complex which we label deoxyHrNO, (ii) NO forms a similar but distinct complex in the presence of fluoride, which we label deoxyHrFNO, (iii) No is directly coordinated to one iron atom of the binuclear pair in these adducts, most likely in a bent end-on fashion, and (iv) the iron atoms in the binuclear sites of both deoxyHrNO and deoxyHrFNO are antiferromagnetically coupled, thereby generating unique electron parmagnetic resonance (EPR) detectable species. The novel EPR signal of deoxyFrNO (deoxyHrFNO) with g.dblvert. = 2.77 (2.58) and g.pnd. = 1.84 (1.80) is explained by the magnetic interaction of the Fe(II) (S'' = 2) and {FeNO}7 (S=3/2) centers observed by Mossbauer spectroscopy. Antiferromagnetic coupling leads to a ground state of Seff = 1/2. Analysis of the EPR parameters using the isotropic spin-exchange Hamiltonian, .cxa.Hex = 2J.cxa.S3/2 .cntdot. .cxa.S2, and including zero-field splitting leads to a coupling constant, -J .apprx. 23 cm-1, for deoxyHrNO. The resonance Raman spectrum of doexyHrNO shows features at 433 and 421 cm-1 that shift downward with 15N16O and that are assigned to stretching and bending modes, respectively, of the {FeNO}7 unit. Sensitivity of the bending mode to D2O suggests that bound NO participates in hydrogen bonding. We propose that the terminal oxygen atom of NO is hydrogen bonded to the proteon of the .mu.-hydroxo bridge in the Fe-(OH)-Fe unit. A bent Fe-N-O geometry is supported by spectroscopic and structural comparisons to synthetic complexes and is consistent with a limiting [FeII,FeIIINO-] formulation for deoxyHrNO. Reversibility of NO binding to deoxyHr is demonstrated by bleaching of the optical and EPR spectra of deoxyHrNO upon additions of excess N3- or CNO-. DeoxyHrNO undergoes autoxidation under anaerobic conditions over the course of several hours. The product of this autoxidation appears to be an EPR-silent NO adduct of semimetHr. The formal one-electron oxidations of the binuclear iron site of deoxyHr by NO and by HNO2 can conceivably occur with no net change in charge on the iron site. In contrast, autoxidation of oxy- to metHr requires a change in net charge on the iron site, which may provide a kinetic barrier.