Structure/function correlations over binuclear non heme iron active sites
Binuclear non heme iron enzymes activate O2 to perform diverse chemistries. Three different structural mechanisms of O2 binding to a coupled binuclear iron site have been identified utilizing variable temperature, variable field magnetic circular dichroism spectroscopy (VTVH MCD) For the m OH bridged Fe(II)2 site in hemerythrin, O2 binds terminally to a five coordinate Fe(II) center as hydroperoxide with the proton deriving from the m OH bridge and the second electron transferring through the resulting m oxo superexchange pathway from the second coordinatively saturated Fe(II) center in a proton coupled electron transfer process. For carboxylate only bridged Fe(II)2 sites, O2 binding as a bridged peroxide requires both Fe(II) centers to be coordinatively unsaturated and has good frontier orbital overlap with the two orthogonal O2 p* orbitals to form peroxo bridged Fe(III)2 intermediates. Alternatively, carboxylate only bridged Fe(II)2 sites with only a single open coordination position on an Fe(II) enable the one electron formation of Fe(III) O2 or Fe(III) NO species. Finally, for the peroxo bridged Fe(III)2 intermediates, further activation is necessary for their reactivities in one electron reduction and electrophilic aromatic substitution, and a strategy consistent with existing spectral data is discussed.
JBIC Journal of Biological Inorganic Chemistry