ASSEMBLY AND CHARACTERIZATION OF AN ACCURATE MODEL FOR THE DIIRON CENTER IN HEMERYTHRIN

Article abstract of DOI:10.1021/ja00324a041

Using either simple ferric salts or the preassembled (μ-oxo)diiron(III) anion, (Cl3FeOFeCl3)^2-, bridged binuclear complexes ((HBpz3)FeO(O2CR)2Fe(HBpz3)), R=H, CH3, C6H5 and HBpz3=tri-1-pyrazolylborate ion, were prepared.X-ray crystallographic studies of the acetate and formate derivatives revealed a structure in which the two iron atoms are linked by a μ-oxo and two μ-carboxylato bridging ligands in a bioctahedral geometry in which the termini are capped by the tridentate tri-1-pyrazolylborate ligands.The geometry of the (Fe2O(O2CR)2) core of these compounds is nearly congruent with that found in the azidomet forms of the marine invertebrate oxygen transport proteins, hemerythrin and myohemerythrin.The (μ-oxo)diiron(III) center is bent, with an average Fe-O bond length of 1.783(3) Angstroem and an average Fe-O-Fe angle of 124.6(1) degree.The vibrational modes of this center have been characterized by extensive Raman, resonance Raman, and Fourier transform infrared (FTIR) spectroscopic studies of the model complexes and their 18O analogues.The latter were prepared by exchange with 18OH2, which is a facile reaction.The asymmetric stretch (ν_as) occurs at 751 cm^-1 and is relatively weak.It underlies a strong HBpz3^- ligand band and shifts under another such band (to 721 cm^-1) in the 18O derivative.This behavior was mapped by difference FTIR spectroscopy.The symmetric stretch and deformation modes occur at 528 and 278 cm^-1, respectively.These vibrational features as well as the magnetic exchange properties, ligand field bands in the optical spectrum (695 nm, ε 70 M_Fe^-1 cm^-1; ca.990 nm, ε 3.5 M_Fe^-1 cm^-1), and Moessbauer spectroscopic parameters (δ=0.52+/-0.03 mm/s; ΔE_Q=1.55+/-0.05 mm/s) are compared to those reported for met- and oxyhemerythrin as well as other proteins believed to contain (μ-oxo)diiron(III) cores.Proton NMR investigations of (Fe2O(O2CCH3)2(HBpz3)2) and its deuterated analogues are reported in which the paramagnetically shifted acetate group methyl proton resonances are identified at -10.5 ppm.This assignment should facilitate location of the analogous resonance in the proteins.Cyclic voltammetric studies of (Fe2O(O2CCH3)2(HBpz3)2) revealed an irreversible reduction accompanied by formation of mononuclear (Fe(HBpz3)2), which itself undergoes a reversible one-electron oxidation.The instability of the (μ-oxo)diiron(II) unit in the model compound parallels that of the reduced (deoxy) form of the protein.Magnetic susceptibility measurements on a SQUID susceptometer over the range 2.9a spin exchange coupling constant J=-121 cm^-1, a value close to that (-134 cm^-1) of methemerythrin.