Exchange coupling constant J of peroxodiferric reaction intermediates determined by Mössbauer spectroscopy

Oxygen activation at a carboxylate-bridged diiron cluster is employed by a number of enzymes for diverse biological functions. The mechanisms by which O₂ is activated at the diferrous clusters have been studied in detail and peroxodiferric reaction intermediates have been observed in several of these diiron proteins. To understand further the magnetic properties of this common reaction intermediate, we have used Mössbauer spectroscopy to determine the magnitude and sign of the exchange coupling constant J (in the exchange Hamiltonian J S₂ S₂) of the peroxodiferric intermediates generated during the reactions of O₂ with two different proteins, the recombinant M ferritin from frog and the site-directed variant W48F/D84E of the R2 subunit of ribonucleotide reductase from Escherichia coli. Both intermediates are antiferromagnetically coupled with a moderate coupling constant J of 50 ± 10 cm^-1 for R2-W48F/D84E and 75 ± 10 cm^-1 for M ferritin. This work demonstrates the capability of Mössbauer spectroscopy to determine exchange coupling constants of diiron complexes, including reaction intermediates. The approach and its limitations are described.