Mössbauer Characterization of an Unusual High-Spin Side-On Peroxo−Fe3+ Species in the Active Site of Superoxide Reductase from Desulfoarculus baarsii. Density Functional Calculations on Related Models†

Superoxide reductase (SOR) is an Fe protein that catalyzes the reduction of superoxide to give H2O2. Recently, the mutation of the Glu47 residue into alanine (E47A) in the active site of SOR from Desulfoarculus baarsii has allowed the stabilization of an iron−peroxo species when quickly reacted with H2O2 [Mathe et al. (2002) J. Am. Chem. Soc. 124, 4966−4967]. To further investigate this non-heme peroxo−iron species, we have carried out a Mossbauer study of the 57Fe-enriched E47A SOR from D. baarsii reacted quickly with H2O2. Considering the Mossbauer data, we conclude, in conjunction with the other spectroscopic data available and with the results of density functional calculations on related models, that this species corresponds to a high-spin side-on peroxo−Fe3+ complex. This is one of the first examples of such a species in a biological system for which Mossbauer parameters are now available: δ/Fe = 0.54 (1) mm/s, ΔEQ = −0.80 (5) mm/s, and the asymmetry parameter η = 0.60 (5) mm/s. The Mossbauer and sp...