Radiolytic reduction at 77 K of oxo/hydroxo-bridged dinuclear iron(III) complexes in frozen solutions forms kinetically stabilized, mixed-valent species in high yields that model the mixed-valent sites of non-heme, diiron proteins. The mixed-valent species trapped at 77 K retain ligation geometry similar to the initial diferric clusters. The shapes of the mixed-valent EPR signals depend strongly on the bridging ligands. Spectra of the Fe(II)OFe(III) species reveal an S = 1/2 ground state with small g-anisotropy as characterized by the uniaxial component (g(z)-g(av)/2<0.03) observable at temperatures as high as ~100 K. In contrast, hydroxo-bridged mixed-valent species are characterized by large g-anisotropy (g(z)-g(av)/2>0.03) and are observable only below 30 K. Annealing at higher temperatures causes structural relaxation and changes in the EPR characteristics. EPR spectral properties allow the oxo- and hydroxo-bridged, mixed-valent diiron centers to be distinguished from each other and can help characterize the structure of mixed-valent centers in proteins.
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Acknowledgements This work was sponsored by the U.S. Department of Energy under contract DE-AC06-76RLO 1830, by Associated Western Universities, Inc. Northwest Division (AWU NW) under Grant DE-FG06-89ER-75522 or DE-FG06-92RL-12451 with the U.S. Department of Energy, and by the NIH through Grant GM-38767. R.M.D. is grateful for participation in the SABIT program. J.A.S. and S.A.D. acknowledge the receipt of Faculty Fellowships from the AWU NW.
- Dinuclear iron
- Mixed-valent species
- Radiolytic reduction