EPR properties of mixed-valent #mu#-oxo and #mu#-hydroxo dinuclear iron complexes produced by radiolytic reduction at 77 K

摘要

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 approx 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.