EPR properties of mixed-valent μ-oxo and μ-hydroxo dinuclear iron complexes produced by radiolytic reduction at 77?K |
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Authors: | R M Davydov Joanne Smieja S A Dikanov Y Zang Lawrence Que Jr M K Bowman |
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Institution: | (1) Macromolecular Structure and Dynamics, WR Wiley Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352-0999, USA e-mail: michael.bowman@pnl.gov, Tel: +1-509-3763299 Fax: +1-509-3762303, US;(2) Institute of Chemical Physics, Russian Academy of Sciences Moscow 117977, Russia, RU;(3) Department of Chemistry, Gonzaga University, Spokane WA 99258, USA, US;(4) Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk 630090, Russia, RU;(5) Department of Chemistry, University of Minnesota Minneapolis, MN 55455, USA, US |
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Abstract: | 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.
Received: 27 June 1998 / Accepted: 25 February 1999 |
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Keywords: | Mixed-valent species Dinuclear iron EPR Radiolytic reduction |
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