Non-Kramers ENDOR and ESEEM of the S = 2 Ferrous Ion of [Fe(II)EDTA] |
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Authors: | Ruitian Song Peter E Doan Ryszard J Gurbiel Bradley E Sturgeon Brian M Hoffman |
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Institution: | Department of Chemistry, Northwestern University, Evanston, Illinois, 60208 |
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Abstract: | We report here the first non-Kramers (NK) ESEEM and ENDOR study of a mononuclear NK center, presenting extensive parallel-mode ESEEM and ENDOR measurements on the St = 2 ferrous center of Fe(II)ethylenediamine-N,N,N′,N′-tetraacetato]2−; Fe(II)EDTA)]2−. The results disclose an anomalous equivalence of the experimental patterns produced by the two techniques. A simple theoretical treatment of the frequency-domain patterns expected for NK-ESEEM and NK-ENDOR rationalizes this correspondence and further suggests that the very observation of NK-ENDOR is the result of an unprecedentedly large hyperfine enhancement effect. The mixed nitrogen–carboxylato oxygen coordination of Fe(II)EDTA]2− models that of the protein-bound diiron centers, although with a higher coordination number. Analysis of the NK-ESEEM measurements yields the quadrupole parameters for the 14N ligands of Fe(II)EDTA]2−, K = 1.16(1) MHz, 0 ≤ η ≤ 0.05, and the analysis indicates that the electronic zero-field splitting tetragonal axis lies along the N–N direction. |
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Keywords: | ENDOR electron-nuclear double resonance ESEEM electron spin-echo envelope modulation non-Kramers [Fe(II)EDTA]2− |
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