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Measured Rates of Fluoride/Metal Association Correlate with Rates of Superoxide/Metal Reactions for Fe^IIIEDTA(H₂O)^- and Related Complexes

Jack S. Summers,*^† Joseph B. Baker,^† Dan Meyerstein,^‡^§ Amir Mizrahi,^‡ Israel Zilbermann,^‡ Haim Cohen,^§ Christopher M. Wilson,^† and Jamie R. Jones^†

Department of Chemistry and Physics, Western Carolina University, Cullowhee, North Carolina 28723, Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel, Biological Chemistry Department, Ariel University Center of Samaria, Ariel, Israel, and Nuclear Research Centre Negev, Beer-Sheva, Israel

J. Am. Chem. Soc., 2008, 130 (5), pp 1727–1734

DOI: 10.1021/ja077193b

Publication Date (Web): January 11, 2008

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

†

Western Carolina University.

‡

Ben-Gurion University of the Negev.

Ariel University Center of Samaria.

Nuclear Research Centre Negev.

, summers@email.wcu.edu

Abstract

The effects of 10 paramagnetic metal complexes (Fe^IIIEDTA(H₂O)^-, Fe^IIIEDTA(OH)^2-, Fe^IIIPDTA^-, Fe^IIIDTPA^2-, Fe^III₂O(TTHA)^2-, Fe^III(CN)₆^3-, Mn^IIEDTA(H₂O)^2-, Mn^IIPDTA^2-, Mn^IIβ-EDDADP^2-, and Mn^IIPO₄^-) on F^- ion ¹⁹F NMR transverse relaxation rates (R₂ = 1/T₂) were studied in aqueous solutions as a function of temperature. Consistent with efficient relaxation requiring formation of a metal/F^- bond, only the substitution inert complexes Fe^III(CN)₆^3- and Fe^IIIEDTA(OH)^2- had no measured effect on T₂ relaxation of the F^- ¹⁹F resonance. For the remaining eight complexes, kinetic parameters (apparent second-order rate constants and activation enthalpies) for metal/F^- association were determined from the dependence of the observed relaxation enhancements on complex concentration and temperature. Apparent metal/F^- association rate constants for these complexes (k_app,F_^-) spanned 5 orders of magnitude. In addition, we measured the rates at which O₂^•^- reacts with Fe^IIIPDTA^-, Mn^IIEDTA(H₂O)^2-, Mn^IIPDTA^2-, and Mn^IIβ-EDDADP^2- by pulse radiolysis. Although no intermediate is observed during the reduction of Fe^IIIPDTA^- by O₂^•^-, each of the Mn^II complexes reacts with formation of a transient intermediate presumed to form via ligand exchange. These reactivity patterns are consistent with literature precedents for similar complexes. With these data, both k_app,O2_^- and k_app,F_^- are available for each of the eight reactive complexes. A plot of log(k_app,O2_^-) versus log(k_app,F_^-) for these eight showed a linear correlation with a slope ≈ 1. This correlation suggests that rapid metal/O₂^•^- reactions of these complexes occur via an inner-sphere mechanism whereas formation of an intermediate coordination complex limits the overall rate. This hypothesis is also supported by the very low rates at which the substitution inert complexes (Fe^III(CN)₆^3- and Fe^IIIEDTA(OH)^2-) are reduced by O₂^•^-. These results suggest that F^- ¹⁹F NMR relaxation can be used to predict the reactivities of other Fe^III complexes toward reduction by O₂^•^-, a key step in the biological production of reactive oxygen species.