Date of Award

1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

William A. Pryor

Abstract

The reaction of nitric oxide (NO) with superoxide forms peroxynitrite, an extremely potent oxidant. Peroxynitrite can be synthesized by the ozonation of the azide ion. This preparation of peroxynitrite is essentially free of hydrogen peroxide and does not contain excess alkali. Peroxynitrites synthesized by independent methods contain very different impurities, but give the same results toward the oxidation of methionine (Met), dimethylsulfoxide, and the nitration of phenols. The oxidations of Met and its analog, 2-keto-4-thiomethylbutanoic acid (KTBA), by peroxynitrite proceed by two competing mechanisms. The first is a non-radical, bimolecular reaction yielding methionine sulfoxide (MetS=O). In the second mechanism, methionine or KTBA reacts with peroxynitrite by a one-electron transfer (ET) reaction that produces ethylene; this reaction is first order in peroxynitrite and zero order in Met. Both ceric and peroxydisulfate ions also oxidize KTBA to ethylene, supporting an ET mechanism. Neither the yields of MetS=O nor the yields of ethylene are affected by several hydroxyl radical scavengers, suggesting that a unimolecular homolysis of peroxynitrite to form the hydroxyl radical is not involved in these reactions. We propose that the reaction of methionine with peroxynitrite to form ethylene involves an excited intermediate of peroxynitrite that has a reactivity similar to the hydroxyl radical. Both peroxynitrous acid and its anion react with methionine to form MetS=O; the second-order rate constants at 25$\sp\circ$C are 2060 $\pm$ 120 and 0.20 $\rm M\sp{-1}s\sp{-1}$, respectively. The first-order rate constants for the decomposition of peroxynitrous acid to give nitric acid is $1.38 \pm 0.02\ {\rm s}\sp{-1}.$ This rate is sufficiently slow so that methionine can trap peroxynitrous acid before it rearranges to nitric acid. The activation parameters for the bimolecular reaction of peroxynitrite with methionine to form MetS=O are: $\Delta{\rm H}\sp\ddagger = 7.6 \pm 0.2$ kcal/mol and $\Delta{\rm S}\sp\ddagger = -19.8 \pm 1$ cal/mol.K at pH 4.7, and $\Delta{\rm H}\sp\ddagger = 7.7 \pm 0.4$ kcal/mol and $\Delta{\rm S}\sp\ddagger = -22.7 \pm 1.3$ cal/mol.K at pH 7.4; and the activation parameters for the unimolecular decomposition of peroxynitrous acid to form nitric acid are: $\Delta{\rm H}\sp\ddagger = 18.1 \pm 0.7$ kcal/mol and $\Delta{\rm S}\sp\ddagger = 1.4 \pm 2$ cal/mol.K at pH 4.7, and $\Delta{\rm H}\sp\ddagger = 17.4 \pm 0.5$ kcal/mol and $\Delta{\rm S}\sp\ddagger = 0.9 \pm 1.7$ cal/mol.K at pH 7.4.

Pages

154

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