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Abstract
This work has been studied with three major iodine oxidation reactions. (I) The oxidation of N-acetylmethionine methyl ester is catalyzed by carboxylate anions to give sulfoxide and acid anhydride. Complicated dependences for the rate constants involve buffer and iodide concentration. We have suggested a mechanism including two consecutive buffer-mediated steps. The Bronsted plot for the catalytic constant from first-order and second-order region of buffer dependence are 1.0 and 1.5 respectively. (II) The oxidation of N-acetylmethionine is also catalyzed by carboxylate anions. Linear catalysis is observed and a Bronsted coefficient = 1.1 is calculated. The reaction is strongly inhibited by iodide anion. In the uncatalyzed reaction of NAM is 1200-fold larger than its methyl ester(NAME). The effective molarity is only 2 M. It is suggested that results from a rate-limiting process involving a tetracoordinate sulfurane. (III) The oxidation of methionine is catalyzed by general bases from as slope = 1 to slope = 0 at pK(,a) = 3. The nonlinear Bronsted plot was interpreted as evidence for a trapping mechanism. The observed changing iodide dependence at high buffer concentration requires an intermediate is a tetracoordinate sulfurane. The uncatalyzed reaction of methionine shows 21-fold rate acceleration than its methyl ester, suggesting carboxylate anion provides electrostatic stabilization of the transition state to the formation of product. Based on these studies, the (alpha)-amino group of methionine attack the sulfur is preferred simply because the high basicity. The kinetic evidence indicates the presence of a sulfurane in the reaction is required.
