Catalysis of the reaction between sorbic acid and thiols by surfactants

Abstract

Sorbic acid reacts slowly with simple thiols at 80°C. The reaction involves nucleophilic attack on the sorbic acid molecule by the thiolate anion. Second order rate constants for the reactions of mercaptoethanol, mercaptoacetic acid, cysteine and glutathione are, respectively, 0.86,0.68,2.3 and2.5 M -~ h-' at 80°C andindependent of pH in the range pH 3.7-5.7. The effects of dodecyltrimethylammonium bromide (DoTAB), sodium dodecyl sulphate (SDS), lecithin, datem ester (diacetyltartaric acid ester of CI8 monoglyceride) and Tween 80 on the initial rates of these reactions at pH 5.0 are reported. The greatest catalytic effect was shown by DoTAB, giving a maximum enhancement of rate by a factor of 21.4 for the mercaptoethanol reaction and 4.6 for the glutathione reaction. SDS was much less effective. The nonionic surfactants all behaved as catalysts. The mechanism of the surfactant-catalysed reaction is discussed. Kinetically derived stoichiometries of binding of the solutes in question to DoTAB micelles depend on surfactant concentration, and for the thiols used (mercaptoacetic acid, cysteine and glutathione) it is found that each thiol molecule is associated with at least 10 surfactant molecules making up the micelles for surfactant concentrations greater than or equal to 50 mM. The implications of micellar catalysis to food additive-food component interactions are considered.

Modeling food matrix effects on chemical reactivity: Challenges and perspectives

“…The distribution of reactants between the aqueous and the micellar phase can result in two distinct "reactors" wherein the same reaction occurs at two different rates because of differences in reactant concentrations. An example is the catalysis of the reaction between sorbic acid and thiols by surfactants (Wedzicha and Zeb, 1990) or bovine serum albumin (Wedzicha and Zeb, 1991). Another relevant example is the partitioning of reactants in emulsions.…”