Kinetic study of copper(II)- and iron(III) catalyzed oxidation of l-ascorbic acid in aspartame containing aerated solution

It is very important to preserve the antioxidant values of fruit juices and nectars in commercialbeverages that may contain natural or added vitamin C (ascorbic acid: AA) and syntheticsweeteners like aspartame (APM). Free transition metal ions like Cu(II) and Fe(III), whenreduced with ascorbate, may give Fenton-type reactions generating hydroxyl radicals thatadversely affect the nutritional quality of beverages and cause tissue damage in consumers.Catalytic oxidation of AA in the presence of APM cannot be followed by direct UVspectrophotometry because of the interfering band of APM in the UV spectral region, andtherefore, spectrophotometric determination of remaining ascorbic acid in solution wasperformed with the copper(II)-neocuproine reagent in ammonium acetate-buffered mediumvia recording of absorbance at 450 nm. The kinetics of uncatalyzed and either Cu(II)- orFe(III) ion- catalyzed oxidation of AA in aerated and acetate buffered aspartame solutionswas studied at 25 oC and ionic strength: I=0.1(KNO3). The rate equation of oxidation in thepresence of cupric and ferric ion as catalyst at various pH levels ( 3,2-5,6) was found to befirst order with respect to AA. At a given pH, AA oxidation rate constants decreased withincreasing APM concentration for a fixed Cu(II), and increased with increasing Cu(II) orFe(III) concentration for a fixed APM level. The effects of aspartame and cupric or ferric ionconcentration on the oxidation rate of AA suggest a mechanism which may involve theformation of a transition complex between monohydrogen ascorbate and metal ion- aspartamechelates in the form of a ternary complex. The differences in the reaction rates observed withCu(II) or Fe(III) ions were due to the stronger metal-complexing ability of aspartamecompared to that of the buffering acetate. Free aquated Cu2+ ion and the weak Cu(II)-acetatecomplex were shown to catalyze AA oxidation to a greater extent than the relatively stableCu(II)-aspartame complex. This finding supports the previously reached conclusion that asthe stability of a Cu(II) complex increases, its catalytic activity as a mediator of electron-transfer in AA oxidation decreases. Since APM is a widely used sweetener in beverages, itmay help to protect their AA values by retarding metal-catalyzed oxidation.Key words: Ascorbic acid, catalyzed oxidation, Cu(II), Fe(III), aspartame.JURY:1. Prof. Dr. İnci SÖNMEZOĞLU (Supervisor) Date: 08.06.20062. Doç. Dr. Filiz İMER Page: 1103. Prof. Dr. Reşat APAK