Spectrophotometric investigation of the catalytic effects of certain copper(II) chelates on the oxidation of ascorbic acid

Vitamin C (ascorbic acid:AA) is an important water soluble antioxidant vitamin in human diet which participate in many biochemical reactions. Copper is trace metal found in many food and pharmaceutical products. It is very important to preserve the antioxidant values of food, that may contain naturel or added vitamin C. Free transtion metal like Cu(II), when reduced with ascorbate, may give Fenton-type reactions generating hidroxyl redicals that adversely affect the nutritional quality of food and cause tissue damage in consumers. Catalytic oxidation of AA in the presence of some amino acids cannot be followed by direct UV spectrophotometry because of some interferences, and therefore, spectrophotometric determination of remaning AA in solution was performed with the copper(II)-neocuproine reagent in amonium acetatebuffered medium via recording of absorbance at 450 nm. The kinetics of uncatalyzed and Cu(II)-ion catalyzed oxidation of AA in aereted and acetate-buffered eight amino acids with different chemical structure (alanine, glycine, aspartic acid, asparagine, glutamic acid, glutamine, phenylalanine and histidine) solutions was studied at 25°C and ionic strenght: I=0,1 (KNO3). The rate equation of oxidation in presence of cupric ion as catalyzed at pH 4,5 was found to be first order with respect to AA. AA oxidation rate constants decreased with increasing amino acid concantration for a fixed Cu(II), and increased with increasing Cu(II) concantration for a fixed amino acid level. The effects of amino acids and cupric ion concantrations on the oxidation rate of AA suggest a mechanism which may involve the formation of transition complex between mono hidrogen ascorbate and copper ion amino acid chelates in the form of a ternary complex. Stability of copper amino acid complex inhibited the catalytic activity of copper in AA oxidation. This finding supports previously reached conclusion. Since foods widely contain amino acids, it may help to protect their AA values by retarding copper catalyzed oxidation. Key words: Ascorbic acid, amino acid, antioxidants, catalysed oxidation, Cu(II), kinetics, food stability.