Tez özetleri Astronomi ve Uzay Bilimleri Anabilim Dalı

Kinetic Investigation Of Copper(II)-Catalyzed Oxidative Degradation Of Vitamin C in the Presence of Protective Fruid Acids

Vitamin C (ascorbic acid: AA) is a water-soluble vitamin found in fruits and vegetables, and is a powerful antioxidant capable of quenching free radicals. It also prevents worsening of taste and colour in various foodstuffs and fruits. It can both be derived from natural sources and chemically synthesized. Since humans cannot synthesize vitamin C in their bodies, they have to supply it from nutritional sources. All fresh vegetables, fruits and meat contain certain amounts of vitamin C. Since this vitamin is sensitive to air oxygen and heat, it can be relatively rapidly degraded during unsuitable conditions of cooking and preservation of food. When transition metal ions like Cu(II) are freely incorporated in fruits, their nutritional quality is adversely affected due to transition metal-catalyzed oxidative degradation of AA. In this study, the variation of ascorbic acid stability in the presence of Cu(II) ions was explored, and the effect of Cu(II) complexes formed with protective fruit acids like citric, oxalic, tartaric, malic, malonic and fumaric acids on the autoxidation of AA was investigated. The aim was to protect AA from catalytic oxidation with the help of these fruit acids complexed with Cu(II). The varying concentration of AA as a function of time was followed with the use of Cu(II)-neocuproine (Nc: 2,9-dimethyl-1,10-phenanthroline) spectrophotometric reagent developed in the Analytical Chemistry Laboratories of Istanbul University and previously used in the CUPRAC (CUPpric Reducing Antioxidant Capacity) assay. The kinetic equation for the Cu(II)-catalyzed oxidation at various pH (3.2-4.5) was found to be first-order with respect to AA concentration. The rate constants of AA oxidation at a given pH increased with Cu(II) concentration for a fixed level of organic acid. The effect of the concentrations of organic acid and Cu(II) on the autoxidation of AA can be interpreted with the plausible formation of a ternary transition complex formed between monohydrogen ascorbate and Cu(II)-organic acid chelate. When different combinations of Cu(II) and organic acids were selected, it was observed that as the organic acid concentration was increased, the Cu(II)-catalyzed autoxidation rate and the corresponding specific rate constants decreased. Since the catalytic oxidation of AA was inhibited in the presence of stable Cu(II)-organic acid complexes, the highest conditional stability constant was encountered in the citric acid complex of Cu(II), resulting in a stronger inhibition with this complex. Since the tested organic fruit acids had a stronger complexation ability for AA than acetate buffer, they were found to better protect AA than acetic acid in the presence of Cu(II). The catalytic activity of Cu(II)-ligand complexes decreases as complex stability increases. The presence of fruit acids in foodstuffs helps to protect nutritional ascorbic acid values by preventing transition metal-catalyzed oxidation reactions.