Kinetics and mechanisms of oxidation of some antioxidants with photochemically generated tert-butoxyl radicals.

Dietary antioxidants play an important role in the prevention of several chronic diseases including cardiovascular diseases, cancer, ageing and diabetes. In order to understand the mechanism of oxidation of antioxidants viz., gallic acid (GA), caffeic acid (CA), rosmarinic acid (RA) and chlorogenic acid (CGA), a systematic kinetic study of these antioxidants with photochemically generated tertiary butoxyl (t-BuO) radicals was carried out. The oxidation of antioxidants by t-BuO radicals was followed spectrophotometrically by measuring the absorbance of GA (266 nm), CA (310 nm), RA (324 nm) and CGA (328 nm) at their respective lambda(max). The initial rates of oxidation of antioxidants were calculated from the plot of absorbance vs time and were found to increase with increase in [antioxidant], [t-BuOOH] and light intensity in all the cases. The quantum yields (phi) were calculated from the initial rates of oxidation of antioxidant and the measured light intensity at 254 nm, the wavelength at which the tert-butyl hydroperoxide (t-BuOOH) was activated to radicals. The quantum yields were found to depend on [antioxidant] and [t-BuOOH], and were independent of light intensity. The order with respect to [antioxidant], [t-BuOOH] were found to be fractional whereas order with respect to intensity was one. The order of reactivity was found to be: CA > CGA > RA > GA. The products were identified by mass spectral data. On the basis of kinetic results and product analysis, probable mechanisms were suggested.