Kinetics and mechanism of protection of adenine from sulphate radical anion by caffeic acid under anoxic conditions

27-30The rates of photo-oxidation of adenine in the presence of peroxydisulphate (PDS) have been determined by measuring the absorbance of adenine at 260.5 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of adenine by sulphate radical anion (SO4•- ) have been determined in the presence of different concentrations of caffeic acid. Increase in the concentration of caffeic acid is found to decrease the rate of oxidation of adenine suggesting that caffeic acid acts as an efficient scavenger of SO4•- and protects adenine from it; SO4•- competes for adenine as well as for caffeic acid. From competition kinetics, the rate constant of SO4•- with caffeic acid has been calculated to be 1.24 ± 0.2 Χ 1010 mol-1dm3s-1. The quantum yields of photo-oxidation of adenine have been calculated from the rates of oxidation of adenine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to SO4•- . The results of experimentally determined quantum yields (φexptl) and the quantum yields calculated (φcl) by assuming that caffeic acid acts only as a scavenger of SO4•- radicals show that φexptl values are lower than φcl values. The φ' values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SO4•- scavenging by caffeic acid, are also found to be greater than φexptl values. These observations suggest that the adenine radicals are repaired by caffeic acid, in addition to scavenging of sulphate radical anions

Photooxidation of substituted purines in presence of peroxydisulphate in aqueous solution

2096-2100Photooxication of purine bases, viz .. caffeine, theobromine, theophylline, xanthine, hypoxanthine, adenine and guanine in aqueous solution has been carried out in presence of peroxydisulphate (PDS). Peroxydisulphate is activated to SO4- at 254 nm. The reactions are followed by measuring the absorbance of purine bases at their respective λmax. The rates of reactions are calculated under different experimental conditions. The light intensity is measured using peroxydisulphate solution as standard chemical actinometer. Using reaction rate and light intensity at 254 nm, the quantum yields are calculated. The rates of photooxidation of purine bases are found to increase with increase in [PDS] and independent of [purine]. The increase of light intensity has been found to increase the rate of oxidation. The quantum yields are found to depend on [purine] but independent of [PDS] and light intensity. On the basis of experimental results a probable mechanism is suggested in which peroxydisulphate on photolysis gives sulphate radical anion which initiates the reaction by capturing an electro n from C8 position of purine to form purine radical cation. This radical cation deprotonates and undergoes further oxidation to give C(8) hydroxy purine

Kinetics and mechanism of oxidation of vitamin E by sulphate radical anion in aqueous acetonitrile solution

303-306The photooxidation of vitamin E (α-tocopherol) in the presence of peroxydisulphate (PDS) in aqueous solution at natural pH (7.5) has been carried out in a quantum yield reactor using a high pressure mercury vapour lamp. The rate of photo oxidation increases with increase in [vitamin E], [PDS] and light intensity. The quantum yield increases with increase in [vitamin E] and is independent of [PDS] and light intensity. The plots of log (rate) versus log [PDS] and the log (rate) versus log [vitamin E] are linear with a slope of less than one indicating fractional order dependence of rate on [PDS] as well as on [vitamin E]. On the basis of experimental results and product analysis a probable mechanism is suggested