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

摘要

The 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 (SOsup4?-/sup ) 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 SOsup4?-/sup and protects adenine from it; SOsup4?-/sup competes for adenine as well as for caffeic acid. From competition kinetics, the rate constant of SOsup4?-/sup with caffeic acid has been calculated to be 1.24 ± 0.2 Χ 10sup10/sup molsup-1/supdmsup3/supssup-1/sup. 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 SOsup4?-/sup . The results of experimentally determined quantum yields (φsubexptl/sub) and the quantum yields calculated (φsubcl/sub) by assuming that caffeic acid acts only as a scavenger of SOsup4?-/sup radicals show that φsubexptl/sub values are lower than φsubcl/sub values. The φ' values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SOsup4?-/sup scavenging by caffeic acid, are also found to be greater than φsubexptl/sub values. These observations suggest that the adenine radicals are repaired by caffeic acid, in addition to scavenging of sulphate radical anions.