Antioxidative properties of Martynoside: pulse radiolysis and laser photolysis study.

Free radical reactions of Martynoside (MAR), a phenylpropanoid glycoside, with a variety of oxidants were studied in the aqueous solution by laser photolysis and pulse radiolysis techniques. The pKa value of MAR in aqueous solution was measured from the pH dependent changes of the UV absorption at 384 nm with value of pKa = 9.2. The phenoxyl radical of MAR which exhibits maximum absorption at 360 nm was generated by one-electron transfer to N3* or Br2*-. Other important properties of phenoxyl radical such as extinction coefficient, formation and decay rate constants were also determined. The reaction rate constant of O2*- with MAR, k = 8.5 x 10(4) dm3 x mol(-1) x s(-1), was measured by the method of competition kinetics. By measuring time-resolved luminescence emission at 1270 nm, the quenching rate constant of singlet oxygen by MAR was obtained to be 3.3 x 10(6) dm3 x mol(-1) x s(-1). Reduction potential of the MAR couple (MAR*/MAR), determined using rutin as reference compound, gave a value E = 0.66 V vs. NHE. The antioxidative properties of MAR were compared with those of some well-known antioxidants.