Pulse radiolysis studies of 3,5-dimethyl pyrazole derivatives of selenoethers

One electron redox reaction of two asymmetric 3,5-dimethyl pyrazole derivatives of selenoethers attached to ethanoic acid (DPSeEA) and propionic acid (DPSePA) were studied by pulse radiolysis technique using transient absorption detection. The reaction of the hydroxyl ((•)OH) radical with DPSeEA or DPSePA at pH 7 produced transients absorbing at 500 nm and at 300 nm, respectively. The absorbance at 500 nm increased with increasing parent concentration indicating formation of dimer radical cations. From the absorbance changes, the equilibrium constants for the formation of dimer radical cation of DPSeEA and DPSePA were estimated as 2020 and 1608 M(-1), respectively. The rate constants at pH 7 for the reaction of the (•)OH radical with DPSeEA and DPSePA were determined to be 9.6 × 10(9) and 1.4 × 10(10) M(-1) s(-1), respectively. The dimer radical cation of DPSeEA and DPSePA decayed by first order kinetics with a rate constant of 2.8 × 10(4) and 5.5 × 10(3) s(-1), respectively. The yield of radical cations of DPSeEA and DPSePA were estimated from the secondary electron transfer reaction, which corresponds to 38% and 48% of (•)OH radical yield, respectively. Some fraction of monomer radical cation undergoes decarboxylation reaction, and the yield of decarboxylation was 25% and 20% for DPSeEA and DPSePA, respectively. These results have implication in understanding their antioxidant activity. The reaction of trichloromethyl peroxyl radical, glutathione, and ascorbic acid further support their antioxidant behavior.