Kinetic Reaction Mechanism of Sinapic Acid Scavenging NO(2) and OH Radicals: A Theoretical Study

The mechanism and kinetics underlying reactions between the naturally-occurring antioxidant sinapic acid (SA) and the very damaging ·NO(2) and ·OH were investigated through the density functional theory (DFT). Two most possible reaction mechanisms were studied: hydrogen atom transfer (HAT) and radical adduct formation (RAF). Different reaction channels of neutral and anionic sinapic acid (SA(-)) scavenging radicals in both atmosphere and water medium were traced independently, and the thermodynamic and kinetic parameters were calculated. We find the most active site of SA/SA(-) scavenging ·NO(2) and ·OH is the –OH group in benzene ring by HAT mechanism, while the RAF mechanism for SA/SA(-) scavenging ·NO(2) seems thermodynamically unfavorable. In water phase, at 298 K, the total rate constants of SA eliminating ·NO(2) and ·OH are 1.30×10(8) and 9.20×10(9) M(-1) S(-1) respectively, indicating that sinapic acid is an efficient scavenger for both ·NO(2) and ·OH.