Kinetic Reaction Mechanism of Sinapic Acid Scavenging NO2 and OH Radicals: A Theoretical Study.

Yang Lu,Hui Zhang,Aihua Wang,Peng Shi,Zesheng Li,Dennis Salahub

doi:10.1371/journal.pone.0162729

Yang Lu, Hui Zhang + Show 4 more

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https://doi.org/10.1371/journal.pone.0162729

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Abstract

The mechanism and kinetics underlying reactions between the naturally-occurring antioxidant sinapic acid (SA) and the very damaging ·NO2 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 ·NO2 and ·OH is the –OH group in benzene ring by HAT mechanism, while the RAF mechanism for SA/SA- scavenging ·NO2 seems thermodynamically unfavorable. In water phase, at 298 K, the total rate constants of SA eliminating ·NO2 and ·OH are 1.30×108 and 9.20×109 M-1 S-1 respectively, indicating that sinapic acid is an efficient scavenger for both ·NO2 and ·OH.

Highlights

Sinapic acid (SA, 3,5-dimethoxy-4-hydroxycinnamic acid) is a naturally-occurring and widespread phenolic acid in the plant kingdom, and can be obtained from various fruits and vegetables such as rye [1], orange, grapefruit, and cranberry [2]
sinapic acid (SA) has an approximately planar structure in which the dihedral angle between benzene ring and carbonyl group is about 179.56°
The optimized geometries of transition states (TSs) and product complexes (PCs) of all channels are shown in Figs 2 and 3

Summary

Introduction

Sinapic acid (SA, 3,5-dimethoxy-4-hydroxycinnamic acid) is a naturally-occurring and widespread phenolic acid in the plant kingdom, and can be obtained from various fruits and vegetables such as rye [1], orange, grapefruit, and cranberry [2]. Hydroxycinnamic acid is a important group of phenolic acids, mainly including caffeic acid, ferulic acid, p-coumaric acid, and sinapic acid, which exist in daily foods such as grape, citrus [8], pear, tomato, and small radish [2]. Antioxidant activities of hydroxycinnamic acids have been studied extensively [9,10,11,12]. SA is suggested as a more efficient antioxidant [13] and superoxide radical scavenger [14] than p-coumaric acid and ferulic acid.

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