Pulse radiolysis studies on redox reactions of gallic acid: one electron oxidation of gallic acid by gallic acid–OH adduct

Abstract

Using the pulse radiolysis technique, studies on reactions of 3,4,5-trihydroxybenzoic acid [gallic acid (GA)] with radical species generated in water are reported. At pH 6.8 and 9.7, OH radicals react with GA to give an adduct initially with rate constants of the order of 1 ] 1010 dm3 mol~1 s~1. This adduct then reacts with parent GA molecules with rate constants of the order 5 ] 108 dm3 mol~1 s~1 to give phenoxyl type radical species having absorption maxima in the 350 nm region. At pH 12 and 13.6, OH/O~radicals directly bring about oxidation of GA. SpeciÐc oxidants like azidyl radical bring about one electron oxidation at neutral and alkaline pHs with almost di †usion controlled rate constants. Rate constants for the reaction of radicals Br 2 ãre found to be lower than those for OH radicals by an order of magnitude. At pH 0, both OH and Cl 2 radicals react with GA to give phenoxyl type radicals. The phenoxyl radicals formed are quite stable at higher pHs, which make GA a good antioxidant. Rate constants for the reactions of with di †erent protolytic e aq forms of GA are determined.

Antioxidant, Cytotoxic Activities, and Structure–Activity Relationship of Gallic Acid‐based Indole Derivatives

“…Among polyphenols, gallic acid (3,4,5-trihydroxybenzoic acid) derivatives are a well-known group of naturally occurring compounds which have been found in many phytomedicines. In recent years, a number of their biological and pharmacological activities have been described resulting in much attention in the development of synthetic gallic acid derivatives [3][4][5][6][7][8][9][10][11]. These compounds have been proved to have various biological properties including neuroprotective, antioxidant [12] and anticancer activities [13].…”

Gallic Acid: A Promising Lead Molecule for Drug Development

“…The alkyl esters of gallic acid have been reported to possess anticancer, antioxidant ability and neuroprotective effect [10], scavenging free radicals [11], inducing apoptosis of cancer cells [12], inhibiting squalene epoxidase [13], interfering the signal pathways involving extract of dried flowers of Nymphaea stellata and Eucalyptus hybrida by HPTLC [30]. Another study showed use of RP-HPLC method for quantification of GA in Symplocos racemose [31].…”

Laser Flash Photolysis Studies on Gallic Acid

“…The formed cation radical GA •＋ (pK a ＝5.0) turned into neutral radical by deprotonation under our experimental condition. According to the report in literature, 15 the species with absorption at about λ max 310 nm was assigned to the neutral radical arising from deprotonation of cation radical. …”

“…15 Richards and Adams 16 have shown that GA is able to repair the tryptophan radical to produce phenoxyl radical. Recently, it was reported that although GA is a free radical scavenger, it may exhibit prooxidant properties under certain conditions.…”