Theoretical and Experimental Investigation on the Oxidation of Gallic Acid by Sulfate Radical Anions

Abstract

By monitoring the decay of SO4*- after flash photolysis of aqueous solutions of S2O82- at different pH values, the kinetics of the reaction of SO4*- radicals with gallic acid and the gallate ion was investigated. The bimolecular rate constants for the reactions of the sulfate radicals with gallic acid and the gallate ion were found to be (6.3 +/- 0.7) x 10(8) and (2.9 +/- 0.2) x 10(9) M(-1) s(-1), respectively. On the basis of the oxygen-independent second-order decay kinetics and on their absorption spectra, the organic radicals formed as intermediates of these reactions were assigned to the corresponding phenoxyl radicals. DFT calculations in the gas phase and aqueous solution support formation of the phenoxyl radicals by H abstraction from the phenols to the sulfate radical anion. The observed recombination of the phenoxyl radicals of gallic acid to yield substituted biphenyls and quinones is also supported by the calculations. HPLC/MS product analysis showed formation of one of the predicted quinones.

A kinetic study of the reactions of sulfate and dihydrogen phosphate radicals with epicatechin, epicatechingallate, and epigalocatechingallate

“…4b; see the following text). Phenoxyl radicals of polyphenols are obtained by the reaction of one-electron oxidants, such as the azidyl [14], carbonate [19], and sulfate radicals [13,24], with these substrates.…”

“…Since the reactions of inorganic radicals with organic molecules important for biological processes are a relevant domain of research, the reaction of sulfate and phosphate radical anions with such molecules has been investigated in the literature [12][13][14]. In an attempt to make a contribution to the free radical chemistry of EC, ECG, and EGCG, in this study we investigated the reactions of sulfate and dihydrogen phosphate radicals with these flavonoids.…”

Radiolytic degradation of gallic acid and its derivatives in aqueous solution

“…• , SO 4 •− and B 2 •− exhibits a maximum around 310 nm and a shoulder at ∼400 nm [12][13][14]. The spectrum of the phenoxyl radical formed from neutral MGA is similar to that of the GA.…”

Isolation of Major Active Antibacterial Compounds of Sumac Fruit (Rhus coriaria L.)

“…Previously, Italian scientists had shown that compound 1 can be produced by in vitro radical oxidation of gallic acid. 18 They used sulfate radical anions to promote the reaction in laboratory conditions (Figure 3). It is probable that the compound 1 has been produced via oxidation reaction in a biological environment.…”