Hydrogen peroxide scavenging, antioxidant and anti-radical activity of some phenolic acids

Abstract

Some water-soluble phenolic acids were investigated as antioxidants, scavengers of hydrogen peroxide (H₂O₂) and scavengers of 1,1-diphenyl-2-picrylhydrazyl radical (DPPH^•). The strongest antioxidant, scavenging of H₂O₂ and DPPH^• radical activity was exhibited by 3,4,5-trihydroxybenzoic (gallic) acid and 1,2,3-trihydroxybenzene (pyrogallol) with three hydroxyl groups bonded to the aromatic ring in an ortho position in relation to each other. Phenolic acids with two hydroxyl groups bonded to aromatic ring in the ortho position, such as 3,4-dihydroxycinnamic (caffeic), 3,4-dihydroxybenzoic (protocatechuic) and 2,3-dihydroxybenzoic (o-pyrocatechuic) acids, showed strong antioxidant and anti-radical activity; however, it was lower than that of 3,4,5-trihydroxybenzoic acid or 1,2,3-trihydroxybenzene. 3,5-Dihydroxybenzoic (α-resorcylic) and 2,4-dihydroxybenzoic (β-resorcylic) acids with two hydroxyls bonded in the meta position in relation to each other showed moderate antioxidant and low DPPH^• and hydrogen peroxide scavenging activity. Compounds with one hydroxyl group such as 3-hydroxybenzoic, 4-hydroxyphenylacetic and 2-hydroxybenzoic (salicylic) acids, exhibited the lowest anti-radical and antioxidant activity. The results obtained show that the antioxidant and anti-radical activity of phenolic acids correlated positively with the number of hydroxyl groups bonded to the aromatic ring. The model of an ortho substitution of hydroxyl groups to the aromatic ring seems to be adequate for antioxidant and H₂O₂ or DPPH^• scavenging activity of phenolic acids.

Introduction

Excessive concentrations of reactive oxygen species in the human body can be involved in a number of pathological events (Werns and Lucchesi, 1989, Auroma, 1994, Kirkinezos and Moraes, 2001, Lee and Wei, 2001). Hydrogen peroxide (H₂O₂), when present in excess, is one of the many compounds that can be injurious for the cells (Halliwell and Gutteridge, 1999a).

Lipid peroxidation is another process that produces many pathological events in the cells and the organism of man (Halliwell and Gutteridge, 1999b, Noguchi and Niki, 1999, Drueke et al., 2001, Spiteller, 2001). This process, causing damage to unsaturated fatty acids, tends to decrease membrane fluidity and leads to many other pathological events.

The above processes can be effectively quenched by plant phenolics (Bahorun et al., 1996, Periera da Silva et al., 2000, Czinner et al., 2001, Lodovici et al., 2001), among them phenolic acids (Croft, 1998, Morton et al., 2000), which are known to be scavengers of various oxygen species, even as toxic as the HO^• radical and singlet oxygen. The efficiency of phenolic compounds as anti-radicals and antioxidants is diverse and depends on many factors, such as the number of hydroxyl groups bonded to the aromatic ring, the site of bonding and mutual position of hydroxyls in the aromatic ring. For example, Rice-Evans et al., 1996, Joyeux et al., 1995 described the advantageous effect of an o-hydroxyl substitution in the aromatic ring for the anti-radical and antioxidant activity of phenolic acids.

The aim of the present study was to examine and compare the inhibition of lipid peroxidation, scavenging of H₂O₂ and scavenging of 1,1-diphenyl-2-picrylhydrazyl radical (DPPH^•) by some water-soluble phenolic acids with different models of hydroxylation of the aromatic ring.