Antioxidant properties of monomeric, oligomeric, and polymeric fractions in hot water extract from Pinus radiata bark

Abstract

Hot water extract (HWE) from Pinus radiata bark was fractionated into monomeric polyphenol (MPP), oligomeric proanthocyanidin (OPA), and polymeric proanthocyanidin (PPA) fractions by monitoring the UV-visible spectrum of the eluted fractions. Nine polyphenols were identified in MPP: three phenolic acids and six flavonoids. The major compounds of MPP were taxifolin (456 μg/mg), catechins (240 μg/mg), and protocatechuic acid (46.2 μg/mg). The OPA components ranged from dimers to hexamers. PPA comprised procyanidin (PC, 94%), a small amount of prodelphinidin (PD, 6%), and also the higher trans configuration, rather than the cis configuration, of C2-C3. The average molecular weight (Mw, Mn), polydispersity (Mw/Mn), and DP of PPA were 3,800 (Mw), 1,200 (Mn), 3.2 and 13, respectively. The potential antioxidant activities of HWE and the three fractions were estimated using proton- or electron-donating assays containing 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical, 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) radical cation, superoxide anion (O2•−), hydrogen peroxide (H2O2), reducing power and lipid peroxidation. All the bark fractions exhibited potent quenching abilities against both commercially available radicals (DPPH•, ABTS•+) and chemically induced radicals (H2O2, O2•−), as well as strong reducing power and inhibitory activity of lipid peroxidation. Particularly, OPA showed a hydrogen peroxide scavenging activity significantly higher (p < 0.05) than that of the other fractions. From 2-thiobarbituric acid reactive substances and ferric thiocyanate assays, PPA was suspected to act by essentially delaying the formation of hydroperoxide by quenching free oxygen radicals that operates in oil-in-water emulsion.