Antioxidative properties of vanillic acid esters in multiple antioxidant assays

The antioxidative properties of vanillic acid esters were systematically evaluated by multiple assays to compare with the well-known antioxidants, vanillic acid and Trolox. We first performed assays with the model radicals, DPPH, galvinoxyl and ABTS cation (ABTS(•+)) types. Methyl vanillate, ethyl vanillate and butyl vanillate showed stronger activity than Trolox in the ABTS(•+)-scavenging assay, but showed no activity in the DPPH radical- and galvinoxyl radical-scavenging assays. In contrast, vanillic acid could quench the three radicals. We then evaluated their antioxidative activities by an ORAC assay and an oxidative hemolysis inhibition assay (OxHLIA), using physiologically relevant peroxyl radicals. Vanillic acid esters and vanillic acid exerted much stronger activity than Trolox in the ORAC assay and OxHLIA. The antioxidative activity by OxHLIA was strongly correlated to the lipophilicity of vanillic acid and its esters. These results indicate that the protective effect of vanillic acid esters against free radical-induced biomembrane damage increased with increasing lipophilicity.