Antioxidant phenolic compounds from walnut kernels (Juglans regia L.)

An activity-directed fractionation and purification process was used to isolate 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging components from Juglans regia kernels. Ethyl acetate and n-butanol fractions showed greater DPPH scavenging activities compared to those of water and petroleum ether fractions. The ethyl acetate fraction was subjected to purification using column chromatography. Seven phenolic compounds, pyrogallol (1), p-hydroxybenzoic acid (2), vanillic acid (3), ethyl gallate (4), protocatechuic acid (5), gallic acid (6) and 3,4,8,9,10-pentahydroxydibenzo[b,d]pyran-6-one (7), containing significant antioxidant activities were isolated and identified in J. regia by spectroscopic methods for the first time in this study. The relative order of DPPH scavenging capacity for these compounds was 7 > 6 ⩾ 4 ⩾ 1 > Trolox ⩾ 5 > 3 > 2. The results of this study suggested that the antioxidant activities of these phenolic compounds may be influenced by the number of hydroxyls in their aromatic rings.