Antioxidant and free radical scavenging activity of flavonol glycosides from different Aconitum species

Bioassay-guided fractionation by 1,1-diphenyl-2-dipicrylhydrazyl (DPPH) radical scavenging test of polar extracts of some Italian Aconitum species (A. napellus subsp. tauricum, A. napellus subsp. neomontanum, A. paniculatum, A. vulparia) led to the isolation of 13 flavonol glycosides: quercetin 3-O-(6-trans-caffeoyl)-β-glucopyranosyl-(1→2)-β-glucopyranoside-7-O-greek small letter alpha-rhamnopyranoside (1), kaempferol 3-O-(6-trans-caffeoyl)-β-glucopyranosyl-(1→2)-β-glucopyranoside-7-O-greek small letter alpha-rhamnopyranoside (2), quercetin 3-O-(6-trans-p-coumaroyl)-β-glucopyranosyl-(1→2)-β-glucopyranoside-7-O-greek small letter alpha-rhamnopyranoside (3), kaempferol 3-O-(6-trans-p-coumaroyl)-β-glucopyranosyl-(1→2)-β-glucopyranoside-7-O-greek small letter alpha-rhamnopyranoside (4), quercetin 7-O-(6-trans-caffeoyl)-β-glucopyranosyl-(1→3)-greek small letter alpha-rhamnopyranoside-3-O-β-glucopyranoside (5), kaempferol 7-O-(6-trans-caffeoyl)-β-glucopyranosyl-(1→3)-greek small letter alpha-rhamnopyranoside-3-O-β-glucopyranoside (6), kaempferol 7-O-(6-trans-p-coumaroyl)-β-glucopyranosyl-(1→3)-greek small letter alpha-rhamnopyranoside-3-O-β-glucopyranoside (7), kaempferol 3-O-β-(2″-acetyl)galactopyranoside (8), kaempferol 3-O-β-(2″-acetyl)galactopyranoside-7-O-greek small letter alpha-arabinopyranoside (9), quercetin 3-O-β-(2″-acetyl)galactopyranoside-7-O-greek small letter alpha-arabinopyranoside (10), quercetin 3,7-di-O-greek small letter alpha-rhamnopyranoside (11), kaempferol 3,7-di-O-greek small letter alpha-rhamnopyranoside (12) and quercetin 3-O-β-glucopyranoside-7-O-greek small letter alpha-rhamnopyranoside (13). Their antioxidant activity (AA) was determined by measuring free radical scavenging activity by DPPH test and the coupled oxidation of β-carotene and linoleic acid assay. The results showed that 5 is the most active compound in the DPPH free-radical scavenging test (IC50 1.9 μM) while in the coupled oxidation of β-carotene and linoleic acid assay compound 1 has the highest inhibitory ratio after 1 h (58.9%). Some structure–activity relationships on the AA were obtained.