Mechanistic study on the radical scavenging activity of viniferins

•The antioxidant activity of δ-viniferin and ε-viniferin is systematically examined.•Hydrogen atom transfer and radical adduct formation pathways are studied.•δ-viniferin exhibits stronger antioxidant activity than ε-viniferin.•The most reactive sites of δ-viniferin and ε-viniferin are 12-OH and 12′-OH.