Due to the large amount of phenolic hydroxyl groups in its structure,lignin has ability to remove oxygen free radicals and is recognized as one type of efficient and natural antioxidants.The excellent antioxidant ability of lignin together with its safety when applied on sunscreen,food and medical,opened new perspectives in its potential application in cosmetic and topical formulations.In this paper,four types of lignin preparations were isolated from poplar,reed,wheat straw and corn stalk residues,respectively.The antioxidant activities of the isolated compounds were tested using 1,1-diphenyl-2-picrylhydrazyl(DPPH)and ferric reducing antioxidant power assays.The contents of phenolic and aliphatic hydroxyl groups in lignin were calculated by the technique of Folin-Ciocalteu method.The correlations between the antioxidant ability of lignin preparations and their functional groups contents were also examined.It was found that the radical scavenging ability of poplar lignin was highest during these four lignin preparations,which was mainly caused by its higher contents of phenolic hydroxyl groups.Six groups of different concentrations in the scope from 0.025 to 0.8 mg/mL of four lignin aqueous were studied.With the increase of concentration,the antioxidant activities of lignin increased or flatten.When the concentration of poplar lignin in ethanol was 0.2 mg/mL,the radical scavenging efficiency was high up to 70％.However,with the similar radical scavenging efficiency,the concentration of the reed lignin,wheat straw lignin or corn stalk residue lignin in ethanol was twice higher than that poplar lignin.
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