title: pH dependent antioxidant activity of lettuce (L. sativa) and synergism with added phenolic antioxidants.
abstract: Influence of pH on the antioxidant activities of combinations of lettuce extract (LE) with quercetin (QC), green tea extract (GTE) or grape seed extract (GSE) was investigated for both reduction of Fremy's salt in aqueous solution using direct electron spin resonance (ESR) spectroscopy and in L-α-phosphatidylcholine liposome peroxidation assay measured following formation of conjugated dienes. All examined phenolic antioxidants showed increasing radical scavenging effect with increasing pH values by using both methods. QC, GTE and GSE acted synergistically in combination with LE against oxidation of peroxidating liposomes and with QC showing the largest effect. The pH dependent increase of the antioxidant activity of the phenols is due to an increase of their electron-donating ability upon deprotonation and to their stabilization in alkaline solutions leading to polymerization reaction. Such polymerization reactions of polyphenolic antioxidants can form new oxidizable -OH moieties in their polymeric products resulting in a higher radical scavenging activity.
authors.affiliation: Ministry of Food, Agriculture and Livestock, Ankara, Turkey.
authors.forename: Arzu
authors.initials: A
authors.lastname: Altunkaya
authors.affiliation: Food Engineering Department, Hacettepe University, Beytepe, Ankara, Turkey. Electronic address: vgokmen@hacettepe.edu.tr.
authors.forename: Vural
authors.initials: V
authors.lastname: Gökmen
authors.affiliation: University of Copenhagen, Faculty of Life Sciences, Department of Food Science, Food Chemistry, Rolighedsvej 30, 1958 Frederiksberg C, Denmark.
authors.forename: Leif H
authors.initials: LH
authors.lastname: Skibsted
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D018545:Lactuca / Q000737:chemistry*; D008081:Liposomes / Q000378:metabolism*; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry*; D010936:Plant Extracts / Q000737:chemistry*
chemical_list: D000975:Antioxidants; D008081:Liposomes; D010636:Phenols; D010936:Plant Extracts
keywords: ESR; Lettuce; Liposome; Polyphenols; Synergism; pH