Pulse radiolysis, electron paramagnetic resonance spectroscopy and theoretical calculations of caffeic acid oligomer radicals.
Seven representative compounds isolated from Salvia officinalis, among them caffeic acid, the dimer rosmarinic acid and oligomers of caffeic acid, were investigated with regard to their antioxidant potential both expressed by the radical scavenging activity and the stability and structure of the intermediate radicals. Pulse-radiolytic investigation revealed very high rate constants with both hydroxyl and azide radicals. Evidence from kinetic modelling calculations suggested unusual complex behavior due to the presence of both O(4)- and O(3)-semiquinones and - in two cases - formation and decay of a hydroxyl radical adduct at the vinyl side chain. EPR spectroscopy studies, which included dihydrocaffeic acid as a model for the saturated side chains of the oligomers, confirmed that the radical structures after oxidation in slightly alkaline solutions are representing dissociated O(4)-semiquinones. While according to calculations based on hybrid density-functional theory the other radical structures are valid intermediates, they cannot be observed except by pulse radiolysis due to their fast decay.
Bors
Wolf
W
Institut für Strahlenbiologie, GSF Forschungszentrum für Umwelt und Gesundheit, GSF Research Center, Ingolstaedter Landstrasse 1, D-85764 Neuherberg, Germany. bors@gsf.de
Michel
Christa
C
Stettmaier
Kurt
K
Lu
Yinrong
Y
Foo
L Yeap
LY
D016227:Benzoquinones / Q000737:chemistry; D002109:Caffeic Acids / Q000737:chemistry* / Q000302:isolation & purification; D002934:Cinnamates / Q000737:chemistry; D053630:Depsides; D004578:Electron Spin Resonance Spectroscopy; D016166:Free Radical Scavengers / Q000737:chemistry* / Q000302:isolation & purification; D017665:Hydroxyl Radical / Q000737:chemistry; D016014:Linear Models; D008433:Mathematics; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry; D011675:Pulse Radiolysis; D027543:Salvia officinalis / Q000737:chemistry*; D000097123:Rosmarinic Acid
D016227:Benzoquinones; D002109:Caffeic Acids; D002934:Cinnamates; D053630:Depsides; D016166:Free Radical Scavengers; D017665:Hydroxyl Radical; D010636:Phenols; D000097123:Rosmarinic Acid; C000995:3,4-dihydroxyphenylpropionic acid; C025232:semiquinone radicals; C042329:phenoxy radical; C040048:caffeic acid