title: Chemical studies on antioxidant mechanisms and free radical scavenging properties of lignans.
abstract: The antioxidant activity, in terms of radical scavenging capacity, of altogether 15 different lignans was measured by monitoring the scavenging of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The effect of differences in skeletal arrangement or the degree of oxidation of the lignans was investigated in a structure-activity relationship study. A large variety in the radical scavenging capacities of the different lignans was observed and related to some structural features. Lignans with catechol (3,4-dihydroxyphenyl) moieties exhibited the highest radical scavenging capacity, while the corresponding guaiacyl (3-methoxy-4-hydroxyphenyl) lignans showed a slightly weaker scavenging capacity. In addition, the butanediol structure was found to enhance the activity, whereas a higher degree of oxidation at the benzylic positions decreased the activity. Additionally, the readily available lignans (-)-secoisolariciresinol, a mixture of hydroxymatairesinol epimers and (-)-matairesinol were studied in more detail, including kinetic measurements and identification of oxidation products in the reactions with DPPH and ABAP (2,2-azobis(2-methylpropionamidine) dihydrochloride. The identification of reaction products, by GC-MS, HPLC-MS and NMR spectroscopy, showed that dimerisation of the two aromatic moieties was the major radical termination reaction. Also, the formation of adducts was a predominant reaction in the experiments with ABAP. The kinetic data obtained from the reactions between the lignans and DPPH indicated a complex reaction mechanism.
authors.affiliation: Department of Organic Chemistry, Process Chemistry Centre, Abo Akademi University, Biskopsgatan 8, FIN-20500, Abo, Finland. paeklund@abo.fi rsjoholm@abo.fi
authors.forename: Patrik C
authors.initials: PC
authors.lastname: Eklund
authors.forename: Otto K
authors.initials: OK
authors.lastname: Långvik
authors.forename: Johan P
authors.initials: JP
authors.lastname: Wärnå
authors.forename: Tapio O
authors.initials: TO
authors.lastname: Salmi
authors.forename: Stefan M
authors.initials: SM
authors.lastname: Willför
authors.forename: Rainer E
authors.initials: RE
authors.lastname: Sjöholm