Antioxidant and free radical-scavenging activity of constituents from two Scorzonera species

The aim of this study was to investigate the secondary metabolites content of Scorzonera papposa DC., an edible plant eaten in the desert region of Jordan and to assess its antioxidant and free radical-scavenging activity. By using this bioassay-oriented approach nine compounds, including the new natural compounds (6-trans-p-coumaroyl)-3-O-β-d-glucopyranosyl-2-deoxy-d-riburonic acid (1), (6-cis-p-coumaroyl)-3-O-β-d-glucopyranosyl-2-deoxy-d-riburonic acid (2a), (6-trans-p-coumaroyl)-3-O-β-d-glucopyranosyl-2-deoxy-d-riburonic acid methyl ester (3), and (6-trans-p-coumaroyl)-3-O-β-d-glucopyranosyl-(5-acetyl)-2-deoxy-d-riburonic acid (4), having the rare deoxy-d-riburonic acid moiety, were isolated. Their structures were elucidated by UV, MS, 1H and 13C NMR and 2D NMR. The antioxidant activity of the S. papposa pure compounds and of related derivatives isolated from another Scorzonera species (S. judaica Eig.) was also tested. The Relative Antioxidant Capacity Index (RACI) was applied as an integrated method to compare the antioxidant activities obtained using different chemical methods.