Antioxidant activities of polyphenols isolated from Mallotus metcalfianus Croizat

Abstract

During our systematic phytochemical examination of Mallotus species, we have studied the chemical composition of Mallotus metcalfianus Croizat (Euphorbiaceae), commonly known as „Ba bet do“ in Vietnam, and evaluated its antioxidant activities by the DPPH test as described by Aquino et al [1]. Two new flavones named luteolin 7-O-(4''-O-(E)-coumaroyl)-β-glucopyranoside) and chrysoeriol-7-O-(4''-O-(E)-coumaroyl)-β-glucopyranoside) and a new flavonolignan, (±)-hydnocarpin 7-O-(4''-O-(E)-coumaroyl)-β-glucopyranoside) were isolated, together with 10 known compounds. This is the first report of these compounds in this species. The structures of the new compounds were established using different spectroscopic methods including extensive 1D- and 2D-NMR spectroscopy. Concerning the antioxidant activities, the ethyl acetate fraction of M. metcalfianus is the most active fraction. Tannins present in this fraction are only responsible for a small part of the activity which seems to be mainly due to flavonoids. We have tested the different pure compounds isolated from M. metcalfianus and different reference samples for their antiradical activities in order to discuss the SAR of these products. To illustrate this discussion, quercitrin, 3-O-β-rhamnosyl-kaempferol and quercetin 3-neohesperidoside are more active than the reference sample, tocopherol. By contrast, the new flavonolignan is not very active. This lack of activity could be explained by the cyclization of the catechol function in ring B of the flavone. Indeed, by comparison with luteolin, luteolin 7-O-(4''-O-(E)-coumaroyl)-β-glucopyranoside) was found to be moderately active as antioxidant in the DPPH assay. The substitution of the flavone by a coumaric acid could explain this decrease of activity, because coumaric acid doesn't show a real antioxidant activity unlike caffeic acid.