Isolation and Structural Confirmation of Xanthone Isomers from Dryopteris ramosa (Hope) C. Chr. and Their In Vitro Antioxidant Mechanism

Abstract

The long history of using plant species as medicines worldwide provides clues for the search for bioactive phytochemicals. One of the important traditional medicinal plant species of Pakistan is Dryopteris ramosa (Dryopteridaceae). The fronds of Dryopteris ramosa are traditionally used for the treatment of gastric ulcer and constipation. The crude methanol extract and aqueous fraction of Dryopteris ramosa exhibited strong free radical scavenging potential. Two xanthonoid compounds, mangiferin and isomangiferin, were isolated from the aqueous fraction of Dryopteris ramosa by chromatographic techniques (CC Sephadex LH20, followed by MPLC and purified by preparative TLC). Their structures were confirmed by spectroscopic analysis MS, NMR (1H, 13C, HSQC and HMBC). This is the first report on isolation of mangiferin and isomangiferin from Dryopteris genus though previously reported in other fern genera, e.g., Cystopteris, Acystopteris, Gymnocarpium and Asplenium. The antioxidant potential of mangiferin (60.34 ± 0.14 μg/mL) and isomangiferin (55.98 ± 0.39 μg/mL) was comparable to ascorbic acid (57.97 ± 0.93 μg/mL), thus representing promising antioxidant agents against oxidative stress. The investigation of the mechanism of free radical scavenging by mangiferin and isomangiferin showed that the catechol moiety was involved in quenching of free radicals. This study justified the traditional use of Dryopteris ramosa confirming the pharmacological properties of isolated compounds. Further phytochemical studies on Dryopteris ramosa are needed for identification of new bioactive phytochemicals in this plant of ethnobotanical relevance.