LC-SPE-NMR-MS analysis of Strychnos usambarensis fruits from Rwanda

Strychnos usambarensis is a Rwandan tree of Loganiaceae family, whose roots and leaves had been widely investigated. They contain a huge number of indolomoterpenic tertiary alkaloids possessing antimalarial [1] and antitumoral properties [2]. Nevertheless, almost no studies had been carried on its fruits until now, except one which showed that demethoxycarbonyl-3,14-dihydro-gambirtannine was the major alkaloid in the apolar extract [3]. Alkaloids and glucoalkaloids have been successfully characterized in fruits of Strychnos usambarensis. Two distinct extracts (methanolic and ethyl acetate) were realized and studied by a hyphenated analytical method, combining high-performance liquid chromatography coupled with solid-phase extraction (SPE), mass spectrometry (HPLC – MS) and NMR spectroscopy. The HPLC gradient system consisted of acetonitrile and heptanesulfonic acid. The analytes were then accumulated by repetitive absorption on SPE cartridges. The trapped compounds were subsequently eluted with CD₃CN for acquisition of NMR data and analysed by electrospray ionization and time of flights mass spectrometry (ESI/Q – TOF/MS). This led to identification among others of palicoside in the methanolic extract, a glucoalkaloid never described in Strychnos usambarensis [4], and akagerine in the ethyl acetate extract, an alkaloid already known in Strychnos usambarensis roots [5,6].

Fig.1: Palicoside

Acknowledgements: This research was supported by the Belgian National Fund for Scientific Research (FNRS – grant N 3.4533.10). M.C. and M.F. are respectively Research Fellow and Senior Research Associate from the FNRS.

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