Structural Characterization of Cucurbitane Triterpenoids in Momordica charantia by Electrospray Ionization Tandem Mass Spectrometry

Bitter melon, Momordica charantia L. (Cucurbitaceae), is a widely cultivated plant and grows in tropical areas, including Asia, East Africa, the Caribbean and South America. The fruits of this plant have been used not only as vegetable, but also as traditional medicine to treat for a bitter stomachic, as a laxative, an anti-diabetic, an anthelmintic in children, treatment of feverish conditions and an antiviral for both measles and hepatitis [1]. Various studies have shown that the saponin fraction of Momordica charantia inhibits the increase of blood glucose and serum neutral fat [2]. Cucurbitane-type triterpenoid saponins isolated from M. charantia were identified using electrospray ionization multi-stage tandem mass spectrometry (ESI-MSⁿ) coupled with an ion-trap analyzer. The fragmentation patterns of these saponins significantly changed in accordance with structural variations of substitution at C-3 and C-19. When the C-19 methyl group is oxidized to the aldehyde or etherified with C-5, the fragmentation behavior of retro-Diels-Alder rearrangement in the B-ring is observed. However, when C-19 forms an acetal moiety, rearrangement is less likely to occur. The glycosidic cleavage at C-3 yielded ions of a neutral loss 162 Da, but if glycosidic cleavage occurrs at C-7, it will give the ions of a neutral loss of 180 Da. If C-25 connects with a hydroxy group, a neutral loss of 60 Da is readily observed. When the hydroxy group at C-25 is replaced by a methoxyl, an ion of loss 30 Da is captured. The results of these studies may benefit the determination of trace triterpenoid saponins of this type in crude plant extracts, and also provide background information to aid the structural investigations of similar glycoconjugates. Acknowledgements: This research is funded in part by “Botanical Dietary Supplements: Science-Base for Authentication” funded by Food and Drug Administration grant number 2 U01 FD 002071-07. References: [1] Beloin N, et al. (2005) J. Ethnopharmacol 96: 49–55. [2] Oishi Y, et al. (2007) Biosci. Biotechnol. Biochem. 71: 735–740.