Abstract
Recently the number of studies investigating triterpenoid saponins has drastically increased due to their diverse and potentially attractive biological activities. Currently the literature contains chemical structures of few hundreds of triterpenoid saponins of plant and animal origin. Triterpenoid saponins consist of a triterpene aglycone with one or more sugar moieties attached to it. However, due to similar physico-chemical properties, isolation and identification of a large diversity of triterpenoid saponins remain challenging. This study demonstrates a methodology to screen saponins using hyphenated analytical platforms, GC-MS, LC-MS/MS, and LC-SPE-NMR/MS, in the example of two different phenotypes of the model plant Barbarea vulgaris (winter cress), glabrous (G) and pubescent (P) type that are known to differ by their insect resistance. The proposed methodology allows for detailed comparison of saponin profiles from intact plant extracts as well as saponin aglycone profiles from hydrolysed samples. Continuously measured 1D proton NMR data during LC separation along with mass spectrometry data revealed significant differences, including contents of saponins, types of aglycones and numbers of sugar moieties attached to the aglycone. A total of 49 peaks were tentatively identified as saponins from both plants; they are derived from eight types of aglycones and with 2–5 sugar moieties. Identification of two previously known insect-deterrent saponins, hederagenin cellobioside and oleanolic acid cellobioside, demonstrated the applicability of the methodology for relatively rapid screening of bioactive compounds.
Highlights
Triterpenoid saponins consist of a triterpene aglycone, with 30 carbon atoms, with one or more sugar moieties attached to the aglycone [1].Triterpenoid saponins are secondary metabolites synthesized in plant and mammalian cells
This study demonstrates the application of hyphenated analytical platforms, including GC-MS, LC-MS/MS, Nuclear magnetic resonance (NMR) and time slice LC-solid phase extraction (SPE)-NMR/MS, for the qualitative and relative quantitative analysis of triterpenoid saponins in plant leaf extracts of B. vulgaris
A single metabolite extraction method based on 85% methanol was developed and employed for three different analytical platforms, including GC-MS, LC-MS and NMR (Figure 1) [14]
Summary
Triterpenoid saponins consist of a triterpene aglycone, with 30 carbon atoms, with one or more sugar moieties (including hexoses, methylpentoses, and pentoses) attached to the aglycone [1]. Molecules 2016, 21, 1614 hydrophilic sugar moieties and the lipophilic triterpenoid aglycone For this reason, the extraction and qualitative and quantitative analysis of saponins might be challenging from complex sample matrices such as plants and must be optimized prior to qualitative and quantitative analysis. Structural characterization of saponins can be further improved when tandem mass spectrometry, for example LC-MS/MS, is applied [12] This will provide unique fragmentation patterns of saponins including the number and type of sugar moieties attached to the aglycone. In order to screen for triterpenoid aglycone molecules, samples are often hydrolysed to cleave off sugar moieties attached to the aglycone This procedure is usually performed either by chemical, using acidic or alkaline, or enzymatic hydrolysis. A significant amount of structural information, MW of aglycones and number and types of sugar moieties can be gained by combining the information obtained from the different platforms
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