Tackling saponin diversity in marine animals by mass spectrometry: data acquisition and integration

Abstract

Saponin analysis by mass spectrometry methods is nowadays progressively supplementing other analytical methods such as nuclear magnetic resonance (NMR). Indeed, saponin extracts from plant or marine animals are often constituted by a complex mixture of (slightly) different saponin molecules that requires extensive purification and separation steps to meet the requirement for NMR spectroscopy measurements. Based on its intrinsic features, mass spectrometry represents an inescapable tool to access the structures of saponins within extracts by using LC-MS, MALDI-MS, and tandem mass spectrometry experiments. The combination of different MS methods nowadays allows for a nice description of saponin structures, without extensive purification. However, the structural characterization process is based on low kinetic energy CID which cannot afford a total structure elucidation as far as stereochemistry is concerned. Moreover, the structural difference between saponins in a same extract is often so small that coelution upon LC-MS analysis is unavoidable, rendering the isomeric distinction and characterization by CID challenging or impossible. In the present paper, we introduce ion mobility in combination with liquid chromatography to better tackle the structural complexity of saponin congeners. When analyzing saponin extracts with MS-based methods, handling the data remains problematic for the comprehensive report of the results, but also for their efficient comparison. We here introduce an original schematic representation using sector diagrams that are constructed from mass spectrometry data. We strongly believe that the proposed data integration could be useful for data interpretation since it allows for a direct and fast comparison, both in terms of composition and relative proportion of the saponin contents in different extracts. Graphical Abstract A combination of state-of-the-art mass spectrometry methods, including ion mobility spectroscopy, is developed to afford a complete description of the saponin molecules in natural extracts.

Ion mobility mass spectrometry of saponin ions

“…The oligosaccharide chain is covalently attached to the C3 of the aglycone and may include xylose, glucose, quinovose, and 3‐ O ‐methylglucose residues. We previously demonstrated the great diversity of the saponin contents in sea cucumber extracts . By submitting the body wall extract prepared for the present study to our MS‐based methodology, we identified 11 saponins (see Table ).…”

“…Monodesmosidic and bidesmosidic saponins present in the sea cucumber, Holothuria forskali , in the quinoa, Chenepodium quinoa , and in the common soy, Glycine max , cover a wide range of monosaccharide residue numbers, from di‐ to hexasaccharide glycones, making these organisms interesting saponin sources in the context of the present investigation. The saponin compositions of the soy, quinoa, and sea cucumber extracts are first assessed by the combination of MALDI‐MS/MS and LC/MS/MS, as reported in our recent paper, to confirm the presence and the structure of the selected saponin ions. Note that saponins are mostly detected upon MALDI‐MS and LC/MS in the positive ion mode as sodium‐cationized [M + Na] + molecules, although protonated [M + H] + and potassium‐cationized [M + K] + saponins are also often detected.…”

“…The second saponin source is the body wall of the sea cucumber ( Holothuria forskali ), a species that has been extensively investigated in our laboratory . Holothuroid saponins are triterpene glycosides.…”

“…For many years, saponin structures have been established based on a time‐consuming procedure combining individual saponin isolation and nuclear magnetic resonance (NMR) measurements . Nowadays, mass spectrometry methods are increasingly involved for their structural identification based on accurate mass measurements (high‐resolution mass spectrometry – HRMS) and collision‐induced dissociation (CID) experiments . Mass spectrometry data are almost always interpreted based on NMR results and it is important to remind here that MS alone is unable to distinguish stereoisomers.…”

“…In a recent publication, we described a global MS‐based methodology for saponin extract analysis that associates matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF), HRMS, liquid chromatography (LC)/MS, CID and IMMS. We took advantage of that paper to report our preliminary results dealing with the structural characterization of saponins by IMMS coupled to computational chemistry . The principal outcome of this initial study was that ion mobility data are not currently sufficient to discriminate different isomers presenting similar structures, which is often the case when considering the extract from a natural source.…”