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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.

Sea urchin pigmentation is mainly due to polyhydroxy-1,4-naphthoquinones called spinochromes. If their molecular structures are well known in test and spines of many species, their abundance and distribution in other body compartments remain unstudied. The aim of this study is to analyse the pigment composition in four body compartments (test/spines, digestive system, gonads and coelomic fluid) of four coloured types of the sea urchin Echinometra mathaei. Qualitative and quantitative measurements by mass spectrometry highlight the existence of 13 different pigments; among which are five isomers of known spinochromes as well as three potentially new ones. The composition comparison shows the largest spinochrome diversity in ‘test/spines’ body compartments. The spinochrome concentrations vary from 48 to 1279 mg kg−1 of dried body compartment. It is the highest in the digestive system, although it is also important in the organic fraction of the ‘test/spines’ body compartment. This observation may be explained by higher exposures of some body compartments to external environments and by the protective role fulfilled by spinochromes against microorganisms, ultraviolet radiation and reactive oxygen species. The ‘black’ type—the most common coloured type in coral reefs—has the highest concentration of spinochromes indicating their importance in Echinoids' fitness by acting as a protective agent.

Saponins are secondary metabolites that are abundant and diversified in echinoderms. Mass spectrometry is increasingly used not only to identify saponin congeners within animal extracts but also to decipher the structure/biological activity relationships of these molecules by determining their inter-organ and inter-individual variability. The usual method requires extensive purification procedures to prepare saponin extracts compatible with mass spectrometry analysis. Here, we selected the sea star Asterias rubens as a model animal to prove that direct analysis of saponins can be performed on tissue sections. We also demonstrated that carboxymethyl cellulose can be used as an embedding medium to facilitate the cryosectioning procedure. Matrix-assisted laser desorption/ionization (MALDI) imaging was also revealed to afford interesting data on the distribution of saponin molecules within the tissues. We indeed highlight that saponins are located not only inside the body wall of the animals but also within the mucus layer that probably protects the animal against external aggressions. Graphical Abstract Saponins are the most abundant secondary metabolites in sea stars. They should therefore participate in important biological activities. Here, MALDI imaging is presented as a powerful method to determine the spatial distribution of saponins within the animal tissues. The inhomogeneity of the intra-organ saponin distribution is highlighted, paving the way for future elegant structure/activity relationship investigations.

Saponins are amphiphilic molecules of pharmaceutical interest and most of their biological activities (i.e., cytotoxic, hemolytic, fungicide, etc.) are associated to their membranolytic properties. These molecules are secondary metabolites present in numerous plants and in some marine animals, such as sea cucumbers and starfishes. Structurally, all saponins correspond to the combination of a hydrophilic glycan, consisting of sugar chain(s), linked to a hydrophobic triterpenoidic or steroidic aglycone, named the sapogenin. Saponins present a high structural diversity and their structural characterization remains extremely challenging. Ideally, saponin structures are best established using nuclear magnetic resonance experiments conducted on isolated molecules. However, the extreme structural diversity of saponins makes them challenging from a structural analysis point of view since, most of the time, saponin extracts consist in a huge number of congeners presenting only subtle structural differences. In the present review, we wish to offer an overview of the literature related to the development of mass spectrometry for the study of saponins. This review will demonstrate that most of the past and current mass spectrometry methods, including electron, electrospray and matrix‐assisted laser desorption/ionization ionizations, gas/liquid chromatography coupled to (tandem) mass spectrometry, collision‐induced dissociation including MS3 experiments, multiple reaction monitoring based quantification, ion mobility experiments, and so forth, have been used for saponin investigations with great success on enriched extracts but also directly on tissues using imaging methods.

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