Determination of saponins in crude plant extracts by liquid chromatography—thermospray mass spectrometry

Decades of Phytochemical Research on African Biodiversity

This review provides a panoramic view of Prof. Kurt Hostettmann's contribution to the study of African medicinal plants as documented in over 85 publications with collaborators from about a dozen African countries. Many novel bioactive secondary metabolites were isolated, their structures elucidated by hyphenated HPLC techniques and their biological activity determined.

Structure analysis of triterpene saponins in Polygala tenuifolia by electrospray ionization ion trap multiple-stage mass spectrometry

Eighteen different triterpene saponins isolated from Polygala tenuifolia were investigated by electrospray ionization ion trap multiple-stage mass spectrometry (ESI-ITMS(n)) in positive and negative ion modes. MS(1)-MS(3)/MS(4) spectra of the both modes were analyzed, and they all gave fragments in line and shared common fragmentation patterns. Key fragments from MS(n) spectra of both the modes and their proposed fragmentation pathways were constructed with examples illustrated for the formation of characteristic fragments in the saponins. Two special fragmentation patterns were proposed: (1) the formation of fragments by cleavage of CH(2)O from Delta(12)-14alpha-CH(2)OH of the oleanene-type saponin aglycone in both positive and negative MS(n) (n > or = 2) modes; (2) the occurrence of fragments by cleavage of CO(2) and 3-glucose as the characteristic structure feature of 23-COOH at the oleanene-type saponin aglycones coupled with 3-Glc substitutes in the negative MS(n) (n > or = 2) modes. Peak intensities in MS(n) spectra were also correlated with structural features and fragmentation preferences of the investigated saponins, which are discussed in detail. In general, fragments formed predominantly by cleavages of glycosidic bonds in the positive mode, while selective cleavages of acyl bonds preceded that of glycosidic bonds in negative MS(n) (n > or = 2) mode, both of which could well be applied to the structural analysis of these saponins. Interpretation of MS(n) spectra presented here provided diagnostic key fragment ions important for the structural elucidation of saponins in P.tenuifolia.

Rapid structural characterization of triterpenoid saponins in crude extract from Symplocos chinensis using liquid chromatography combined with electrospray ionization tandem mass spectrometry

Triterpenoid saponins in bioactive crude extract from Symplocos chinensis were rapidly identified using electrospray ionization multi-stage tandem mass spectrometry (ESI-MSn) and liquid chromatography coupled with sequential mass spectrometry (LC-MSn). According to the characteristic fragmentation behavior of known glucuronide-type triterpenoid saponins isolated from this plant, a total of fourteen constituents in the crude extract were structurally characterized on the basis of their retention time and tandem mass spectrometric analysis, including five pairs of naturally occurring isomers. Except one known saponin formerly obtained, the other constituents were new compounds. The analytical method of LC-MSn combined with ESI-MSn in positive and negative ion modes has been developed for the direct structural elucidation of triterpenoid saponins of this kind in plant extracts.

Chromatographic determination of plant saponins

The methods used for saponin determination in plant materials are presented. It is emphasised that the biological and spectrophotomeric methods still being used for saponin determination provide, to some extent, valuable results on saponin concentrations in plant material. However, since they are sensitive to the structural variation of individual saponins they should be standardized with saponin mixtures isolated from the plant species in which the concentration is measured. However, one plant species may contain some saponins which can be determined with a biological test and others which cannot. That is why biological and colorimetric determinations do not provide accurate data and have to be recognized as approximate. Thin-layer chromatography on normal and reversed-phases (TLC, HPTLC, 2D-TLC) provides excellent qualitative information and in combination with on-line coupling of a computer with dual-wavelength flying-spot scanner and two-dimensional analytical software can be used for routine determination of saponins in plant material. The densitometry of saponins has been very sensitive, however, to plate quality, spraying technique and the heating time and therefore appropriate saponin standards have to be run in parallel with the sample. Gas-liquid chromatography has limited application for determination since saponins are quite big molecules and are not volatile compounds. Thus, there are only few applications of GC for determination of intact saponins. The method has been used for determination of TMS, acetyl or methyl derivatives of an aglycones released during saponin hydrolysis. However, structurally different saponins show different rates of hydrolysis and precise optimisation of hydrolysis conditions is essential. Besides, during hydrolysis a number of artefacts can be formed which can influence the final results. High performance liquid chromatography on reversed-phase columns remains the best technique for saponin determination and is the most-widely used method for this group of compounds. However, the lack of chromophores allowing detection in UV, limits the choice of gradient and detection method. The pre-column derivatisation with benzoyl chloride, coumarin or 4-bromophenacyl bromide has been used successfully in some cases allowing UV detection of separation. Standardisation and identification of the peaks in HPLC chromatograms has been based on comparison of the retention times with those observed for authentic standards. But new hyphenated techniques, combining HPLC with mass spectrometry and nuclear magnetic resonance are developing rapidly and allow on-line identification of separated saponins. Capillary electrophoresis has been applied for saponin determination only in a limited number of cases and this method is still being developed.

On-line identification of phytochemical constituents in botanical extracts by combined high-performance liquid chromatographic-diode array detection-mass spectrometric techniques

It is necessary to determine all of the phytochemical constituents of botanical extracts in order to ensure the reliability and repeatability of pharmacological and clinical research, to understand their bioactivities and possible side effects of active compounds and to enhance product quality control. HPLC chromatographic fingerprints can be applied for this kind of documentation. Combined HPLC-diode array detection-MS techniques can provide on-line UV and MS information for each peak in a chromatogram. In most cases, direct identification of the peaks is possible, based on comparison with published data or with standard compounds. This review will primarily focus on electrospray and thermospray ionization MS and their applications for the qualitative analyses of phenolic compounds, saponins, alkaloids and other classes of natural products in botanical extracts. Twenty-one of the most commonly used herbal examples, their phytochemical analyses and characteristics of their mass spectra are described.

Rapid identification of saponins in plant extracts by electrospray ionization multi-stage tandem mass spectrometry and liquid chromatography/tandem mass spectrometry

Electrospray ionization multi-stage tandem mass spectrometry (ESI-MS(n)) and liquid chromatography coupled with on-line mass spectrometry (LC/MS/MS) were applied to characterize saponins in crude extracts from Panax ginseng. The MS(n) data of the [M - H](-) ions of saponins can provide structural information on the sugar sequences of the saccharide chains and on the sapogins of saponins. By ESI-MS(n), non-isomeric saponins and isomeric saponins with different aglycones can be determined rapidly in plant extracts. LC/MS/MS is a good complementary analytical tool for determination of isomeric saponins. These approaches constitute powerful analytical tools for rapid screening and structural assignment of saponins in plant extracts.

Analysis of saponins from black bean by electrospray ionization and fast atom bombardment tandem mass spectrometry

Saponins from black bean (Vigna mungo L. Hepper) were analyzed using positive and negative ion fast atom bombardment mass spectrometry (FAB-MS) and liquid chromatography/mass spectrometry. Methanol was used to extract the saponins from defatted black bean, which was partially purified by extraction with n-butanol, and the extract was dialyzed with 3000 M(r) cut-off tubing. The dialyzate was analyzed using mass spectrometry. According to FAB-MS/MS, mixtures from black bean contain soyasaponin I as the predominant saponin. In addition, MS/MS analysis was performed in which the structures of saponins of black bean cotyledon were determined to be soyasaponin I, soyasaponin II, soyasaponin V, 3-O-[alpha-L-rhamnopyranosyl-(1 --> 2)-beta-D-galactopyranosyl-(1 --> 2)-beta-D-glucuronopyranosyl]complogenin (saponin A) and 3-O-[alpha-L-rhamnopyranosyl-(1 --> 2)-beta-D-glucopyranosyl-(1 --> 2)-beta-D-glucopyranosyl]oleanolic acid (saponin B). For the black bean shell and the root of black bean sprout, analysis confirmed the saponins of soyasaponin I, soyasaponin II, soyasaponin V, saponin A, saponin B, acetylsoyasaponin A(4) and soyasaponin beta(g). Moreover, all the studied saponins were found in the stem and leaves of the black bean sprouts, except soyasaponin beta(g) and acetylsoyasaponin A(4), respectively.