Identification of common glycosyl groups of flavonoid O-glycosides by serial mass spectrometry of sodiated species

Binding of Quercetin Derivatives toward G-Tetrads as Studied by the Survival Yield Method

Recently, much interest has been devoted to finding effective G-quadruplex ligands, both of synthetic or natural origins, which may be of potential use in the field of cancer therapy. Among compounds of natural origin, a common flavonol quercetin has attracted notable attention. Yet, only a modest number of papers have been concerned with a comparison of quercetin conjugates binding to G-quadruplexes. In this study, we applied the survival yield (SY) method in order to compare the stability of G-tetrad complexes with quercetin and its conjugates, namely, 3-O-glycosides and O-methylated conjugates. According to the determined values of Ecomδ50, flavonol glycosides bind most effectively with G-tetrads, whereas, among flavonols, 3-O-methylquercetin makes the most effective bonds. Because the aglycone structure is of crucial importance for biological processes, 3-O-methylquercetin seems to be a suitable candidate for anticancer therapeutics, and the extracts from the plants, which contain high amounts of 3-O-methylquercetin or its glycosides, should be considered as interesting materials for preparation of pharmaceuticals or dietary supplements.

Antioxidant and Antiproliferative Activities of Phenolic Extracts of Eriobotrya japonica (Thunb.) Lindl. Fruits and Leaves

Increasing interest in new sources of secondary metabolites as biologically active substances has resulted in an advanced study of many plant species. Loquat (Eriobotrya japonica (Thunb.) Lindl. = Rhaphiolepis bibas (Lour.) Galasso & Banfi, Rosaceae family), an evergreen, subtropical fruit tree, native to China and Japan, but cultivated in southern countries of Europe, is a species commonly used in folk medicine and may be an excellent source of bioactive compounds. Therefore, the aim of the present study was to evaluate the profile of the phenolic constituents of E. japonica fruits and leaves originating from Tuscany (Italy), as well as their in vitro antioxidant and chemopreventive activities on human cancer cell lines breast adenocarcinoma (MCF-7), colon adenocarcinoma (Caco-2 and HT-29), and glioblastoma (U87MG). Results revealed that the extract of leaves displayed higher antioxidant and anticancer potential than the fruit extract and contained 25 individual phenolic compounds that have been characterized and quantified by the UPLC-PDA-MS method. The antiproliferative activity was correlated with the content of polyphenolic compounds indicating that both fruits and leaves are a good source of antioxidants and may be exploited as nutraceuticals enriching food or as components for the cosmetic/pharmaceutical industry.

Chenopodium murale Juice Shows Anti-Fungal Efficacy in Experimental Oral Candidiasis in Immunosuppressed Rats in Relation to Its Chemical Profile

Chenopodium murale (Syn. Chenopodiastrum murale) (amaranthaceae) is used in the rural Egypt to treat oral ulcers in newborn children. The current study aimed to discover new natural products suitable for treating candidiasis disease with minimal side effects. Characterization of bioactive compounds by LC-QTOF-HR-MS/MS from Chenopodium murale fresh leaves' juice (CMJ) was carried out in order to elucidate their potential anti-fungal and immunomodulatory effects in oral candidiasis in immunosuppressed rats. An oral ulcer candidiasis model was created in three stages: (i) immunosuppression by drinking dexamethasone (0.5 mg/L) for two weeks; (ii) Candida albicans infection (3.00 × 10⁶ viable cell/mL) for one week; and (iii) treatment with CMJ (0.5 and 1.0 g/kg orally) or nystatin (1,000,000 U/L orally) for one week. Two doses of CMJ exhibited antifungal effects, for example, through a significant reduction in CFU/Petri (236.67 ± 37.86 and 4.33 ± 0.58 CFU/Petri), compared to the Candida control (5.86 × 10⁴ ± 1.21 CFU/Petri), p ≤ 0.001. In addition, CMJ significantly induced neutrophil production (32.92% ± 1.29 and 35.68% ± 1.77) compared to the Candida control level of 26.50% ± 2.44. An immunomodulatory effect of CMJ at two doses appeared, with a considerable elevation in INF-γ (103.88 and 115.91%), IL-2 (143.50, 182.33%), and IL-17 (83.97 and 141.95% Pg/mL) compared with the Candida group. LC-MS/MS analysis operated in negative mode was used for tentative identification of secondary (SM) metabolites based on their retention times and fragment ions. A total of 42 phytoconstituents were tentatively identified. Finally, CMJ exhibited a potent antifungal effect. CMJ fought Candida through four strategies: (i) promotion of classical phagocytosis of neutrophils; (ii) activation of T cells that activate IFN-γ, IL-2, and IL-17; (iii) increasing the production of cytotoxic NO and H₂O₂ that can kill Candida; and (iv) activation of SOD, which converts superoxide to antimicrobial materials. These activities could be due to its active constituents, which are documented as anti-fungal, or due to its richness in flavonoids, especially the active compounds of kaempferol glycosides and aglycone, which have been documented as antifungal. After repetition on another type of small experimental animal, their offspring, and an experimental large animal, this study may lead to clinical trials.

Chemical Profiling, Bioactive Properties, and Anticancer and Antimicrobial Potential of Juglans regia L. Leaves

The aim of this study was to assess the biological potential of the polyphenolic fraction isolated from J. regia leaves, collected in the Subcarpathian region (Poland). The phenolic profile was determined using the UPLC-PDA-MS/MS method. Biological activity was determined by evaluating the antioxidant, anticancer, antibacterial, and antifungal effects. Prior to this study, the purified polyphenolic fraction was not been tested in this regard. A total of 40 phenolic compounds (104.28 mg/g dw) were identified, with quercetin 3-O-glucoside and quercetin pentosides dominating. The preparation was characterized by a high ability to chelate iron ions and capture O₂^•- and OH^• radicals (reaching IC₅₀ values of 388.61, 67.78 and 193.29 µg/mL, respectively). As for the anticancer activity, among the six tested cell lines, the preparation reduced the viability of the DLD-1, Caco-2, and MCF-7 lines the most, while in the antibacterial activity, among the seven tested strains, the highest susceptibility has been demonstrated against K. pneumoniae, S. pyogenes, and S. aureus. Depending on the needs, such a preparation can be widely used in the design of functional food and/or the cosmetics industry.

Secondary or Specialized Metabolites, or Natural Products: A Case Study of Untargeted LC–QTOF Auto-MS/MS Analysis

The large structural diversity of specialized metabolites represents a substantial challenge in untargeted metabolomics. Modern LC–QTOF instruments can provide three- to four-digit numbers of auto-MS/MS spectra from sample sets. This case study utilizes twelve structurally closely related flavonol glycosides, characteristic specialized metabolites of plant tissues, some of them isomeric and isobaric, to illustrate the possibilities and limitations of their identification. This process requires specific software tools that perform peak picking and feature alignment after spectral deconvolution and facilitate molecular structure base searching with subsequent in silico fragmentation to obtain initial ideas about possible structures. The final assignment of a putative identification, so long as spectral databases are not complete enough, requires structure searches in a chemical reference database, such as SciFindern, in attempts to obtain additional information about specific product ions of a metabolite candidate or check its feasibility. The highlighted problems in this process not only apply to specialized metabolites in plants but to those occurring in other organisms as well. This case study is aimed at providing guidelines for all researchers who obtain data from such analyses but are interested in deeper information than just Venn diagrams of the feature distribution in their sample groups.

Characterisation of phenylethanoid glycosides by multiple-stage mass spectrometry

RATIONALE

Although phenylethanoid glycosides (PhGs) occur widely in plants, their characterisation by liquid chromatography/mass spectrometry (LC/MS) is less well studied than other phenolic glycosides such as flavonoid glycosides. The multiple-stage mass spectrometry (MSⁿ ) experiments required to improve the annotation of common verbascoside-type PhGs are described here.

METHODS

Deprotonated, ammoniated and sodiated molecules of nine PhGs were subjected to low-energy collision-induced dissociation (CID) in a hybrid ion trap/orbitrap mass spectrometer. Most experiments were recorded at nominal mass using the linear ion trap analyser for wider applicability in the plant metabolomics community. Data interpretation was supported by high-resolution orbitrap scanning of product ions. Comparative data was acquired on the same instrument by performing higher-energy collisional dissociation (HCD) in the C-trap.

RESULTS

Low-energy CID-MS² of the deprotonated and ammoniated molecules generated diagnostic product ions from which the molecular masses of the phenolic acid and phenylethanoid moieties, respectively, could be determined. The sugar at C-3' of the core glucose was preferentially lost from the sodiated molecule following CID-MS² , while CID-MSⁿ produced a sodiated product ion from ring cleavage of the core glucose bearing the sugar at C-6'. Evidence of a disaccharide substitution came from a sodiated disaccharide residue in CID-MSⁿ spectra.

CONCLUSIONS

The consistency of PhG dissociation following low-energy CID-MSⁿ of various ions is sufficient to enable annotation of verbascoside-type PhGs in LC/MS analyses of crude plant extracts. This can be achieved on a low-resolution instrument capable of MSⁿ .

Structural Characterization of Flavonoid Glycoconjugates and Their Derivatives with Mass Spectrometric Techniques

Mass spectrometry is currently one of the most versatile and sensitive instrumental methods applied to structural characterization of plant secondary metabolite mixtures isolated from biological material including flavonoid glycoconjugates. Resolution of the applied mass spectrometers plays an important role in structural studies of mixtures of the target compounds isolated from biological material. High-resolution analyzers allow obtaining information about elemental composition of the analyzed compounds. Application of various mass spectrometric techniques, including different systems of ionization, analysis of both positive and negative ions of flavonoids, fragmentation of the protonated/deprotonated molecules and in some cases addition of metal ions to the studied compounds before ionization and fragmentation, may improve structural characterization of natural products. In our review we present different strategies allowing structural characterization of positional isomers and isobaric compounds existing in class of flavonoid glycoconjugates and their derivatives, which are synthetized in plants and are important components of the human food and drugs as well as animal feed.

Rapid Identification of Flavonoid Constituents Directly from PTP1B Inhibitive Extract of Raspberry (Rubus idaeus L.) Leaves by HPLC-ESI-QTOF-MS-MS

Many potential health benefits of raspberry (Rubus idaeus L.) leaves were attributed to polyphenolic compounds, especially flavonoids. In this study, the methanol extract of R. idaeus leaves showed significant protein tyrosine phosphatase-1B (PTP1B) inhibitory activity with IC50 value of 3.41 ± 0.01 µg mL(-1) Meanwhile, a rapid and reliable method, employed high-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry, was established for structure identification of flavonoids from PTP1B inhibitive extract of R. idaeus leaves using accurate mass measurement and characteristic fragmentation patterns. A total of 16 flavonoids, including 4 quercetin derivatives, 2 luteolin derivatives, 8 kaempferol derivatives and 2 isorhamnetin derivatives, were identified. Compounds 3: and 4: , Compounds 6: and 7: and Compounds 15: and 16: were isomers with different aglycones and different saccharides. Compounds 8: , 9: and 10: were isomers with the same aglycone and the same saccharide but different substituent positions. Compounds 11: and 12: were isomers with the same aglycone but different saccharides. Compounds 2: , 8: , 9: and 10: possessed the same substituent saccharide of glycuronic acid. Most of them were reported inR. idaeus for the first time.

Using the knowns to discover the unknowns: MS-based dereplication uncovers structural diversity in 17-hydroxygeranyllinalool diterpene glycoside production in the Solanaceae

Exploring the diversity of plant secondary metabolism requires efficient methods to obtain sufficient structural insights to discriminate previously known from unknown metabolites. De novo structure elucidation and confirmation of known metabolites (dereplication) remain a major bottleneck for mass spectrometry-based metabolomic workflows, and few systematic dereplication strategies have been developed for the analysis of entire compound classes across plant families, partly due to the complexity of plant metabolic profiles that complicates cross-species comparisons. 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) are abundant defensive secondary metabolites whose malonyl and glycosyl decorations are induced by jasmonate signaling in the ecological model plant Nicotiana attenuata. The multiple labile glycosidic bonds of HGL-DTGs result in extensive in-source fragmentation (IS-CID) during ionization. To reconstruct these IS-CID clusters from profiling data and identify precursor ions, we applied a deconvolution algorithm and created an MS/MS library from positive-ion spectra of purified HGL-DTGs. From this library, 251 non-redundant fragments were annotated, and a workflow to characterize leaf, flower and fruit extracts of 35 solanaceous species was established. These analyses predicted 105 novel HGL-DTGs that were restricted to Nicotiana, Capsicum and Lycium species. Interestingly, malonylation is a highly conserved step in HGL-DTG metabolism, but is differentially affected by jasmonate signaling among Nicotiana species. This MS-based workflow is readily applicable for cross-species re-identification/annotation of other compound classes with sufficient fragmentation knowledge, and therefore has the potential to support hypotheses regarding secondary metabolism diversification.

Phenylethanoid glycosides in tepals of Magnolia salicifolia and their occurrence in flowers of Magnoliaceae

Phenylethanoid glycosides were among the major UV-absorbing components in 80% aq. CH3OH extracts of the tepals of Magnolia salicifolia (Siebold & Zucc.) Maxim. (Magnoliaceae; Magnolia subgenus Yulania). Structural characterisation of isolated compounds by spectroscopic and chemical methods revealed three previously unrecorded examples, yulanoside A, yulanoside B and 2'-rhamnoechinacoside, and the known compounds echinacoside and crassifolioside; chromatographic methods also identified verbascoside in the tepal extract. Yulanoside A is the first reported example of a phenylethanoid pentaglycoside, namely hydroxytyrosol 1-O-{β-D-glucopyranosyl-(1→4)-β-D-glucopyranosyl-(1→6)-[3,4-dihydroxycinnamoyl-(→4)][α-L-rhamnopyranosyl-(1→3)][α-L-rhamnopyranosyl-(1→2)]-β-D-glucopyranoside}. A survey of Magnolia sensu lato and Liriodendron (the two genera of Magnoliaceae) suggested that yulanoside A and its deglucosyl derivative (yulanoside B) were a feature of the tepal chemistry of Magnolia subgenus Yulania (except Magnolia acuminata, the sole member of section Tulipastrum, which did not accumulate phenylethanoid glycosides). The two species of Liriodendron and examined examples of Magnolia subgenus Magnolia sections Magnolia and Rytidospermum (subsection Oyama) also accumulated phenylethanoid glycosides in their tepals and in these species, and in subgenus Yulania, the major compounds were one or more of echinacoside, 2'-rhamnoechinacoside, crassifolioside and verbascoside. Levels of phenylethanoid glycosides were found to be much lower in species studied from Magnolia sections Gwillimia, Macrophylla and Rytidospermum (subsection Rytidospermum), although yulanoside A was detectable in M. macrophylla and this may have some bearing on the placement of section Macrophylla, which is currently uncertain. In the isolates of yulanoside B and echinacoside, minor phenylethanoid glycosides were determined to be analogues of these compounds with β-D-xylose at C-3' of the primary glucose rather than α-L-rhamnose.

Phytochemical analysis of Pfaffia glomerata inflorescences by LC-ESI-MS/MS

Pfaffia glomerata contains high levels of β-ecdysone, which has shown a range of beneficial pharmacological effects. The present study demonstrated that inflorescences of P. glomerata contain other important bioactive compounds in addition to β-ecdysone. The identification of compounds from inflorescences using liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) was performed for the first time. The eight compounds identified were β-ecdysone, flavonoid glycosides such as quercetin-3-O-glucoside, kaempferol-3-O-glucoside and kaempferol-3-O-(6-p-coumaroyl)-glucoside, oleanane-type triterpenoid saponins such as ginsenoside Ro and chikusetsusaponin IV, in addition to oleanonic acid and gluconic acid. This study provided information on the phytochemicals contained in P. glomerata inflorescences revealing the potential application of this plant part as raw material for the phytotherapeutic and cosmetic industries.

Enhanced profiling of flavonol glycosides in the fruits of sea buckthorn (Hippophae rhamnoides)

Use of enhanced LC-MS/MS methods to identify common glycosyl groups of flavonoid glycosides enabled better characterization of the flavonoids in fruits of sea buckthorn (Hippophae rhamnoides). The saccharide moieties of 48 flavonol O-glycosides detected in a methanol extract were identified by these methods. Several of the flavonol glycosides were acylated, two of which were isolated and found to be new compounds. Their structures were determined using spectroscopic and chemical methods as isorhamnetin 3-O-(6-O-E-sinapoyl-β-D-glucopyranosyl)-(1→2)-β-D-glucopyranoside-7-O-α-L-rhamnopyranoside (24) and isorhamnetin 3-O-(6-O-E-feruloyl-β-D-glucopyranosyl)-(1→2)-β-D-glucopyranoside-7-O-α-L-rhamnopyranoside (30). Analysis of the acylated glycosyl groups of 24 and 30 by serial mass spectrometry provided evidence to suggest the acylation position of 11 other minor flavonol glycosides acylated with hydroxycinnamic or hydroxybenzoic acids. The nitric oxide scavenging activities of 24 and 30 were compared with those of other flavonoids and with ascorbic acid and the potassium salt of 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (carboxy-PTIO).

Highly glycosylated flavonols with an O-linked branched pentasaccharide from Iberis saxatilis (Brassicaceae)

Four flavonol glycosides isolated from non-flowering leafy shoots of Iberis saxatilis (Brassicaceae) were characterised by spectroscopic and chemical methods as saxatilisins A-D, the 3-O-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)[β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside, 3-O-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)[α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside, 3-O-(6-O-E-sinapoyl)-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)[β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside, and 3-O-(6-O-E-feruloyl)-β-D-glucopyranosyl-(1→3)-α-L-rhamnopyranosyl-(1→2)[β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside of isorhamnetin (3,5,7,4'-tetrahydroxy-3'-methoxyflavone), respectively. Analysis of (2)J(HC) correlations detected with the H2BC (heteronuclear two-bond correlation) pulse sequence aided the unambiguous assignment of glycosidic resonances in the (1)H and (13)C NMR spectra of these compounds. Saxatilisins A, C, and D, are the first flavonol glycosides to be described with a pentasaccharide chain at a single glycosylation site. Several pentaglycosides of kaempferol and quercetin, tentatively assigned as saxatilisin analogues from LC-MS/MS analyses, were present as minor constituents of the extracts.

Distinct chemotypes of Tephrosia vogelii and implications for their use in pest control and soil enrichment

Tephrosia vogelii Hook. f. (Leguminosae) is being promoted as a pest control and soil enrichment agent for poorly-resourced small-scale farmers in southern and eastern Africa. This study examined plants being cultivated by farmers and found two chemotypes. Chemotype 1 (C1) contained rotenoids, including deguelin, rotenone, sarcolobine, tephrosin and α-toxicarol, required for pest control efficacy. Rotenoids were absent from chemotype 2 (C2), which was characterised by prenylated flavanones, including the previously unrecorded examples (2S)-5,7-dimethoxy-8-(3-hydroxy-3-methylbut-1Z-enyl)flavanone, (2S)-5,7-dimethoxy-8-(3-methylbut-1,3-dienyl)flavanone, (2S)-4'-hydroxy-5-methoxy-6″,6″-dimethylpyrano[2″,3″:7,8]flavanone, (2S)-5-methoxy-6″,6″-dimethyl-4″,5″-dihydrocyclopropa[4″,5″]furano[2″,3″:7,8]flavanone, (2S)-7-hydroxy-5-methoxy-8-prenylflavanone, and (2R,3R)-3-hydroxy-5-methoxy-6″,6″-dimethylpyrano[2″,3″:7,8]flavanone. The known compounds (2S)-5-methoxy-6″,6″-dimethylpyrano[2″,3″:7,8]flavanone (obovatin 5-methyl ether) and 5,7-dimethoxy-8-(3-hydroxy-3-methylbut-1Z-enyl)flavone (Z-tephrostachin) were also found in C2. This chemotype, although designated Tephrosia candida DC. in collections originating from the World Agroforestry Centre (ICRAF), was confirmed to be T. vogelii on the basis of morphological comparison with verified herbarium specimens and DNA sequence analysis. Sampling from 13 locations in Malawi where farmers cultivate Tephrosia species for insecticidal use indicated that almost 1 in 4 plants were T. vogelii C2, and so were unsuitable for this application. Leaf material sourced from a herbarium specimen of T. candida contained most of the flavanones found in T. vogelii C2, but no rotenoids. However, the profile of flavonol glycosides was different to that of T. vogelii C1 and C2, with 6-hydroxy-kaempferol 6-methyl ether as the predominant aglycone rather than kaempferol and quercetin. The structures of four unrecorded flavonol glycosides present in T. candida were determined using cryoprobe NMR spectroscopy and MS as the 3-O-α-rhamnopyranosyl(1→6)-β-galactopyranoside-7-O-α-rhamnopyranoside, 3-O-α-rhamnopyranosyl(1→2)[α-rhamnopyranosyl(1→6)]-β-galactopyranoside, 3-O-α-rhamnopyranosyl(1→2)[α-rhamnopyranosyl(1→6)]-β-galactopyranoside-7-O-α-rhamnopyranoside, and 3-O-α-rhamnopyranosyl(1→2)[(3-O-E-feruloyl)-α-rhamnopyranosyl(1→6)]-β-galactopyranosides of 6-hydroxykaempferol 6-methyl ether. Tentative structures for a further 37 flavonol glycosides of T. candida were assigned by LC-MS/MS. The correct chemotype of T. vogelii (i.e. C1) needs to be promoted for use by farmers in pest control applications.