Identification of cardiac glycosides in fractions from Periploca forrestii by high-performance liquid chromatography/diode-array detection/electrospray ionization multi-stage tandem mass spectrometry and liquid chromatography/nuclear magnetic resonance

Cardiac glycosides are a class of naturally occurring compounds that are characterized by some interesting biological activities and are widely distributed in the plant kingdom and can also be found in some animals. There is an interest in the chemical characterization of these molecules due to their toxicity and their use in medicines. In the study reported here, a combination of electrospray ionization tandem mass spectrometry with high-performance liquid chromatography equipped with diode-array detector (HPLC-DAD/ESI-MSⁿ), and hyphenation to both liquid chromatography and nuclear magnetic resonance spectroscopy (HPLC/NMR) were utilized for the on-line analyses of cardiac glycosides from Periploca forrestii. The fragmentation patterns and ¹H NMR spectra of nine isolated cardiac glycosides were investigated; their fragmentation rules and ¹H NMR spectral characteristics were summarized and applied to the structural identification of similar constituents in fractions from P. forrestii. As a result, a total of nine trace cardiac glycosides were tentatively determined by analyses of accurate molecular masses, representative fragment ions and characteristic ¹H NMR signals provided by HPLC/high-resolution mass spectrometry (HRMS), HPLC-DAD/ESI-MSⁿ and HPLC/¹H NMR experiments, respectively. Of these, eight (2–9) are new compounds and one (1) is reported from P. forrestii for the first time. Results of the present study can benefit the rapid identification and targeted isolation of new cardiac glycosides from crude plant extracts.     

Resin glycosides from the aerial parts of Operculina turpethum

Three glycosidic acids, turpethic acids A−C, and two intact resin glycosides, turpethosides A and B, all having a common pentasaccharide moiety and 12-hydroxy fatty acid aglycones of different chain lengths, were obtained from the aerial parts of Operculina turpethum. Their structures were elucidated by spectroscopic analyses and chemical correlations. The aglycones were characterized as 12-hydroxypentadecanoic acid in two compounds, 12-hydroxyhexadecanoic acid in two other components, and 12-hydroxyheptadecanoic acid in the fifth compound, which were all confirmed by synthesis. The absolute configurations of these aglycones were all established as S by Mosher’s method. These compounds represent the first examples of resin glycosides with a monohydroxylated 12-hydroxy fatty acid as an aglycone, and one compound is the first described resin glycoside having a hydroxylated C₁₇ fatty acid as its aglycone.     

Iridoids from the flowers of Gardenia jasminoides Ellis and their chemotaxonomic significance

Phytochemical investigation of the flowers of Gardenia jasminoides Ellis (Rubiaceae Gardenia) resulted in the isolation and identification of four iridoid aglycones (1–4) and eleven iridoid glycosides (5–15). This is the first report of the occurrence of these compounds in the genus Gardenia: garjasmine (1), dunnisin (2), α-gardiol (3), β-gardiol (4), diffusoside A (6), diffusoside B (7), genameside C (13), and deacetylasperulosidic acid (14). The chemotaxonomic significance and biosynthetic pathways of these iridoid aglycones (1–4) and iridoid glycosides (5–15) are summarized. Iridoid aglycones and iridoid glycosides are considered as important chemotaxonomic markers in Rubiaceae and this work indicates that the iridoid aglycones (1–4) have a limited distribution in the Rubiaceae.     

Computational annotation of plant metabolomics profiles via a novel network-assisted approach

Mass spectrometry (MS) has become the analytical method of choice in plant metabolomics. Nevertheless, metabolite annotation remains a major challenge and implies the integration of structural searches in compound libraries with biological knowledge inferred from metabolite regulation studies. Here we propose a novel integrative approach to process and exploit the rich structural information contained in in-source fragmentation patterns of high-resolution LC–MS profiles. In this approach, a correlation matrix is first calculated from individual mass features extracted by xcms processing. Mass feature co-regulation patterns corresponding to metabolite in-source fragmentation are then detected and assembled into compound spectra using the R package CAMERA and processed for in silico fragment-based structure elucidation using MetFrag. We validate the performance of this approach for the rapid annotation of the twelve largest compound spectra, including four O-acyl sugars and six 17-hydroxygeranyllinalool diterpene glycosides in metabolic profiles of insect-attacked Nicotiana attenuata leaves. Additionally, we demonstrate the power of refining MetFrag metabolite annotations based on co-regulation patterns between known and unknown compounds in the correlation matrix and proposed structural annotations on two previously un-characterized O-acyl sugars. In summary, this novel approach facilitates compound annotation from in-source fragmentation patterns using correlation between intensities of mass features of one or several metabolites. Additionally, this analysis provides further support that insect herbivory activates major metabolic reconfigurations in N. attenuata leaves.     

Cytotoxic cardiac glycosides and coumarins from Antiaris toxicaria

Eight new cardiac glycosides/aglycones (antiaritoxiosides A–G, 1–7, and antiarotoxinin B, 8), two new coumarins (anticarins A–B, 41–42), and two new flavanones (antiarones L–K, 43–44) were isolated from trunk bark of Antiaris toxicaria together with 53 known compounds. The new structures were established by extensive analysis of spectroscopic data. Compound 1 (10-carboxy and 3α-hydroxy) and compounds 3–6 (10-hydroxy) contain unique substituents that are rarely found in cardiac glycosides. The cytotoxic effects of isolated compounds against ten human cancer cell lines, KB, KB-VIN, A549, MCF-7, U-87-MG, PC-3, 1A9, CAKI-1, HCT-9 and S-KMEL-2, were tested using the sulforhodamine B assay. Five compounds (12, 16, 20, 22, and 31) showed significant cytotoxicity against all ten cancer cell lines, with notable potency at the ng/mL level against some cell lines, which merits further development as clinical trial candidates.     

Metabolite profiling of polar steroid constituents in the Far Eastern starfish Aphelasterias japonica using LC–ESI MS/MS

Numerous polar steroids are characteristic metabolites of starfish which determine physiological activities of their extracts. The Far Eastern starfish Aphelasterias japonica is a rich source of different steroid glycosides and polyhydroxysteroids. For detailed analysis of complicated mixture of steroids from this species, isolated by solid-phase extraction, a liquid chromatography–electrospray tandem mass spectrometry (LC–ESI MS/MS) approach was selected and applied. The characteristic fragmentations in ESI MS/MS spectra of steroid glycosides allowed determining types of aglycones, presence of sulfate groups, sugar sequences as well as branching. In addition, main structural features of polyhydroxylated polar steroids including position of hydroxylation and level of sulfation were also established. Totally, 68 metabolites, comprising of 33 asterosaponins, 28 sulfated polyhydroxysteroid mono- and biosides and 7 sulfated polyhydroxysteroid compounds were found by this method. In addition to 15 previously isolated compounds from A. japonica, many new steroid glycosides including asterosaponins with unusual carbohydrate chains were discovered and characterized by their ESI product ion mass spectra. Some details of biosynthesis of polyhydroxylated steroids and their conjugated forms in the species studied such as a role of sulfation and order of introduction of hydroxyl group in A. japonica were proposed.     

Brevistylumsides A and B,diterpene glycosides from Illicium brevistylum

A phytochemical investigation of 80% ethanol extract of Illicium brevistylum (Illiciaceae) resulted in the isolation of two new diterpene glycosides (1 and 2), together with three known diterpenes (3–5). The structures of the new compounds were deduced on the basis of comprehensive spectroscopic analysis including IR, HR-ESI–MS, ¹H NMR, ¹³C NMR, DEPT, COSY, HMBC and HSQC. The structures of known compounds 3–5 were identified by comparison of their spectral data with those reported in the literature.     

A new guaianolide glucoside from aerial parts of Lactuca aculeata

From aerial parts of Lactuca aculeata one new and ten known sesquiterpene lactone aglycones and glycosides were isolated, including four lactucin-type guaianolides, six costus lactone-type guaianolides and one melampolide-type germacranolide. The structure of the new compound was determined as 11β-hydroxycrepidiaside B by high resolution mass spectrometry and 1D- and 2D-NMR spectroscopy.     

Hylaranins: prototypes of a new class of amphibian antimicrobial peptide from the skin secretion of the oriental broad-folded frog,Hylarana latouchii

Amphibian skin secretions contain a broad spectrum of biologically active compounds, particularly antimicrobial peptides, which are considered to constitute a first line of defence against bacterial infection. Here we describe the identification of two prototype peptides representing a novel structural class of antimicrobial peptide from the skin secretion of the oriental broad-folded frog, Hylarana latouchii. Named hylaranin-L1 (GVLSAFKNALPGIMKIIVamide) and hylaranin-L2 (GVLSVIKNALPGIMRFIAamide), both peptides consist of 18 amino acid residues, are C-terminally amidated and are of unique primary structures. Their primary structures were initially deduced by MS/MS fragmentation sequencing from reverse-phase HPLC fractions of skin secretion that demonstrated antimicrobial activity. Subsequently, their precursor-encoding cDNAs were cloned from a skin secretion-derived cDNA library and their primary structures were confirmed unequivocally. Synthetic replicates of both peptides exhibited broad-spectrum antimicrobial activity with mean inhibitory concentrations (MICs) of 34 μM against Gram-negative Escherichia coli, 4.3 μM against Gram-positive Staphylococcus aureus and 4–9 μM against the yeast, Candida albicans. Both peptides exhibited little haemolytic activity (<6 %) at the MICs for S. aureus and C. albicans. Amphibian skin secretions thus continue to provide novel antimicrobial peptide structures that may prove to be lead compounds in the design of new classes of anti-infection therapeutics.     

Isolation and purification of flavonoid glucosides from Radix Astragali by high-speed counter-current chromatography

High-speed counter-current chromatography methods, combined with resin chromatography were applied to the separation and purification of flavonoid glycosides from the Chinese medicinal herb, Radix Astragali. Five flavonoid glycosides, namely calycosin-7-O-β-d-glucoside, ononin, (6aR, 11aR)-9,10-dimethoxypterocarpan-3-O-β-d-glucoside, (3R)-2′-hydroxy-3′,4′-dimethoxyisoflavan-7-O-β-d-glucoside and calycosin-7-O-β-d-glucoside-6′′-O-acetate, were obtained. Among them, calycosin-7-O-β-d-glucoside-6′′-O-acetate was preparatively separated from Radix Astragali for the first time. Their structures were identified by ESI–MS, ¹H NMR, ¹³C NMR, and 2D NMR.     

Identification of maloyl glucans from Euphorbia tirucalli by ESI-(−)-FT-ICR MS analyses

Euphorbia tirucalli aerial parts are popularly used in Brazil for cancer treatment. The elution of the aqueous extract of the plant on silica gel C-18 cartridge furnished a water-soluble fraction, which was analyzed directly into the electrospray ionization (ESI) source combined with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and characterized as a mixture of malic acid glycosides 1–5. The compounds were detected in their deprotonated form [M−H]⁻, where their exact mass (mass error lower than 1 ppm), molecular formula (C_nH_hO_o), double bond equivalent (DBE) and connectivity were determined from ESI-(−)-MS and ESI-(−)-MS/MS experiments. The presence of malic acid and glucose, as part of the structures, could originate from crassulacean acid metabolism (CAM) of the plant.     

Spirostanol tetraglycosides from Ypsilandra thibetica

Phytochemical reinvestigation on the whole plants of Ypsilandra thibetica obtained five new spirostane glycosides, ypsilandrosides H-L (1-5), and a known saponin polyphylloside III (6). Among them, 1 and 2 are the first spirostane glycosides which possess novel 5(6 → 7) abeo-steroidal aglycones. Compounds 3 and 4 are rare saponins whose aglycones contain a hydroxyl group at C-7. Their structures were elucidated on the basis of MS, 1D and 2D NMR spectroscopic analysis and chemical evidences. The isolated compounds were evaluated for their cytotoxic activity on five tumor cell lines.     

Selenoglucosinolates and their metabolites produced in Brassica spp. fertilised with sodium selenate

Glucosinolates are sulphur-containing glycosides found in many Brassica spp. that are important because their aglycone hydrolysis products protect the plant from herbivores and exhibit anti-cancer properties in humans. Recently, synthetically produced selenium analogues have been shown to be more effective at suppressing cancers than their sulphur counterparts. Although selenium is incorporated into a number of Brassica amino acids and peptides, firm evidence has yet to be presented for the presence of selenium in the glucosinolates and their aglycones in planta. In this study broccoli and cauliflower florets, and roots of forage rape, all obtained from plants treated with sodium selenate, were analysed for the presence of organoselenides. GC–MS analysis of pentane/ether extracts identified six organoselenium compounds including selenium analogues of known myrosinase-derived Brassica volatiles: 4-(methylseleno)butanenitrile, 5-(methylseleno)pentanenitrile, 3-(methylseleno)propylisothiocyanate, 4-(methylseleno)butylisothiocyanate, and 5-(methylseleno)pentylisothiocyanate. LC–MS analysis of ethanolic extracts identified three selenoglucosinolates: 3-(methylseleno)propylglucosinolate (glucoselenoiberverin), 4-(methylseleno)butylglucosinolate (glucoselenoerucin), and 5-(methylseleno)pentylglucosinolate (glucoselenoberteroin). LC–MS/MS analysis was used to locate the position of the selenium atom in the selenoglucosinolate and indicates preferential incorporation of selenium via selenomethionine into the methylselenyl moiety rather than into the sulphate or β-thioglucose groups. In forage rape, selenoglucosinolates and their aglycones (mainly isothiocyanates), occurred at concentrations up to 10% and 70%, respectively, of their sulphur analogues. In broccoli, concentrations of the selenoglucosinolates and their aglycones (mainly nitriles) were up to 60% and 1300%, respectively of their sulphur analogues. These findings indicate the potential for the incorporation of high levels of selenium into Brassica glucosinolates.     

New phenolic glycosides isolated from Penthorum chinense Pursh

A phytochemical investigation from 80% ethanol extract of Penthorum chinense Pursh (Saxifragaceae) resulted in the isolation of three new phenolic glycosides (1–3), together with three known phenolic glycosides (4–6). The structures of the new compounds were deduced from their comprehensive spectroscopic analysis including IR, HR-EI-MS, ¹H NMR, ¹³C NMR, DEPT, COSY, HMBC and HMQC. And the structures of known compounds 4–6 were identified by comparison of their spectral data with those reported in the literature.     

Trinorguaian and germacradiene type sesquiterpenes along with flavonoids from the herbs of Pimpinella cappadocica Boiss. & Bal.

A trinorguaian-type sesquiterpenoid named kanalpin and two acylated flavonol glycosides named erzurumin and ilicanin have been isolated from Pimpinella cappadocica Boiss. & Bal., together with three known sesquiterpenoids and six known flavonoid glycosides. The structures of these compounds were determined by extensive spectroscopic (UV, IR, EI–MS, APCI–MS, HR-ESI-MS, 1D and 2D NMR) analyses. Antioxidant capacity of isolated secondary metabolites containing extracts of P. cappadocica was evaluated.     

Blechnosides A and B: Ecdysteroid glycosides from Blechnum minus

The structures of two new ecdysteroid glycosides from Blechnum minus have been shown, on the basis of chemical, mass spectral, ¹H NMR and ¹³CNMR spectral evidence, to be 2-deoxyecdysone 3-β-d-glycopyranoside (blechnoside A) and 2-deoxyecdysone 25-β-d-glucopyranoside (blechnoside B).     

Spectral trees as a robust annotation tool in LC–MS based metabolomics

The identification of large series of metabolites detectable by mass spectrometry (MS) in crude extracts is a challenging task. In order to test and apply the so-called multistage mass spectrometry (MS ⁿ ) spectral tree approach as tool in metabolite identification in complex sample extracts, we firstly performed liquid chromatography (LC) with online electrospray ionization (ESI)?CMS ⁿ , using crude extracts from both tomato fruit and Arabidopsis leaf. Secondly, the extracts were automatically fractionated by a NanoMate LC-fraction collector/injection robot (Advion) and selected LC-fractions were subsequently analyzed using nanospray-direct infusion to generate offline in-depth MS ⁿ spectral trees at high mass resolution. Characterization and subsequent annotation of metabolites was achieved by detailed analysis of the MS ⁿ spectral trees, thereby focusing on two major plant secondary metabolite classes: phenolics and glucosinolates. Following this approach, we were able to discriminate all selected flavonoid glycosides, based on their unique MS ⁿ fragmentation patterns in either negative or positive ionization mode. As a proof of principle, we report here 127 annotated metabolites in the tomato and Arabidopsis extracts, including 21 novel metabolites. Our results indicate that online LC?CMS ⁿ fragmentation in combination with databases of in-depth spectral trees generated offline can provide a fast and reliable characterization and annotation of metabolites present in complex crude extracts such as those from plants.     

Four new phenylpropanoid glycosides from Clausena dunniana var. robustaand their antiinflammatory activities

Four new phenylpropanoid glycosides, clausenaglycosides A–D (1–4), along with fourteen known compounds (5–18) were isolated from the leaves and twigs of Clausena dunniana var. robusta. The structures of the isolated compounds were determined by a combination of 1D and 2D NMR, MS, and CD spectroscopic data. The NF-κB inhibitory activity was evaluated in HeLa cells. Compound 17 displayed a moderate inhibition against NF-κB activation.     

Mass spectrometric characterization of siderophores produced by <Emphasis Type="Italic">Pseudomonas taiwanensis</Emphasis> VLB120 assisted by stable isotope labeling of nitrogen source

The structures of three previously unknown siderophores produced by the fluorescent, biotechnologically relevant Pseudomonas taiwanensis (P. taiwanensis) VLB120 bacteria were elucidated by means of hydrophilic interaction liquid chromatography (HILIC) hyphenated to high-resolution tandem mass spectrometry (HRMS/MS). They could be verified as iron(III)- and aluminum(III) complexes as well as the protonated molecules of the siderophores formed by in-source fragmentation. The siderophores were separated according to their different acyl side chains and additionally according their central ions. One of the siderophores was identified as pyoverdine with a malic acid (hydroxy succinic acid) amide side chain and a peptide moiety consisting of Orn-Asp-OHAsn-Thr-AcOHOrn-Ser-cOHOrn. The other analytes were assigned to an azotobactin with the identical peptide chain linked to the characteristic chromophoric unit and a pyoverdine with a variation in the amino acid sequence. Proline is directly linked to the pyoverdine chromophore instead of ornithine. Acidic and enzymatic hydrolyses were carried out to analyze the individual amino acids. Beside OHAsn, each amino acid of the peptide part was identified by HILIC–HRMS and comparison to authentic standards. Additionally, ¹⁵N-labeled cellular supernatants were analyzed by means of HRMS/MS. The results of the MS/MS experiments complemented by accurate mass data facilitated elucidation of the structures studied in this work and provided further confirmation of the three recently described pyoverdines of P. taiwanensis VLB120 (Baune et al. in Biometals 30:589–597, 2017.  https://doi.org/10.1007/s10534-017-0029-7).