Fragmentation study of protonated chalcones by atmospheric pressure chemical ionization and tandem mass spectrometry

Structure-based identification and pathway elucidation of flavonoids in Camptotheca acuminate

Flavonoid metabolism in Camptotheca acuminate remained an untapped area for years. A tandem MS approach was used and focused on the mining and characterizing of flavonoids in mature C. acuminate. Fifteen new flavonoids and forty-three known flavonoids, including fifteen flavone analogs, sixteen flavonol analogs, seven flavanone analogs, six chalcone analogs, four xanthone analogs, ten flavane analogs were mined and identified based on their MS/MS fragments. Fifty-three of them were firstly characterized in C. acuminate. Eight biosynthetic precursors for these flavonoids were also identified. We constructed a specific metabolic map for flavonoids according to their relative contents in the flowers, fruits, stems, and leaves of C. acuminate. Furthermore, the most probable genes involved in chalcone biosynthesis, flavonoid hydroxylation, methylation, and glycosylation were further mined and fished in the gene reservoir of C. acuminate according to their conserved domains and co-expression analysis. These findings enable us to acquire a better understanding of versatile flavonoid metabolism in C. acuminate.

Chiral probe for mass spectrometric identification and quantitation of levodropropizine and its enantiomer, dextrodropropizine

Acyl chlorides react rapidly with hydroxyl and amino groups in the absence of catalysts and therefore hold great promise for chiral mass spectrometry. Herein, a tandem mass spectrometry method based on derivatization with (-)-camphanic acid chloride as a simple chiral probe was developed for the highly sensitive detection and quantitation of levodropropizine and its enantiomer, namely, dextrodropropizine. The diastereomeric derivatization products were quantified based on the relative abundances of their fragment ions produced upon collision-induced dissociation in positive-ion mode. The reactive site was elucidated using nuclear magnetic resonance spectroscopy and two-dimensional total correlation spectroscopy, and the reaction mechanism was proved by stoichiometry studies. The degree of isomer recognition was investigated at different collision energies and determined as R_chiral  ≈ 1.5. Thus, this study gives the way to the mass spectrometric identification and quantitation of levodropropizine and its enantiomer, and the developed method represents a new and practical technique for rapid and sensitive determination and quality control of enantiomers of chiral drugs.

Synthesis and characterization of a series of N,N'-substituted urea derivatives by using electrospray ionization tandem mass spectrometry: Differentiation of positional isomers

RATIONALE

Characterization of N,N'-substituted ureas was found to be challenging by nuclear magnetic resonance (NMR) spectroscopy, particularly N-di- and tri-alkylated ureas because of the absence of adjacent protons. In the present study, electrospray ionization tandem mass spectrometry has been used to differentiate positional isomeric pairs and to characterize a series of N,N'-substituted ureas, as these compounds have significant importance for drug discovery. Additionally, urea is an essential functionality in several bioactive compounds as well as a variety of clinically approved therapies.

METHODS

High-resolution electrospray ionization tandem mass spectrometry (ESI-HR-MS/MS) has been used to characterize a series of N,N'-substituted urea derivatives and differentiate two pairs of positional isomers. The data was acquired by Xcaliber application in positive ionization mode.

RESULTS

ESI-HR-MS/MS spectra of [M + H]⁺ ions of the positional isomeric urea derivatives 8a and 8b show distinct fragmentation patterns. For example, the MS/MS spectrum of the [M + H]⁺ ion of isomer 8a displays the abundant fragment ion at m/z 285.1595, which was totally absent in isomer 8b. This would be plausibly formed by the cleavage of the C-N bond of the urea group with the elimination of the isocyanate moiety. In contrast, the MS/MS spectrum of the [M + H]⁺ ion of isomer 8b shows an intense ion at m/z 311.1389 which is completely absent in isomer 8a which would be formed by the cleavage of the C-N bond attached to the ring nitrogen. Similarly, another pair of positional isomers, 8c and 8d, have been clearly distinguished by their fragmentation behaviour. In addition, a series of N,N'-substituted urea derivatives were studied to investigate the impact of different substitution on the fragmentation behaviour.

CONCLUSIONS

The present study demonstrates that ESI-HR-MS/MS can be used to differentiate pairs of N,N'-substituted urea positional isomers and characterize a series of derivatives. It was observed that a characteristic fragment ion was formed by the C-N bond cleavage with the elimination of an isocyanate moiety. The proposed mechanism of fragmentation was supported by the change in the fragmentation pathway upon alkylation of the NH. In order to generalize this fragmentation pattern, a series of N-alkylated ureas was synthesized and studied by MS/MS.

Profiling the Philippine Blue: Liquid chromatography/mass spectrometry-based metabolomics study on Philippine Indigofera

RATIONALE

High-throughput liquid chromatography/mass spectrometry (LC/MS) analysis presents an interesting platform for natural dyes research. A particular example is the assessment of the dynamic changes in fermentation mixtures of Philippine Indigofera, and in the investigation of commercially available indigo prepared using traditional and optimized methods.

METHODS

Leaves from Indigofera tinctoria and Indigofera suffruticosa were subjected to methanolic extraction and aqueous fermentation for 48 h. Indigo powders prepared following 2-day and 15-day fermentation were also subjected to profiling using ultra-high-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/QTOF-MS). MS² spectra were annotated through a library search in the community-curated Global Natural Products Social Molecular Networking (GNPS). Spectra with no library hits in GNPS were annotated by analysis of their fragmentation pathways.

RESULTS

UHPLC/MS-based detection and fragmentation analysis led to characterization of leucoindigo and the unreported tryptanthrin intermediate, 5a-hydroxy-5,5a-dihydroindolo[2,1-b]quinazoline-6,12-dione, in the fermentation extract of I. tinctoria leaves. Indigo-associated metabolites were absent in an Indigofera specimen in Laguna Province, which explained why it did not produce blue dye. Locally produced indigo was abundant in indigotin and indirubin, differentiated based on product ions with the corresponding predicted fragmentation pattern. The relative intensity of indigotin, however, decreased with the traditional process of extended fermentation to produce indigo.

CONCLUSIONS

The study is the first to demonstrate simultaneous MS-based analysis of reaction intermediates, indigotin dye, side products, and catabolites on actively transforming fermentation extracts of I. tinctoria. New results include annotated mass spectra for leucoindigo, and for the unreported 5a-hydroxy-5,5a-dihydroindolo[2,1-b]quinazoline-6,12-dione, which is probably an intermediate in tryptranthrin synthesis. The proposed fragmentation schemes could guide the annotation of analogous compounds in complex mixtures.

Fragmentation pathways and differentiation of positional isomers of sorafenib and structural analogues by ESI-IT-MSn and ESI-Q-TOF-MS/MS coupled with DFT calculations

Sorafenib is an orally active multikinase inhibitor for the treatment of renal cell carcinoma. A series of sorafenib structural analogues were investigated in this work for their gas-phase fragmentation behaviors using electrospray ionization ion trap mass spectrometry and quadrupole time-of-flight mass spectrometry in the positive mode. The possible fragmentation pathways were proposed based on ESI-MS/MS data and theoretical calculation. Different from the typical α-cleavage of amide, consecutive reactions that involved elimination of H₂ O and CH₃ NC were observed for 2-pyridinecarboxamide derivatives, which were followed by the formation of a stabilized 7-membered ring carbocation by loss of CO. Two possible protonation sites occurred at carbonyl oxygen atoms for aryl-urea derivatives and the α-cleavage of urea was the main fragmentation pathways, which was followed by the formation of stable benzo [d] oxazole ring characteristic to aryl-urea derivatives. The relative abundance of characteristic fragment ions and the energy-resolved breakdown curves were used to distinguish the 4 sets of positional isomers of sorafenib and analogues. The methodology and results of the present work would contribute to the chemical structure identification of other structural analogues and the potential impurities presented in active pharmaceutical ingredients and drug formulations.

Application of ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry in identification of three isoflavone glycosides and their corresponding metabolites

RATIONALE

Metabolites of isoflavones have attracted much attention in recent years due to their potential bioactivities. However, the complex constituents of the metabolic system and the low level of metabolites make them difficult to analyze. A mass spectrometry (MS) method was applied in our identification of metabolites and study of their fragmentation pathways due to the advantages of rapidity, sensitivity, and low level of sample consumption.

METHODS

Three isoflavone glycosides and their metabolites were identified using ultra-performance liquid chromatography coupled with quadrupole time-of-flight tandem mass spectrometry (UPLC/QTOF-MS). These metabolites were obtained by anaerobically incubating three isoflavone glycosides with human intestinal flora. The characteristic fragments of isoflavone glycosides and their metabolites were used for the identification work.

RESULTS

Two metabolites from ononin, three metabolites from irilone-4'-O-β-D-glucoside, and five metabolites from sissotrin were identified respectively by the retention time (RT), accurate mass, and mass spectral fragmentation patterns. The losses of the glucosyl group, CO from the [M+H]⁺ ion were observed for all the three isoflavone glycosides. The characteristic retro-Diels-Alder (RDA) fragmentation patterns were used to differentiate the compounds. The metabolic pathways of the three isoflavone glycosides were proposed according to the identified chemical structures of the metabolites.

CONCLUSIONS

A selective, sensitive and rapid method was established for detecting and identifying three isoflavone glycosides and their metabolites using UPLC/QTOF-MS. The established method can be used for further rapid structural identification studies of metabolites and natural products. Furthermore, the proposed metabolic pathways will be helpful for understanding the in vivo metabolic process of isoflavone.

Fragmentation pathways of 2-substituted pyrrole derivatives using electrospray ionization ion trap and electrospray ionization quadrupole time-of-flight mass spectrometry

RATIONALE

Pyrrole derivatives are of considerable importance and are present in a wide range of natural products and used extensively in drug discovery. Fragmentation pathway studies play an important role in the structural identification of pyrrole derivatives. As a part of our ongoing work on heterocycles, fragmentation pathways of 2-substituted pyrrole derivatives were investigated by mass spectrometry (MS).

METHODS

Twelve pyrrole derivatives were synthesized and analyzed. Low-resolution fragmentation ions of all the compounds were generated by ion trap mass spectrometry (ITMS(n) ) with an electrospray ionization (ESI) source in positive mode. Hybrid quadrupole time-of-flight mass spectrometry (QTOFMS) was used to determine the elemental compositions of the resultant product ions.

RESULTS

The side-chain substituents at the 2-position influence the fragmentation pathways. Typical losses of H2 O, aldehydes and pyrrole moieties from the [M + H](+) ion are observed for the compounds with side chains bearing aromatic groups at the 2-position of the pyrrole. However, losses of H2 O, alcohols and C3 H6 are the main cleavage pathways for compounds 6 and 12 with nonphenyl-substituted side chains at the 2-position.

CONCLUSIONS

Typical fragmentation mechanisms of 2-substituted pyrrole derivatives are proposed and elucidated based on the observations of ITMS(n) and QTOFMS spectra. The results showed that the fragmentation pathways were remarkably influenced by the side-chain substituents at the 2-position of pyrrole. This investigation should have value in the structural identification of this series of molecules or compounds with similar structures.

Kinetic and thermodynamic control of protonation in atmospheric pressure chemical ionization

For p-(dimethylamino)chalcone (p-DMAC), the N atom is the most basic site in the liquid phase, whereas the O atom possesses the highest proton affinity in the gas phase. A novel and interesting observation is reported that the N- and O-protonated p-DMAC can be competitively produced in atmospheric pressure chemical ionization (APCI) with the change of solvents and ionization conditions. In neat methanol or acetonitrile, the protonation is always under thermodynamic control to form the O-protonated ion. When methanol/water or acetonitrile/water was used as the solvent, the protonation is kinetically controlled to form the N-protonated ion under conditions of relatively high infusion rate and high concentration of water in the mixed solvent. The regioselectivity of protonation of p-DMAC in APCI is probably attributed to the bulky solvent cluster reagent ions (S(n)H(+)) and the analyte having different preferred protonation sites in the liquid phase and gas phase.

Unravelling ionization and fragmentation pathways of carotenoids using orbitrap technology: a first step towards identification of unknowns

Vegetables are a major source of carotenoids and carotenoids are identified as potentially important natural antioxidants that may aid in the prevention of several human chronic degenerative diseases. Characterization of carotenoids in organic biological matrices is a crucial step in any research valorization trajectory. This study reports for the first time the use of high mass resolution and exact mass orbitrap technology for the elucidation of carotenoid fragmentation pathways. This contributes to the generation of new tools for identifying unknown carotenoids based on fragmentation patterns. Two different chromatographic methods making use of different mobile phases resulted in the generation of different ion species because of the large influence of the mobile phase solvent composition on ionization. It was shown that depending on the molecular ion species that are generated (protonated ions or radical molecular ions), different fragments are formed when applying higher energy collisional dissociation. Fragmentation and the abundance of fragments provide valuable structural information on the type of functional groups, the polyene backbone and the location of double bonds in ring structures of carotenoids. Furthermore, coherence between specific substructures in the molecules and characteristic fragmentation patterns was observed allowing the assignment of fragmentation patterns for carotenoid substructures that can theoretically be extrapolated to carotenoids with similar (sub)structures. Differentiation between isomeric carotenoids by compound specific fragments could however not be made for all the isomeric groups under study. As a wide variety of isomeric forms of carotenoids exist in nature, the combination of good chromatographic separation with high resolution mass spectrometry and other complementary qualitative structure elucidation techniques such as a photo diode array detector and/or nuclear magnetic resonance spectroscopy are indispensable for unambiguous identification of unknown carotenoids.

Tandem mass spectrometry based investigation of cinnamylideneacetophenone derivatives: valuable tool for the differentiation of positional isomers

Cinnamylideneacetophenones have been extensively used as versatile starting materials in numerous different transformations. The structural characterization of this type of compounds is, therefore, of crucial importance since it can give information on the chemistry, reactivity and also the potential biological activity of this type of compounds. Thus, 24 derivatives were systematically studied by tandem mass spectrometry (MS(2)) with electrospray ionization (ESI), in positive ion mode. The protonated molecules, [M + H](+), formed under ESI conditions were induced to dissociate and the fragmentation patterns were studied. The information collected provided important structural information on the type of substituents present and constitute an important database concerning this family of compounds. Overall, it was found that the substitution pattern of the cinnamylideneacetophenone derivatives changes the ESI-MS(2) fragmentation considerably. These results indicate that ESI-MS(2) is a useful technique for distinguishing positional isomers of these cinnamylideneacetophenone derivatives.

Structural identification of Diindole agonists of the aryl hydrocarbon receptor derived from degradation of indole-3-pyruvic acid

Aerobic incubation of the tryptophan transamination/oxidation product indole-3-pyruvic acid (I3P) at pH 7.4 and 37 degrees C yielded products with activity as Ah receptor (AHR) agonists. The extracts were fractionated using HPLC and screened for AHR agonist activity. Two compounds were identified as agonists: 1,3-di(1H-indol-3-yl)propan-2-one (1) and 1-(1H-indol-3-yl)-3-(3H-indol-3-ylidene) propan-2-one (2), with the potency of 2 being 100-fold > 1 [ Nguyen et al. ( 2009 ) Chem. Res. Toxicol. , DOI: 10.1021/tx900043s . ]. Both 1 and 2 showed UV spectra indicative of indole. The molecular formulas were established by high-resolution mass spectrometry (HRMS), and the structures were determined by a combination of NMR methods, including (1)H, natural abundance (13)C, and two-dimensional methods. An intermediate in the oxidation of I3P to 1 is 3-hydroxy-2,4-di(1H-indol-3-yl)butanal (HRMS established the presence of a compound with the formula C(20)H(19)N(2)O(2)). Compound 1 was converted to 2 in air or (faster) with mild oxidants, and 2 could be further oxidized to 1,3-di(3H-indol-3-ylidene)propan-2-one. Determination of the structures allowed estimation of the molar Ah receptor agonist activity of these natural products, similar in potency to known classical AHR inducers.

Gas-phase nazarov cyclization of protonated 2-methoxy and 2-hydroxychalcone: an example of intramolecular proton-transport catalysis

Upon CA, ESI generated [M + H](+) ions of chalcone (benzalacetophenone) and 3-phenyl-indanone both undergo losses of H(2)O, CO, and the elements of benzene. CA of the [M + H](+) ions of 2-methoxy and 2-hydroxychalcone, however, prompts instead a dominant loss of ketene. In addition, CA of the [M + H](+) ions of 2-methoxy-beta-methylchalcone produces an analogous loss of methylketene instead. Furthermore, the [M + D](+) ion of 2-methoxychalcone upon CA eliminates only unlabeled ketene, and the resultant product, the [M + D - ketene](+) ion, yields only the benzyl-d(1) cation upon CA. We propose that the 2-methoxy and 2-hydroxy (ortho) substituents facilitate a Nazarov cyclization to the corresponding protonated 3-aryl-indanones by mediating a critical proton transfer. The resultant protonated indanones then undergo a second proton transport catalysis facilitated by the same ortho substituents producing intermediates that eliminate ketene to yield 2-methoxy- or 2-hydroxyphenyl-phenyl-methylcarbocations, respectively. The basicity of the ortho substituent is important; for example, replacement of the ortho function with a chloro substituent does not provide an efficient catalyst for the proton transports. The Nazarov cyclization must compete with an alternate cyclization, driven by the protonated carbonyl group of the chalcone that results in losses of H(2)O and CO. The assisted proton transfer mediated by the ortho substituent shifts the competition in favor of the Nazarov cyclization. The proposed mechanisms for cyclization and fragmentation are supported by high-mass resolving power data, tandem mass spectra, deuterium labeling, and molecular orbital calculations.

Electron ionization mass spectra and tautomerism of substituted 2-phenacylquinolines

Tautomerism has been studied conventionally in solutions or in the solid state. However, the importance of mass spectrometry in the gas phase was realized relatively late. 2-Phenacylquinolines are known to undergo ketimine-enaminone tautomerism. The ratio of tautomers is dependent on the nature of the phenyl ring substituent and the Hammett substituent constants sigma. Theoretical calculations indicate the presence of ketimine and enaminone tautomers in the gas phase. The electron ionization mass spectra of eight 2-phenacylquinolines (ketimine form) were recorded at 70 eV in order to determine the fragmentation routes and to screen for the presence of their enaminone tautomers, (Z)-2-benzoylmethylene-1,2-dihydroquinolines, in the gas phase. The relative abundances or total ion currents of some ions correlated with the Hammett substituent constants and Hammett-Brown constants. The product ions [M-CO](+.) and [M-HCO](+) were observed. A reaction mechanism is suggested for the formation of these ions, requiring skeletal rearrangements. The results furnish information relating to tautomerism in the gas phase.

Structural characterization of nitrated 2'-hydroxychalcones by electrospray ionization tandem mass spectrometry

Isomeric 2'-hydroxychalcones bearing nitro and methoxy groups in different positions of their skeleton were analyzed by tandem mass spectrometry (MS/MS) with electrospray ionization (ESI), in positive mode. Collision-induced dissociation of the protonated molecules, [M + H](+), formed under electrospray conditions were studied and it was found that the product ion spectra of these chalcones presented different fragmentation patterns depending on the position of the substituents on the molecule. The product ion spectra (ESI- MS/MS) of the B ring ortho-nitro substituted 2'-hydroxychalcone and of the 4'-methoxychalcones showed loss of OH, 2OH and combined losses of OH and H(2)O. These fragment ions were absent in the spectra of the respective meta- and para isomers. The observed differences in the product ion spectra of these nitrochalcones allowed identification of the o-nitro derivatives. Distinction between the meta- and para derivatives was not achieved. Chalcones bearing 6'-methoxy substituents showed distinct fragmentation from the one observed for their isomers, 4'-methoxychalcones, since they present only one fragment ion, a typical ((0,alpha)A - H)(+) and, therefore, do not allow detailed structural information to be obtained, nor to differentiate between the o-, m- or p-nitro isomers. Overall, it was found that small changes in the substitution pattern of chalcones change their fragmentation considerably in the ESI-MS/MS, and that these features permit the differentiation of specific isomers of these 2'-hydroxynitrochalcones.

Analysis of minor flavonoids in Piper hostmannianum var. berbicense using liquid chromatography coupled with atmospheric pressure chemical ionization mass spectrometry

The fragmentations of hydroxylated flavanones, chalcones and dihydrochalcones were investigated by direct loop injection using an ion trap mass spectrometry equipped with atmospheric pressure chemical ionization (APCI) probe. Some of them have been isolated from the leaves of Piper hostmannianum var. berbicense and standards were used to confirm their fragmentation behaviour. In negative ion mode, fragmentations of these three types of flavonoids revealed specific diagnostic ions which allowed us to identify aglycones in a crude plant extract. The major fragment ion obtained in MS/MS experiment for methoxylated chalcones is the neutral loss of a methyl radical whereas a H(2)O molecule is lost in the case of methoxylated dihydrochalcones. Methoxylated chalcones and flavanones isomers could be differentiated by the relative intensity ratio of [M-H-CH(3)]*(-) and [M-H-C(2)H(2)O](-) ions. Based on UV and MS data, a decision tree that includes UV lambda(max) absorptions and MS/MS diagnostic ions was built in order to obtain structural information of unknown compounds present in the extract. This tree was used to identify flavonoids in the ethyl acetate extract of P. hostmannianum var. berbicense leaves after analysis by high-performance liquid chromatography-diode array detection-atmospheric pressure chemical ionization ion trap multistage mass spectrometry. A total of 11 flavonoids were tentatively characterized based on the MS fragmentations pattern observed in MS(n) experiments.

Unimolecular dissociation of protonated trans-1,4-diphenyl-2-butene-1,4-dione in the gas phase: rearrangement versus simple cleavage

Fragmentation mechanisms of trans-1,4-diphenyl-2-butene-1,4-dione were studied using a variety of mass spectrometric techniques. The major fragmentation pathways occur by various rearrangements by loss of H(2)O, CO, H(2)O and CO, and CO(2). The other fragmentation pathways via simple alpha cleavages were also observed but accounted for the minor dissociation channels in both a two-dimensional (2-D) linear ion trap and a quadrupole time-of-flight (Q-TOF) mass spectrometer. The elimination of CO(2) (rather than CH(3)CHO or C(3)H(8)), which was confirmed by an exact mass measurement using the Q-TOF instrument, represented a major fragmentation pathway in the 2-D linear ion trap mass spectrometer. However, the elimination of H(2)O and CO becomes more competitive in the beam-type Q-TOF instrument. The loss of CO is observed in both the MS(2) experiment of m/z 237 and the MS(3) experiment of m/z 219 but via the different transition states. The data suggest that the olefinic double bond in protonated trans-1,4-diphenyl-2-butene-1,4-dione plays a key role in stabilizing the rearrangement transition states and increasing the bond dissociation (cleavage) energy to give favorable rearrangement fragmentation pathways.