Main Fragmentation Pathways Research Articles

Liquid chromatography–mass spectrometry combined with xanthine oxidase inhibition profiling for identifying the bioactive constituents from Cistanche deserticola

Xanthine oxidase (XO) inhibitor can block the synthesis of uric acid and is regarded as one of the therapeutic approaches of gout. The anti-XO constituents in the stem of Cistanche deserticola were characterized though LC–MS combined with xanthine oxidase inhibition profiling method. Seven phenylethanoid glycosides (1–7) and two phenylpropanoid glycosides (8–9) were identified from the bioactive extracts. Isoacteoside (1), kankanoside G (4) and cistanoside F (8) displayed crucial activity against XO with the IC50 values of 46.91, 85.31 μM and 36.41 μM respectively. The main mass fragmentation pathways of phenylethanoid glycosides were the losses of glucosyl, rhamanopyranosyl and phenylethoxyl groups in positive ion mode and the elimination of caffeoyl unit in negative ion mode. It shows that the stem of Cistanche deserticola is a functional food which performs potent anti-gout activity.

Detection and Verification of Sibutramine Adulterated in Herbal Slimming Supplements Using Electrospray Ionization Mass Spectrometry

Sibutramine, a drug for weight loss, has been implicated in numerous deaths in recent years and has been sold to the public in adulterated herbal slimming supplements. In order to efficiently detect and verify the existence of sibutramine in samples, it was essential to develop a mass spectrometry method for detecting sibutramine, the drug described in this study. Sibutramine was extracted from herbal material matrixes by sonication. Separation was achieved via liquid chromatography and mass spectrometry analysis was conducted by electrospray ionization triple‐quadrupole mass spectrometry in the multiple reaction monitoring mode (MRM). The main fragmentation pathways were proposed and verified by Isotopic labeling of sibutramine hydrochloride. These pathways involved the rearrangement of hydrogen, methyl migration and an unstable multi‐membered‐ring transition structure. We found four high‐abundance fragment ions and created a method to verify the presence of sibutramine in herbal slimming supplements using MRM. This verification method offers high quality data with high sensitivity, low limits of detection and clear, easily interpretable results that are well suited for the verification of sibutramine hydrochloride in herbal slimming supplements of complex mixtures.

A detailed experimental and computational study of monocarbohydrazones

The substituent and solvent effect on intramolecular charge transfer (ICT) of twelve monocarbohydrazones (mCHs) were studied using experimental and theoretical methodology. The effects of specific and non-specific solvent–solute interactions on the UV–Vis absorption maxima shifts were evaluated using linear free energy relationships (LFERs) principles, i.e. using the Kamlet-Taft and Catalán models. Linear free energy relationships in the form of single substituent parameter equation (SSP) was used to analyze substituent effect on UV–Vis, NMR and pKa change. According to crystallographic data and quantum chemical calculations, the trans (E) form was found to be more stable. A photochromism of compounds with 2-hydroxyphenyl and 2-pyridylimino groups substituted at imine carbon atom results in E/Z isomerization due to creation of intermolecular hydrogen bond in E and Z form, respectively. Multiple stage mass spectrometry (MS-MSn) analysis was applied to define main fragmentation pathways. Furthermore, the experimental findings were interpreted with the aid of ab initio MP2/6–311G(d,p) and time-dependent density functional (TD-DFT) methods. TD-DFT calculations were performed to quantify the efficiency of intramolecular charge transfer (ICT) with the aid of the charge-transfer distance (DCT) and the amount of transferred charge (QCT) calculation. It was found that both substituents and solvents influence electron density shift i.e. extent of conjugation, and affect ICT character in the course of excitation.

Mass spectra of new heterocycles: XVI. Electron impact study of alkyl 5-aminothiophene-2-carboxylates

Electron impact mass spectra of alkyl 4-alkoxy-5-amino-3-methylthiophene-2-carboxylates were studied for the first time. These compounds, except for 4-(1-ethoxyethoxy) and 4-(ferrocenylmethoxy) derivatives, give rise to a stable molecular ion whose decomposition follows three pathways. The main fragmentation pathway of the molecular ion is elimination of alkyl radical from the 4-alkoxy group, the second pathway involves expulsion of alkoxy group from the ester moiety, and the third pathway is decomposition of the thiophene ring. The molecular ions of 4-(1-ethoxyethoxy)thiophenes decompose mainly via elimination of ethyl vinyl ether molecule with formation of [M–VinOEt]+ · odd-electron ion, and fragmentation of the latter follows general pathways. In the mass spectra of 4-(ferrocenylmethoxy)thiophenes the most abundant are ferrocenylmethyl ion with m/z 199 (I rel 100%) and fragment ions derived therefrom.

Photodecay Pathways of Stored, Size-Selected Lead Clusters Pbn+, n = 6–41 and Pbn–, n = 9–56

Fragmentation of cationic (Pbn+, n = 6–41) and anionic (Pbn–, n = 9–56) lead clusters by photoabsorption at 532 nm is investigated. For small (Pbn+, n < 19, Pbn–, n < 16) and large (Pbn+, n > 24, Pbn–, n > 30) lead clusters the fragmentation is dominated by evaporation of monomers, but at the intermediate sizes the main fragmentation pathways seem to include the breaking off of a neutral heptamer, Pb7. Under the same conditions as for all other clusters being investigated, Pb14+ shows a peculiar behavior, as no decay products are detected.

Mass spectra of new heterocycles: XV. Fragmentation of 1-substituted 3-alkoxy-2-(propargylsulfanyl)- and 3-alkoxy-2-(allenylsulfanyl)-1H-pyrroles under electron impact

The electron impact mass spectra of 1-R-substituted 3-alkoxy-2-(propargylsulfanyl)- and 3-alkoxy-2-(allenylsulfanyl)-1H-pyrroles (R = Me, i-Pr, s-Bu, Ph) have been studied for the first time. These compounds give rise to stable molecular ions whose primary fragmentation follows three competing pathways: cleavage of the C–O bonds with expulsion of alkyl radical, cleavage of the C–S bonds with formation of [M–C3H3]+ ions, and cleavage of the C–N bonds with synchronous hydrogen transfer to give odd-electron [M–CnH2n]+ · ion. The main fragmentation pathway of 2-(propargylsulfanyl) derivatives is cleavage of the C–S bond with formation of [M–C3H3]+ ion.

Dereplication of Flavonoid Glycoconjugates from Adenocalymma imperatoris-maximilianii by Untargeted Tandem Mass Spectrometry-Based Molecular Networking.

The interpretation of large datasets acquired using high performance liquid chromatography coupled with tandem mass spectrometry represents one of the major challenges in natural products research. Here we propose the use of molecular networking to rapid identify the known secondary metabolites from untargeted MS/MS analysis of Adenocalymma imperatoris-maximilianii plant extracts. The leaves, stems and roots of A.imperatoris-maximilianii were extracted using different solvents according to Snyder selectivity triangle. The samples were analyzed by HPLC coupled with ion trap mass spectrometer in a collision-induced dissociation MS/MS configuration in both positive and negative electrospray ionization modes. Molecular networking simultaneously organized the spectra by cosine similarity. The chemical identification was performed based on the systematic study of the main fragmentation pathways observed for the resulting network. The untargeted tandem mass spectrometry-based molecular networking allowed for the identification of 63 metabolites, mainly mono-, di- and tri-, C- and/or O-glycosyl flavones. Molecular networking was capable not only to dereplicate known flavonoids, but also to point out related prenyl derivatives, described for the first time in Adenocalymma species. The gas-phase reaction route to form the characteristic [M-H2O-(30/60/90)]+ fragments in C-glycosyl flavones was suggested as sequential sugar ring opening followed by retro-aldol elimination involving aldose-ketose isomerization. The use of molecular networking with LC-CID-MS/MS assisted the identification of various isomeric and isobaric flavonoid glycoconjugates by establishing clusters according to the fragmentation similarities. Additionally, the proposed cross-ring sugar cleavages can contribute to the identification of C-glycosides by MS/MS analysis.

Characterization of 2,3-diarylxanthones by electrospray mass spectrometry: gas-phase chemistry versus known antioxidant activity properties.

Xanthones (XH) are a class of heterocyclic compounds widely distributed in nature that hold numerous noteworthy biological and antioxidant activities. Therefore, it is of utmost importance to achieve relevant detailed structural information to understand and assist prediction of their biological properties. The potential relationship between radical-mediated xanthone chemistry in the gas phase and their promising antioxidant activities has not been previously explored. Protonated xanthones XH1-9 were generated in the gas phase by electrospray ionization (ESI) and the main fragmentation pathways of the protonated XH1-9 formed due to collision-induced dissociation (CID) were investigated. In the CID-MS/MS spectra of [M+H](+) ions of XH1, XH2 and XH4 the product ions formed due to H2 O elimination corresponding to the base peak of the spectra. For the remaining six xanthones (XH3, XH5-9), showing the most promising biological profile, the product ion produced with the highest relative abundance (RA) corresponded to the one formed through concomitant loss of H2 O plus CO. Indicative of an inexistent or lower biological activity is the combined loss of CO plus O unique to the CID-MS/MS spectra of XH1, XH2, XH4, and XH5. The product ion formed by loss of 64Da (concomitant loss of two molecules of H2 O plus CO) is only observed for xanthones containing a catechol unit (XH3 and XH6-9). This product ion has the highest RA for the most potent scavenger of reactive oxygen and nitrogen species XH9 that contains two of these catechol moieties. A strong relationship between some of the biological activities of the studied 2,3-diarylxanthones and their ESI-MS/MS fragmentation spectra was found. The multivariate statistical analysis results suggest that the selected MS features are related to the important biological features. Copyright © 2016 John Wiley & Sons, Ltd.

General synthesis of 4-aryloxy-6-methylpyrimidin-2-amines and their fragmentation under positive electrospray ionization

A number of 4-aryloxy-6-methylpyrimidin-2-amines were synthesized by reaction of 2-amino-6-methylpyrimidin-4-yl 4-methylbenzenesulfonate with phenols. The main fragmentation pathway of these compounds under positive electrospray ionization is decomposition of the heterocycle.

Liquid chromatography-tandem mass spectrometry determination of the degradation products of V-type nerve agnents, N,N-dialkylaminoethanesulfonic acids

Degradation products of V-type nerve agents are important markers of these toxic chemical warfare agents, hence their detection and identification is of high importance from verification point of view of Chemical Weapons Convention (CWC). We focused on detailed investigation of fragmentation of N, N-dialkylaminoethanesulfonic acids, degradation products of V-type nerve agents using liquid chromatography-tandem mass spectrometry (LC−MS/MS), a LCQ Deca XP max ion trap mass spectrometer via an electrospray ionization in ESI mode. Capillary and tube lens voltages were optimized to give maximum response to m/z 210 [M + H]+ ion of N,N-diisopropylaminoethanesulfonic acid. The main fragmentation pathways are loss of alkyl groups attached to the nitrogen atoms. Within the aims of CWC, LC−ESI-MSn represents an important tool to control the production of prohibited compounds among others due to a possibility of easy monitoring of their content in environmental matrices. The knowledge of the fragmentation pathway will be useful for the unambiguous identification of degradation products of V-type nerve agents in human urine matrices as well as environmental matrices in the Organization for the Prohibition of Chemical Weapons proficiency test.

EIMS Fragmentation Pathways and MRM Quantification of 7α/β-Hydroxy-Dehydroabietic Acid TMS Derivatives.

EI mass fragmentation pathways of TMS derivatives οf 7α/β-hydroxy-dehydroabietic acids resulting from NaBH(4)-reduction of oxidation products of dehydroabietic acid (a component of conifers) were investigated and deduced by a combination of (1) low energy CID-GC-MS/MS, (2) deuterium labeling, (3) different derivatization methods, and (4) GC-QTOF accurate mass measurements. Having identified the main fragmentation pathways, the TMS-derivatized 7α/β-hydroxy-dehydroabietic acids could be quantified in multiple reaction monitoring (MRM) mode in sea ice and sediment samples collected from the Arctic. These newly characterized transformation products of dehydroabietic acid constitute potential tracers of biotic and abiotic degradation of terrestrial higher plants in the environment.

Structural analysis of metabolites of asiatic acid and its analogue madecassic acid in zebrafish using LC/IT-MSn.

Although zebrafish has become a significant animal model for drug discovery and screening, drug metabolism in zebrafish remains largely unknown. Asiatic acid (AA) and madecassic acid (MA), two natural pentacyclic triterpenoids mainly obtained from Centella asiatica (L.) Urban, have been found to possess many pharmacological effects. This study is to probe the metabolic capability of zebrafish via investigation of the drug metabolism of AA and MA in zebrafish, using a sensitive LC/IT-MSn method. In addition, the main fragmentation pathways of AA and MA were reported for the first time. Nineteen metabolites of AA and MA were firstly identified after zebrafish was exposed to the drug, which all were the phase I metabolites and mainly formed from hydroxylation, dehydrogenation, hydroxylation and dehydrogenation, dihydroxylation and dehydrogenation, and dehydroxylation reaction. The results indicated that zebrafish possessed strong metabolic capacity, and the metabolites of AA and MA were formed via similar metabolic pathways and well matched with the known metabolic rules in vivo and in vitro, which supports the widely use of this system in drug metabolism research. This investigation would also contribute to the novel information on the structural elucidation, in vivo metabolites and metabolic mechanism of pentacyclic triterpenoids.

Screening of tricyclic quinazoline alkaloids in the alkaloidal fraction of Adhatoda beddomei and Adhatoda vasica leaves by high-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry.

Adhatoda beddomei and Adhatoda vasica are popular Ayurvedic medicinal plants in India, belonging to the family Acanthaceae. Tricyclic quinazoline alkaloids are found to be the most abundant in these plants which are responsible for broad-spectrum medicinal properties. This study aims to seek identification and characterization of those alkaloids based on their fragmentation patterns. A method was developed to elucidate the main fragmentation pathways of tricyclic quinazoline alkaloids in positive ion mode using high-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (HPLC/ESI-QTOF-MS/MS). Chromatographic separation was carried on a Supelco Discovery HS C18 column (15 cm × 4.6 mm, 3 µm) with 0.1% formic acid aqueous solution and acetonitrile as a mobile phase. In full scan mass spectra, protonated molecules were observed for all the quinazoline alkaloids. Ring cleavages of the tricyclic quinazoline moiety were observed in MS(2) spectra and the characteristic ions provide valuable structural information of these alkaloids. Fragmentation pathways and fragment ion structures were proposed in two groups of quinazoline alkaloids. The established fragmentation patterns have been successfully used to identify 23 tricyclic quinazoline alkaloids in the alkaloidal fraction of A. beddomei and A. vasica.

Decarboxylation versus Acetonitrile Loss in Silver Acetate and Silver Propiolate Complexes, [RCO2Ag2(CH3CN)n]+ (where R = CH3 and CH3C≡C; n = 1 and 2)

Gas-phase experiments using collision-induced dissociation in an ion trap mass spectrometer have been used in combination with density functional theory (DFT) calculations (at the B3LYP/SDD6–31+G(d) level of theory) to examine the competition between decarboxylation and loss of a coordinated acetonitrile in the unimolecular fragmentation reactions of the silver acetate and silver propiolate complexes, [RCO2Ag2(CH3CN)n]+ (where R = CH3 and CH3C≡C; n = 1 and 2), introduced into the gas-phase via electrospray ionisation. When R = CH3, loss of acetonitrile is the sole reaction channel observed for both complexes (n = 1 and 2), consistent with DFT calculations, which highlight that the barriers for decarboxylation 2.18 eV (n = 2) and 1.96 eV (n = 1) are greater than the binding energies of the coordinated acetonitriles (1.60 eV for n = 2; 1.64 eV for n = 1). In contrast, when R = CH3C≡C, decarboxylation is the main fragmentation pathway observed for both complexes (n = 1 and 2), with loss of acetonitrile only being a minor product channel. This is consistent with DFT calculations, which reveal that the barriers for decarboxylation are 1.17 eV (n = 2) and 1.16 eV (n = 1), which are both below the binding energies of the coordinated acetonitriles (1.55 eV for n = 2; 1.56 eV for n = 1). The barrier for decarboxylation of [CH3C≡CCO2Ag2]+ is 1.22 eV, which is less than the 2.06 eV reported for decarboxylation of [CH3CO2Ag2]+ (Al Sharif et al. Organometallics, 2013, 32, 5416). The observed ease of decarboxylation of silver propiolate complexes in the gas-phase is consistent with the recently reported use of silver salts in metal catalysed decarboxylative C–C and C–X bond forming reactions of propiolic acids.

Residues investigation of fluoroquinolones and sulphonamides and their metabolites in bovine milk by quantification and confirmation using ultra-performance liquid chromatography–tandem mass spectrometry

A novel UPLC–MS/MS method for quantification and confirmation of eight fluoroquinolones, five sulphonamides (SAs) and four acetyled metabolites in milk is developed. Their main fragmentation pathways were presented. The limits of quantification were in the range of 0.01–0.29μgkg−1 for FQs and 0.13–1.68μgkg−1 for SAs. Mean recoveries ranging from 61% to 115% were achieved at spiked level of 0.5–100μgkg−1. The intra- and inter-day precisions were in the range of 6.3–10.7% and 9.0–13.4%, respectively. This novel approach has high speed and sensitivity, and can ensure the multiclass and multiresidue analysis of bovine milk at low μgkg−1 level. The content of each analyte residue is much lower than the maximum residue limits. Some residues in milk were confirmed in according to the Commission Decision 657/2002/EC.

Chemical fingerprint analysis and characterization of monacolins, pigments, citrinin, sudan red G from red yeast rice samples using liquid chromatography-accurate QToF mass spectrometry

Red yeast rice (RYR) is prepared by fermentation of rice with various strains of the yeast Monascus spp belonging to family Aspergillaceae, the genus Monascus. Depending on the Monascus strains and the fermentation conditions, the products may contain polycetides called monacolins or polyketides (pigments), which are secondary metabolites. The preparations of red yeast rice have being used in food as a preservative, spice, coloring, rice wine or been sold as dietary supplements to lower blood levels of cholesterol and related lipids. The UHPLC-QToF-MS analysis of monacolins, pigments, citrinin and sudan red G from various RYR samples and dietary supplements was performed. A separation by LC was achieved using a reverse-phase column and a gradient of water/acetonitrile each containing formic acid as the mobile phase. MS-MS detection was used to provide structural information. Under typical ESI-MS conditions, monacolins produced abundant ions corresponding to the protonated molecules, sodium ion adducts and dimers [2 M+H]+. The elimination of the ester side chain followed by dehydration and dissociation of the lactone moiety were observed as the main fragmentation pathways for most of the monacolin-type of compounds.

Structural characterization of secoiridoid glycosides by high-performance liquid chromatography/electrospray ionization mass spectrometry.

The fragmentation behavior of seven secoiridoid glycosides (SIGs) isolated from Gentiana triflora Pall., a traditional Chinese medicine, was investigated by electrospray ionization ion trap mass spectrometry (ESI-IT-MS). Multistage electrospray ionization mass spectrometry (ESI-MS(n)) experiments both in positive- and negative-ion mode were used to elucidate the main fragmentation pathways of these compounds. In full scan mass spectra, different deprotonated or protonated molecules were observed for compounds with different basic structure. Ring cleavages of the aglycon moiety were observed in MS(2) spectra and the characteristic ions provided valuable information on the basic structural skeletons. Their fragmentation pathways and fragment ion structures were proposed. The established fragmentation patterns have been successfully used to identify eleven SIGs in the extract of Gentiana triflora Pall.

Structures and Unimolecular Reactivity of Gas-Phase [Zn(Proline-H)]+ and [Zn(Proline-H)(H2O)]+

A combination of IRMPD spectroscopy, collision-induced dissociation, deuterium isotopic substitution, and computational chemistry was used to determine the structure and unimolecular chemistry of [Zn(Pro-H)](+) and the singly hydrated complex in the gas phase. Five competing dissociation channels were observed: loss of H2O, CO, CO2, and HCOOH and the main fragmentation pathway, loss of neutral Zn. By comparing the IRMPD spectrum with the predicted IR spectra of the lowest energy structures, it was confirmed that [Zn(Pro-H)](+) complex is deprotonated at the amine moiety, and a hydrogen from either C2 or C5 migrated to Zn(2+). In this H-type complex, ZnH(+) was chelated between the amine nitrogen and the carbonyl oxygen. Calculations of the potential energy surface revealed that the loss of neutral zinc is energetically more favorable than the loss of dehydrogenated proline leading to ZnH(+) product. Furthermore, calculations on all five primary decomposition routes, all beginning with the lowest energy structure, revealed that loss of Zn has the lowest energy requirement, consistent with it being the most abundant product of unimolecular dissociation following collisional or IR multiphoton activation. For the singly hydrated complex, [Zn(Pro-H)(H2O)](+), IRMPD spectroscopy confirms a structure with water added to the H-type structure and intramolecularly hydrogen bonded to the deprotonated amine site. This structure is not the lowest-energy [Zn(Pro-H)(H2O)](+) isomer, but it is the one where water is added to the lowest energy [Zn(Pro-H)](+) isomer.

Gas-Phase Formation and Fragmentation Reactions of the Organomagnesates [RMgX2]−

A range of mononuclear organomagnesates [RMgX2]− were generated in the gas phase by decarboxylation of the magnesium carboxylate precursors [RCO2MgX2]− (R = Me, Et, Pr, iPr, tBu, vinyl, allyl, HC≡C, Ph, PhCH2, PhCH2CH2; X = Cl, Br, I). The gas-phase formation and unimolecular reactivity of these organomagnesates were examined using a combination of experiments carried out in linear ion trap and triple-quadrupole mass spectrometers and DFT calculations. Halide loss was found to directly compete with decarboxylation in the formation of mononuclear [RMgX2]−. However, sterically unhindered, stable R– substituents and strong Mg–Cl bonds can be employed to facilitate the decarboxylation reaction at the expense of the halide loss channel. Thus, in the case of R = HC≡C, PhCH2, decarboxylation is the main fragmentation pathway. The resultant mononuclear organomagnesates [RMgX2]− were mass-selected, and their unimolecular chemistry was examined. Four competing fragmentations were observed: bond homolysis, bond heterolysis, halide loss, and β-hydride transfer. Which of these competing reactions dominates depends on the nature of R and X. A conjugatively stabilized R• allows the observation of [MgX2]•–, whereas the presence of a β-hydride generates [HMgX2]−. Weaker Mg–X bonds (e.g., Br and I) promote the formation of X– upon CID.

Neighboring Effect in Fragmentation Pathways of Cage Guanylhydrazones in the Gas Phase

ESI-MS/MS investigation of the mono- and bis(guanylhydrazone) derivatives 1-5 based on adamantane and pentacycloundecane (PCU) skeleton was described. Elimination of neutral guanidine is the most abundant reaction channel in the case of 2,4-adamantyl and PCU derivatives 4 and 5, while the elimination of CH2N2 fragment is preferred for other compounds. This was attributed to the cage opening of adamantane or PCU skeletons in the former case leading to the formation of the cyclohexyl- or cyclopropylcarbinyl carbocation stabilized by the conjugation with the guanylhydrazone subunit. The main fragmentation pathways observed experimentally were analyzed by using DFT calculations. All investigated bis(guanylhydrazone)s formed dications and their abundances were found to be proportional to the interguanidine distance in the considered ions. Calculation of the first and the second proton affinities supported qualitative interpretation of the dication abundance. Close contact of two guanidine subunits is thus confirmed to be crucial in determining preferential fragmentation pathway and to suppress formation of the dication.