Fragmentation of deprotonated cyclic dipeptides by electrospray ionization mass spectrometry

The fragmentation pathways of deprotonated cyclic dipeptides have been studied by electrospray ionization multi‐stage mass spectrometry (ESI‐MSⁿ) in negative mode. The results showed that the fragmentation pathways of deprotonated cyclic dipeptides depended significantly on the different substituents, the side chains of amino acid residues at the diketopiperazine ring. In the spectra of deprotonated cyclic dipeptides, the ion [M? H? substituent radical]^? was firstly observed in the ESI mode. The characteristic fragment ions [M? H? substituent radical]^? and [M? H? (substituent? H)]^? could be used as the symbols of particular cyclic dipeptides. The hydrogen/deuterium (H/D) exchange experiment, the high‐resolution mass spectrometry (Q‐TOF) and theoretical calculations were used to rationalize the proposed fragmentation pathways and to verify the differences between the fragmentation pathways. The relative Gibbs free energies (ΔG) of the product ions and possible fragmentation pathways were estimated using the B3LYP/6–31++G(d, p) model. The results have some potential applications in the structural elucidation and interpretation of the mass spectra of homologous compounds and will enrich the gas‐phase ESI‐MS ion chemistry of cyclic dipeptides. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrospray ionization multistage tandem mass spectrometry of penta- and hexa-substituted aryloxycyclotriphosphazenes

Several penta- and hexa-substituted aryloxycyclotriphosphazenes were synthesized and investigated by electrospray ionization tandem mass spectrometry (ESI-MSⁿ). Their main fragmentation pathways are proposed based on the MSⁿ and accurate mass data. An apparent hydrolysis reaction is an important fragmentation process exhibited in the ESI-MS/MS spectra for all of them. Also interesting is the intramolecular electrocyclic ring closure observed in ESI-MS/MS spectra of them. These observations may have some potential applications in the distinction between the mass spectra of penta- and hexa-substituted hexachlorocyclotriphosphazene derivatives.     

QCMS2 as a new method for providing insight into peptide fragmentation: The influence of the side‐chain and inter–side‐chain interactions

The identification of peptides and proteins from tandem mass spectra is a difficult task and multiple tools have been developed to aid this identification. We present a new method called quantum chemical mass spectrometry for materials science (QCMS²), which is based on quantum chemical calculations of bond orders, reaction, and transition‐state energies at the DFT/B3LYP/6‐311+G* level of theory. The method was used to describe the fragmentation pathways of five X‐His‐Ser tripeptides with X = Asn, Asp, Glu, Ser, and Trp, thereby focusing on the influence of the side chain and inter–side‐chain interactions on the fragmentation. The main features in the mass spectra of the five tripeptides were correctly reproduced, and a number of fragments were assigned to fragmentations involving the side chain and the influence of inter–side‐chain interactions. Product ion spectra were recorded to evaluate the capabilities and limitations of QCMS² and a number of conventional tools.     

二肽衍生物的电喷雾质谱研究

基于HIV整合酶核心结构域,合成了以HIV整合酶为靶标的二肽衍生物,采用多级质谱技术（二级、三级）研究二肽衍生物在质谱条件下的化学键断裂途径,发现主要的断裂方式为：氨基与羰基间的NH-CO键的断裂以及N-(苯并噻唑-2-基)甲酰氨基与亚甲基间的CO-C间的断裂。     

Identification of isobaric product ions in electrospray ionization mass spectra of fentanyl using multistage mass spectrometry and deuterium labeling

Isobaric product ions cannot be differentiated by exact mass determinations, although in some cases deuterium labeling can provide useful structural information for identifying isobaric ions. Proposed fragmentation pathways of fentanyl were investigated by electrospray ionization ion trap mass spectrometry coupled with deuterium labeling experiments and spectra of regiospecific deuterium labeled analogs. The major product ion of fentanyl under tandem mass spectrometry (MS/MS) conditions (m/z 188) was accounted for by a neutral loss of N‐phenylpropanamide. 1‐(2‐Phenylethyl)‐1,2,3,6‐tetrahydropyridine (1) was proposed as the structure of the product ion. However, further fragmentation (MS³) of the fentanyl m/z 188 ion gave product ions that were different from the product ion in the MS/MS fragmentation of synthesized 1, suggesting that the m/z 188 product ion from fentanyl includes an isobaric structure different from the structure of 1. MS/MS fragmentation of fentanyl in deuterium oxide moved one of the isobars to 1 Da higher mass, and left the other isobar unchanged in mass. Multistage mass spectral data from deuterium‐labeled proposed isobaric structures provided support for two fragmentation pathways. The results illustrate the utility of multistage mass spectrometry and deuterium labeling in structural assignment of isobaric product ions. Copyright © 2010 John Wiley & Sons, Ltd.     

VUV photoionisation of free azabenzenes: Pyridine, pyrazine, pyrimidine, pyridazine and s-triazine

Photoionisation mass spectrometry was used to obtain the fragmentation pathways of pyridine, pyridazine, pyrimidine, pyrazine and s-triazine molecules upon absorption of 23.0, 15.7 and 13.8 eV synchrotron photons. The ionic fragments observed vary from molecule to molecule, however C₂H₂⁺, HCN⁺and HCNH⁺ are common to all five molecules at the three photon energies. Furthermore, the presence of C₂H₂N₂⁺, C₃H₃N⁺ and C₄H₄⁺ in the spectra of some of the molecules suggests dissociation pathways via loss of HCN moieties. The respective parent cations, m/q=79, 80 and 81 have a greater yield at low photon energies when compared to the most intense fragment peak in each spectra. We recorded two of the fragment cation yields, as well as the parent photoion yield curves of pyridine, pyridazine, and pyrimidine in the 8–30 eV range. The formation of abundant cation fragments show a strong propensity of the molecules for dissociation after the absorption of VUV photons higher than 14 eV. The differences in relative fragment yields from molecule to molecule, and when changing the excitation energy, suggest significant bond rearrangements and nuclear motion during the dissociation time. Thus, bond cleavage is dependent on the photon energy deposited in the molecule and on intramolecular reactivity. With the aid of photoion yield curves and energy estimations we have assigned major peaks in the spectra and discussed their fragmentation pathways.     

Ionspray mass spectrometric and tandem mass spectrometric investigations of N-aliyl-N′-arylsulphonylthioureas, -isothioureas and -guanidines

Mass and tandem mass Spectrometric results for N-allyl-N′-arylsulphonylthioureas,-isothioureas and -guanidines using the pneumatically assisted electrospray (ionspray) technique are presented. The mass Spectrometric experiments show dominant quasi-molecular peaks of the monomers and signals of dimers and, more weakly, fragments. Full mass spectra are available from amounts below 500 fmol, indicating a high sensitivity. By tandem mass spectrometry of the protonated molecular ions and their fragments (sequential dau ghter-ion scanning) and using the chlorine and bromine isotope patterns, the fragmentation pathways were elucidated.     

Electron ionisation mass spectral studies of bridgehead‐fused Δ2‐norbornanethiazolines

The electron ionisation (EI) mass spectra of a series of bridgehead‐fused Δ²‐norbornanethiazolines, a new class of bridgehead‐norbornane derivatives, have been studied and their cleavage mechanisms rationalised on the basis of the substituent shifts as well as on the identification of relevant peaks through accurate mass measurements and collision‐induced dissociation tandem mass spectrometric experiments. The fragmentation patterns of isomeric pairs of 6,6‐ and 10,10‐dimethylnorbornanethiazolines are almost identical, probably due to an initial isomerisation of molecular ion previous to the fragmentation. In general, the dominant peaks in the spectra of all the studied compounds originate from initial α‐cleavages of C(5)–C(6) or C(1)–C(10) bonds, followed by concomitant homolytic cleavage of C(1)–C(9) and C(7)–C(10) bonds. The driving force for this fragmentation pathway, directed by the gem‐dimethyl group, is the formation of a highly stabilised thiazolilmethyl cation which constitutes the base peak in all the spectra and allows the identification of these interesting ligands. Copyright © 2009 John Wiley & Sons, Ltd.     

Basic mass spectrometric investigations of amino acid-fluorescamine derivatives

Mass spectra of fluorescamine derivatives of seventeen of the naturally occuring amino acids have been studied using electron ionization mass spectrometry. Seven fragmentation processes of fluorophors are proposed and elucidated by accurate mass measurements, stable isotope incorpor-ation and low resolution spectra. Two major pathways contributed most of the predominant peaks in the mass spectra. All fragmentations involve specific rearrangements of the spiro ring, while some lose the amino acid sidechain. Identification of the amino acid is unequivocally established in the low resolution spectra. Loss of one oxygen atom from the dehydrated fluorophors was found for all compounds studied. The site of this loss was determined by incorporation of ¹⁸O into the carbonyl groups of fluorescamine.     

Analysis of α‐acyloxyhydroperoxy aldehydes with electrospray ionization–tandem mass spectrometry (ESI‐MSn)

A series of α‐acyloxyhydroperoxy aldehydes was analyzed with direct infusion electrospray ionization tandem mass spectrometry (ESI/MSⁿ) as well as liquid chromatography coupled with the mass spectrometry (LC/MS). Standards of α‐acyloxyhydroperoxy aldehydes were prepared by liquid‐phase ozonolysis of cyclohexene in the presence of carboxylic acids. Stabilized Criegee intermediate (SCI), a by‐product of the ozone attack on the cyclohexene double bond, reacted with the selected carboxylic acids (SCI scavengers) leading to the formation of α‐acyloxyhydroperoxy aldehydes. Ionization conditions were optimized. [M + H]⁺ ions were not formed in ESI; consequently, α‐acyloxyhydroperoxy aldehydes were identified as their ammonia adducts for the first time. On the other hand, atmospheric‐pressure chemical ionization has led to decomposition of the compounds of interest. Analysis of the mass spectra (MS² and MS³) of the [M + NH₄]⁺ ions allowed recognizing the fragmentation pathways, common for all of the compounds under study. In order to get detailed insights into the fragmentation mechanism, a number of isotopically labeled analogs were also studied. To confirm that the fragmentation mechanism allows predicting the mass spectrum of different α‐acyloxyhydroperoxy aldehydes, ozonolysis of α‐pinene, a very important secondary organic aerosol precursor, was carried out. Spectra of the two ammonium cationized α‐acyloxyhydroperoxy aldehydes prepared with α‐pinene, cis‐pinonic acid as well as pinic acid were predicted very accurately. Possible applications of the method developed for the analysis of α‐acyloxyhydroperoxy aldehydes in SOA samples, as well as other compounds containing hydroperoxide moiety are discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Electron ionization mass spectrometric study of substituted alloxazine‐5‐oxides and iso‐alloxazine‐5‐oxide

The fragmentation pathways in electron ionization (EI) mass spectra of a series of new N(5)‐oxides of alloxazines and iso‐alloxazine are presented, and compared with those of substituted alloxazines and iso‐alloxazine. The EI mass spectra of these compounds showed characteristic fragmentation pathways A, B and C, started by the ejection of atomic oxygen, a HNCO molecule and an OH ^. radical, respectively. On the basis of B/E and B²/E spectra, the mechanism of elimination of the OH ^. radical is discussed. The influence of the methyl substituent in the benzene ring of alloxazine on the mass fragmentation pathways is described. Copyright © 2009 John Wiley & Sons, Ltd.     

Negative-ion fast atom bombardment mass spectrometry of N-phosphoamino acids

The fragmentation patterns of N-phosphoamino acids in negative-ion fast atom bombardment mass spectrometry (FABMS) showed different characteristics to those in positive-ion FABMS. Six typical N-diisopropyloxyphorphorylamino acids all had intense [M ? 1]^? peaks, and they underwent similar fragmentation pathways. In general, the elimination of one alkene molecule followed by the loss of one molecule of alcohol occurred. They also favoured an N → O rearrangement reaction, followed by fragmentation to (RO)₂ PO₂^? and (RO) (HO)PO₂^?.     

Stereochemical differentiation of C‐7 hydroxyltaxane isomers by electrospray ionization mass spectrometry

In this study, different electrospray ionization mass spectrometric (ESI‐MS) methods were utilized to analyze several pairs of taxane stereoisomers including paclitaxel and 7‐epi‐paclitaxel. Both ESI‐MS and tandem mass spectrometry (MS/MS) techniques provided stereochemically dependent mass spectra in negative‐ion mode, and all studied stereoisomers could be easily distinguished based on their characteristic ions or distinct fragmentation patterns. MS/MS experiments for several taxane analogues at various collision energies were performed to elucidate potential dissociation pathways. The gas‐phase deprotonation potentials were also calculated to estimate the most thermodynamically favorable deprotonation site using DFT B3LYP/6‐31G(d). The results of the theoretical studies agreed well with the fragmentation patterns of paclitaxel and 7‐epi‐paclitaxel observed from MS/MS experiments. In addition, it was found that liquid chromatography (LC)/ESI‐MS was a useful and sensitive technique for assignment of C‐7 taxane stereoisomers from realistic samples. Copyright © 2009 John Wiley & Sons, Ltd.     

Analysis of lignans in Schisandra chinensis and rat plasma by high‐performance liquid chromatography diode‐array detection,time‐of‐flight mass spectrometry and quadrupole ion trap mass spectrometry

High‐performance liquid chromatography/diode‐array detection (HPLC/DAD), time‐of‐flight mass spectrometry (HPLC/TOFMS) and quadrupole ion trap mass spectrometry (HPLC/QIT‐MS) were used for separation, identification and structural analysis of lignans in Schisandra chinensis and rat plasma after oral administration of the herbal extract. Six lignans in Schisandra chinensis extract were identified unambiguously by comparing the retention time, their characteristic ultraviolet (UV) absorption and accurate mass measurement. A formula database of known lignans in Schisandra chinensis was established, against which the other 15 lignans were identified effectively based on the accurate extract masses and formulae acquired by HPLC/TOFMS. In order to distinguish the isomers, multi‐stage mass spectrometry (ion trap mass spectrometry, MSⁿ) was also used. The fragmentation behavior of the lignans in the ion trap mass spectrometer was studied by the six lignan standards, and their fragmentation rules in MSⁿ spectra were summarized. These deduced fragmentation rules of lignans were successfully implemented in distinguishing the three groups of isomers in Schisandra chinensis by HPLC/QIT‐MS. By using the three different analytical techniques, 21 lignans in Schisandra chinensis were identified within 30 min. After oral administration of the extract, 11 lignans in rat plasma were detected and identified by comparing their retention time, characteristic UV absorption and accurate mass measurement of peaks in HPLC/TOFMS chromatograms of the herbal extract. Finally, HPLC/TOFMS fingerprints of Schisandra chinensis in vitro and rat plasma in vivo were established. It is concluded that a rapid and effective method based on three analytical techniques for identification of chemical components was established, which is useful for rapid identification of multiple components in Schisandra chinensis in vitro and in vivo. In addition, it can provide help for further pharmacology and action mechanism study of lignans in Schisandra chinensis. Copyright © 2009 John Wiley & Sons, Ltd.     

Vacuum-ultraviolet photoionization and mass spectrometric characterization of lignin monomers coniferyl and sinapyl alcohols

The fragmentation mechanisms of monolignols under various energetic processes are studied with jet-cooled thermal desorption molecular beam (TDMB) mass spectrometry (MS), 25 keV Bi(3)(+) secondary ion MS (SIMS), synchrotron vacuum-ultraviolet secondary neutral MS (VUV-SNMS) and theoretical methods. Experimental and calculated appearance energies of fragments observed in TDMB MS indicate that the coniferyl alcohol photoionization mass spectra contain the molecular parent and several dissociative photoionization products. Similar results obtained for sinapyl alcohol are also discussed briefly. Ionization energies of 7.60 eV ± 0.05 eV for coniferyl alcohol and <7.4 eV for both sinapyl and dihydrosinapyl alcohols are determined. The positive ion SIMS spectrum of coniferyl alcohol shares few characteristic peaks (m/z = 137 and 151) with the TDMB mass spectra, shows extensive fragmentation, and does not exhibit clear molecular parent signals. VUV-SNMS spectra, on the other hand, are dominated by the parent ion and main fragments also present in the TDMB spectra. Molecular fragmentation in VUV-SNMS spectra can be reduced by increasing the extraction delay time. Some features resembling the SIMS spectra are also observed in the desorbed neutral products. The monolignol VUV-SNMS peaks shared with the TDMB mass spectra suggest that dissociative photoionization of ion-sputtered neutral molecules predominate in the VUV-SNMS mass spectra, despite the extra internal energy imparted in the initial ion impact. The potential applications of these results to imaging MS of biomolecules are discussed.     

Effect of electrospray ionization conditions on low-energy tandem mass spectra of peptides

The effect of electrospray ionization (ESI) conditions on low-energy tandem mass spectra of peptides in the relative molecular mass range 400–1200 was examined. For singly charged peptide ions the source skimmer potential (which determines the degree of acceleration of the ions through the intermediate pressure region in the source) can strongly influence the extent of fragmentation observed in tandem mass spectra, especially at low collision energies. For each peptide there is an optimum skimmer potential which represents a balance between generating ions with sufficient internal energy for subsequent tandem mass spectrometric experiments and inducing the onset of other processes such as source fragmentation. The fragmentation which can be achieved in tandem mass spectra with high skimmer potentials differs from ESI source fragmentation for the same peptides. We have found that fragmentation in ESI mass spectra depends both on skimmer potential and on solvent pH, presumably because the latter determines the proportion of doubly charged species generated from a given peptide. Low-energy tandem mass spectra of peptides following ESI are equally as sensitive to peptide structure and the type of adduct studied (e.g. [M + H]⁺ vs. [M + NH₄]⁺) as tandem mass spectra obtained following older ionization methods such as fast atom bombardment.     

Determination of cannabinoids from a surrogate hops matrix using multiple reaction monitoring gas chromatography with triple quadrupole mass spectrometry

Cannabinoids are the primary bioactive constituents of Cannabis sativa and Cannabis indica plants. In this work, gas chromatography in conjunction with triple quadrupole mass spectrometry in multiple reaction monitoring mode was explored for determination of cannabinoids from a surrogate hops matrix. Gas chromatography with mass spectrometry is a reasonable choice for the analysis of these compounds; however, such methods are susceptible to false positives for Δ9‐tetrahydrocannabinol, due to decarboxylation of Δ9‐tetrahydrocannabinolic acid, its acid precursor, in the hot injection port. To avoid this transformation, the carboxyl group of Δ9‐tetrahydrocannabinolic acid was protected through a silylation reaction. Multiple reaction monitoring transitions for both unmodified and silylated cannabinoids were developed and the fragmentation pathways for the different species were assigned. Precision and accuracy were evaluated for cannabinoids spiked into hops at different levels. The developed methods provided good linearity (R² > 0.99) for all the cannabinoids with a linear range from 0.15 to 20 mg/L, and with limits of detection in the orders of low‐ to mid‐picogram on column. The recoveries for the cannabinoids were generally between 75 and 120%. Precisions (<6% coefficient of variation) were within acceptable ranges.     

Energy‐resolved technique for discovery and identification of malonyl‐triterpene saponins in Caulophyllum robustum by UHPLC‐electrospray Fourier transform mass spectrometry

Malonyl‐triterpene saponins (MTSs) attract scientific attentions because of their structural diversities and valuable bioactivities. However, its thermal instability brings a huge amount of challenges for isolation and purification of this class of compounds. To our best knowledge, there has been no report on isolation and analysis of MTSs from genus Caulophyllum. In this study, a strategy combining data acquisition using an energy‐resolved technique and the narrow widow extracted ion chromatograms as data mining method was developed for discovery and identification of MTSs in Caulophyllum robustum hair roots by ultra high liquid chromatography coupled to electrospray ionization Fourier transform mass spectrometry. The method was performed at an independent MS full scan using our bottom‐up energies by in‐source collision induced dissociations with 0, 25, 50 and 100 eV in both positive and negative modes. Precursor ion as well as fragment ion information was simultaneously collected from four energy‐resolved MS spectra in a single run of 18 min. The fragmentation pathways of intact deprotonated, protonated and sodium ions of MTSs were proposed for the structural elucidation of Caulophyllum MTSs. A flowchart involving a stepwise procedure based on key fragments from ESI^?/ESI⁺‐FT‐MS^{(1, 1)} to MS^{(1, 4)} spectra was constructed for the identification of structural elements in the MTSs. As a result, a total of 23 MTSs were discovered and tentatively identified, which had not been reported from Caulophyllum species before. All of these were potentially new compounds. This study provides an excellent example for discovery and identification of MTSs in herb medicines. Copyright © 2016 John Wiley & Sons, Ltd.     

Derivatization and mass spectrometric investigations of substituted benzeneboronic acids. The use of linked scanning during gas chromatography mass spectrometry

Substituted benzeneboronic acids are important intermediates in the synthesis of support matrices for affinity chromatography but their analysis by mass spectrometry is hindered by thermal reactions in the ion source. A simple derivatization with 1,2- or 1,3-diols removes this difficulty and imparts sufficient volatility for application of gas chromatography/mass spectrometry. The mass spectra of the resulting boronate esters are discussed with reference to high resolution measurements, isotope labelling studies and observation of metastable ions. ortho Substituents are shown to interact strongly during fragmentation. Linked scanning at constant B/E was used to characterize fragmentation pathways and the compatibility of linked scanning and GC/MS is reported.     

Electron impact mass spectrometry of phenyl thiophosphoroorganic amides

The interpretation of the electron impact mass spectra of methyl phenyl phosphinomorpholinylamidothioate, O-methyl phenyl phosphonomorpholinylamidothioate, phenyl phosphonomorpholinylamidochloridothioate and O-ethyl phenyl phosphonomorpholinyhimidothioate is presented. Elucidation of the fragmentation pathways was aided by exact mass measurements and observation of peaks due to metastable transitions detected for the first and last of the above compounds.