Structural characterization of steroidal saponins by electrospray ionization and fast-atom bombardment tandem mass spectrometry

The structural characterization of four steroidal saponin compounds involving two and three sugar groups, namely spirostanol saponins and furostanol saponins, were investigated by positive ion fast-atom bombardment (FAB), electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) techniques. Important structural information was obtained from collision-induced dissociation (CID) and FAB-MS spectra with different liquid matrices. It was found that a characteristic fragmentation involving the loss of 144 Da arising from the cleavage of the E-ring was observed when there was no sugar chain at the C-26 position. When a glucoside group was substituted at the C-26 position, this C-26 sugar moiety was preferentially eliminated. All of these compounds produced a major product ion with a stable skeleton structure at m/z 255. The results of this paper can assist structural analysis of mixtures of steroidal saponins.     

Structural characterization of iridoid glucosides by ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry

The mass spectral fragmentation behavior of ten iridoid glucosides (IGs) has been studied using electrospray ionization (ESI), collision-induced dissociation (CID), and quadrupole time-of-flight tandem mass spectrometry (Q-TOF MS/MS). In the negative ESI mass spectra, the deprotonated [M-H](-) ion was observed for all of the ten IGs except gardoside methyl ester, while the formate adduct [M+HCOO](-) ion appeared to be favored by the presence of a methyl ester or a lactone group in the C-4 position when formic acid was added to the mobile phase. The CID MS/MS spectra of the [M-H](-) ions have been used for structural elucidation. Ring cleavages of the aglycone moiety have been observed in the MS/MS spectra, corresponding to (1,4)F(-), (2,6)F(-), (2,7)F(-), and (2,7)F(0) (-) ions, based on accurate mass measurements and the elemental compositions of the product ions. These characteristic ions gave valuable information on the basic structural skeletons. Furthermore, on the basis of the relative abundances of the fragment ions (1,4)F(-) and (2,7)F(-), different sub-classes, such as cyclopentane-type and 7,8-cyclopentene-type IGs, can be differentiated. Ring cleavage of the sugar moieties was also observed, yielding useful information for their characterization. In addition, the neutral losses, such as H(2)O, CO(2), CH(3)OH, CH(3)COOH, and glucosidic units, have proved useful for confirming the presence of functional substituents in the structures of the IGs. Based on the fragmentation patterns of these standard IGs, twelve IGs have been characterized in an extract of Hedyotis diffusa Willd. by means of ultra-performance liquid chromatography/Q-TOF MS/MS, of which six have been unambiguously identified and the other six have been tentatively identified.     

The early glycation products of the Maillard reaction: mass spectrometric characterization of novel imidazolidinones derived from an opioid pentapeptide and glucose

Glucose-substituted imidazolidinones related to the endogenous opioid peptide leucine-enkephalin have been investigated using fast atom bombardment tandem mass spectrometry (FAB-MS/MS) and electrospray ionization tandem mass spectrometry (ESI-MS/MS). In addition to Amadori compounds, the studied imidazolidinones represent a novel type of the early glycation products formed in the Maillard reaction. To obtain insight into the fragmentation behavior of these carbohydrate-peptide adducts, we also studied synthetic precursors of the glucose-substituted imidazolidinones as well as the corresponding isopropylidene derivatives. The collision-induced dissociation (CID) spectra of [M + H](+) ions of all these imidazolidinones have been compared. Detailed analysis showed that fragmentation of each compound generates two ions at m/z 566 and m/z 598 which are characteristic and undoubtedly confirm the imidazolidinone-type structure. These two significant ions were identified as the M + 10 and M + 42 modifications of the N-terminus of the parent opioid pentapeptide effected by the carbohydrate moiety. Furthermore, the ion at m/z 178 is identified as the M + 42 modification of the immonium ion of the N-terminal amino acid (tyrosine) also effected by the carbohydrate moiety. They can be used as diagnostic ions for imidazolidinone-type compounds in studying the Maillard reaction. Thus, we have demonstrated the utility of FAB-MS/MS and ESI-MS/MS in the structural determination and identification of such novel peptide-carbohydrate adducts, useful in understanding the details of the mechanism of non-enzymatic glycation in vivo.     

Study on fragmentation behavior of 5/7/6-type taxoids by tandem mass spectrometry

The mass spectrometric behaviors of seven compounds, namely four 4beta(20),5-oxetane 5/7/6-type taxoids (i.e. taxayuntin A, taxayuntin B, taxayuntin and taxayuntin C) and three 4(20)-methylene 5/7/6-type taxoids (i.e. brevifoliol, taxchinin A and 7-acetyl-10-deacetyl-7-debenzoylbrevifoliol) have been investigated by the positive ion FAB-MS/MS technique. The fragmentation has been correlated with the types and positions of substituents of these compounds. It has been found that with the OH group at the C-10 position, taxayuntin A, taxayuntin B and 7-acetyl-10-deacetyl-7-debenzoylbrevifoliol are dominated by the loss of H2O, while with the BzO group at the C-10 position, taxayuntin, taxayuntin C, brevifoliol and taxchinin A preferentially eliminate the BzO group. In addition, C-2 is an active site, and neutral loss from the C-2 position readily occurs. The four 4beta(20),5-oxetane 5/7/6-type taxoids produce the terminal product ion with a stable conjugated system at m/z 311, while the 4(20)-methylene 5/7/6-type taxoids brevifoliol and 7-acetyl-10-deacetyl-7-debenzoylbrevifoliol produce this ion at m/z 237, and taxchinin A at m/z 253. Interestingly, characteristic fragment ions involving the loss of a 118 u group were observed for taxayuntin, and a possible fragmentation mechanism is given. The major fragmentation pathways and mechanisms of ion formation for the compounds are proposed on the basis of CID spectra and accurate mass measurements. The results of this paper will be helpful for structural analysis of analogs.     

Structural determination of monoacylglycerols extracted from marine sponge by fast atom bombardment tandem mass spectrometry

Five new monoacylglycerols (MAGs) were isolated from the marine sponge Stelletta sp. by reversed-phase high-performance liquid chromatography and analyzed by positive ion fast atom bombardment mass spectrometry (FAB-MS). FAB mass spectra of these compounds produced abundant sodium-adducted molecules [M+Na]+ from a mixture of 3-nitrobenzyl alcohol and sodium iodide. The structural elucidation of these sponge MAGs was carried out by FAB tandem mass spectrometry (MS/MS). To find diagnostic ions for the characterization of the MAGs, authentic MAGs were initially analyzed by collision-induced dissociation (CID) MS/MS. The CID MS/MS of [M+Na]+ precursor ions resulted in the formation of numerous characteristic product ions via a series of dissociative processes. The product ions formed by charge-remote fragmentation (CRF) provided important information for the characterization of acyl chains substituted at the glycerol backbone, and product ions at m/z 84, 97, 113 and 139 were diagnostic for the sodiated glycerol backbone. On the basis of these fragmentation patterns, the structures of five MAGs extracted from marine sponge were elucidated. In addition, high-resolution mass measurement was performed to obtain the elemental compositions of the MAGs.     

Energy-dependent collision-induced dissociation study of buprenorphine and its synthetic precursors

The collision-induced dissociation (CID) of protonated buprenorphine ([M+H](+) ) and four related compounds was studied by electrospray quadrupole/time-of-flight mass spectrometry (ESI-QTOF MS). The fragmentation pathways were investigated by using energy-dependent CID and pseudo-MS(3) (in-source CID combined with tandem mass spectrometry (MS/MS)) methods. The first steps of the fragmentation are the parallel losses of the substituents from the non-aromatic ring moieties. Depending on the applied collision energies, a large number of further fragment ions arising from the cross-ring cleavages of the core-ring structure were observed. Based on the experimental results, a generalized fragmentation scheme was developed for the five buprenorphine derivatives highlighting the differences for the alternatively substituted compounds. The collision-energy-dependent fragmentation profile of buprenorphine is visualized in a two-dimensional plot to aid its fingerprint identification.     

Analysis of fungal cyclopentenone derivatives from Hygrophorus spp. by liquid chromatography/electrospray-tandem mass spectrometry

Fruitbodies of the genus Hygrophorus (Basidiomycetes) contain a series of anti-biologically active compounds. These substances named hygrophorones possess a cyclopentenone skeleton. LC/ESI-MS/MS presents a valuable tool for the identification of such compounds. The mass spectral behaviour of typical selected members of this group under positive and negative ion electrospray conditions is discussed. Using the ESI collision-induced dissociation (CID) mass spectra of the [M + H]+ and [M - H]- ions, respectively, the compounds can be classified with respect to the substitution pattern at the cyclopentenone ring and the type of oxygenation at C-6 (hydroxy/acetoxy or oxo function) of the side chain. The elemental composition of the fragment ions was determined by ESI-QqTOF measurements. Thus, in case of the negative ion CID mass spectra an unusual loss of CO2 from the deprotonated molecular ions could be observed.     

Structural determination of saponins extracted from starfish by fast atom bombardment collision-induced dissociation mass spectrometry.

Three saponins were extracted and isolated from starfish by reversed-phase high performance liquid chromatography (HPLC), and analyzed by fast atom bombardment mass spectrometry (FAB-MS). Their molecular weight information could be obtained by the presence of abundant [M+Na]+ ions and weak [M+H]+ ions in FAB-MS spectra. Moreover, high resolution mass measurements of their [M+Na]+ ions were performed at the resolution of 10000 to elucidate the element composition of extracted saponins. The collision-induced dissociation (CID) of sodium-adducted molecules [M+Na]+ yielded diverse product ions via dissociated processes. In the collision-induced dissociation (CID)-MS/MS analysis of [M+Na]+ ion, the sulfate-containing saponins produced characteristic ions such as SO4Na+, [NaHSO4+Na]+, [M+Na-sugar]+ and [M+Na-2sugar]+ ions, whereas the sulfate-free compound showed characteristic ions produced by cleavage of sugar moiety and side chain of aglycone. The fragmentation patterns could provide information on the linkage position of sugar groups in aglycone and sulfate groups.     

Fragmentation study of iridoid glucosides through positive and negative electrospray ionization, collision-induced dissociation and tandem mass spectrometry

Mass spectrometric methodology based on the combined use of positive and negative electrospray ionization, collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS) has been applied to the mass spectral study of a series of six naturally occurring iridoids through in-source fragmentation of the protonated [M+H]+, deprotonated [M--H]- and sodiated [M+Na]+ ions. This led to the unambiguous determination of the molecular masses of the studied compounds and allowed CID spectra of the molecular ions to be obtained. Valuable structural information regarding the nature of both the glycoside and the aglycone moiety was thus obtained. Glycosidic cleavage and ring cleavages of both aglycone and sugar moieties were the major fragmentation pathways observed during CID, where the losses of small molecules, the cinnamoyl and the cinnamate parts were also observed. The formation of the ionized aglycones, sugars and their product ions was thus obtained giving information on their basic skeleton. The protonated, i.e. [M+H]+ and deprotonated [M--H]-, ions were found to fragment mainly by glycosidic cleavages. MS/MS spectra of the [M+Na]+ ions gave complementary information for the structural characterization of the studied compounds. Unlike the dissociation of protonated molecular ions, that of sodiated molecules also provided sodiated sugar fragments where the C0+ fragment corresponding to the glucose ion was obtained as base peak for all the studied compounds.     

Mass spectral characterization of tetracyclines by electrospray ionization,H/D exchange,and multiple stage mass spectrometry

Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N2 and with ND3 as the nebulizer gas. CID mass spectra were obtained for [M + H]+ and [M - H]- ions and the exchanged analogs, [M(Dx) + D]+ and [M(Dx) - D]- , and produced by ESI using a Sciex API-III(plus) and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MS(N) spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H2O (D2O) and NH3 (ND3) if there is a tertiary OH at C-6. The loss of H2O (D2O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH3 (ND3) initially. MSN experiments showed easily understandable losses of HDO, HN(CH3)2, CH3 - N=CH2, and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH3 (ND3) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol.     

Gas Phase Chemistry of Li+ with Amides: the Observation of LiOH Loss in Mass Spectrometry

Collision-induced dissociation (CID) of Li(+) adducts of three sets of compounds that contains an amide bond, including 2-(4, 6-dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamide, its derivatives and simpler structures was investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Observed fragment ions include those that reflect loss of LiOH. Other product ions result from the Smiles rearrangement and direct C-S bond cleavage. MS/MS of H/D exchange products demonstrated occurrence of a 1,3-H shift from the amide nitrogen atom to the phenyl ring of these compounds. The LiOH loss from Li(+) adducts of amides was further examined by CID of [M + Li](+) ions of N-phenylbenzamide and N-phenylcinnamide. Loss of LiOH was essentially the sole fragmentation reaction observed for the former. For the latter, both losses of LiOH and H(2)O were discovered. The presence of electron-donating substituents of the phenyl ring of these compounds was found to facilitate elimination of LiOH, while that loss was retarded by electron-withdrawing substituents. Proposed fragment ion structures were supported by elemental compositions deduced from ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR-MS/MS) m/z value determinations. Density functional theory-based (DFT) calculations were performed to evaluate potential mechanisms for these reactions.     

Structural determination of the novel fragmentation routes of morphine opiate receptor antagonists using electrospray ionization quadrupole time-of-flight tandem mass spectrometry

Electrospray ionization quadrupole time-of-flight (ESI-QqToF) mass spectra of naltrindole hydrochloride 1, naltriben mesylate 2, and naltrexone hydrochloride 3, a common series of morphine opiate receptor antagonists, were recorded using different declustering potentials. Low-energy collision-induced dissociation (CID) MS/MS experiments established the fragmentation routes of these compounds. In addition, re-confirmation of the various established fragmentation routes was effected by conducting a series of ESI-CID-QqTof-MS/MS experiments using non-conventional quasi MS(n) (up to MS8) product ion scans, which were initiated by CID in the atmospheric pressure/vacuum interface using a higher declustering potential. Precursor ion scan analyses were also performed with a conventional quadrupole-hexapole-quadrupole tandem mass spectrometer and allowed the confirmation of the genesis of some diagnostic ions.     

金鸡纳生物碱电喷雾质谱裂解规律研究

利用电喷雾离子阱质谱（ES-ITMS）技术进行了金鸡纳中4种喹啉类生物碱（奎宁、奎尼定、辛可宁和辛可尼定）的电喷雾质谱研究。在优化的质谱条件下,正离子全扫描时,4种生物碱都易形成带一个质子的分子离子;进一步碰撞诱导解离四种母离子,二级质谱表明羟基丢失、奎宁环断裂及重排为其主要裂解方式。     

Structural determination of cyclitol derivatives by fast-atom bombardment tandem mass spectrometry

Three cyclitol derivatives were isolated from the marine sponge Sarcotragus sp. by reversed-phase high-performance liquid chromatography and analyzed by fast-atom bombardment mass spectrometry (FAB-MS). Their structural elucidation was carried out with FAB tandem mass spectrometry (FAB-MS/MS). FAB-MS spectra produced a significant abundance of the sodium adducts [M+Na]+ and [M+2Na-H]+ from a mixture of m-NBA and NaI. In addition, trifluoroacetylation of the cyclitol derivatives was used for confirmation of the presence of the cyclitol ring. High abundance [M-5H+5CF3CO+Na]+ ions were observed in the FAB-MS spectra of the trifluoroacetyl-cyclitol derivatives. Collision-induced dissociation (CID) of the [M+Na]+ ions produced diverse product ions via a series of dissociative processes. Charge-remote fragmentation (CRF) patterns of [M+Na]+ ions were very useful for the identification of product ions which are characteristic for the cyclitol ring and long hydrocarbon chains substituted at the glycerol backbone. Moreover, the CID-MS/MS spectra of the [M+Na]+ ions yielded characteristic product ions at m/z 53, 83, 113, 155 and 171 for the cyclitol moiety, and at m/z 213, 229 and 245 for the glycerol backbone attached to the cyclitol ring.     

Fragmentation of protonated dipeptides containing arginine. Effect of activation method

The fragmentation reactions of the protonated dipeptides Gly-Arg and Arg-Gly have been studied using collision-induced dissociation (CID) in a quadrupole ion trap, by in-source CID in a single-quadrupole mass spectrometer and by CID in the quadrupole cell of a QqTOF mass spectrometer. In agreement with earlier quadrupole ion trap studies (Farrugia, J. M.; O'Hair, R. A. J., Int. J. Mass Spectrom., 2003, 222, 229), the CID mass spectra obtained with the ion trap for the MH(+) ions and major fragment ions are very similar for the two isomers indicating rearrangement to a common structure before fragmentation. In contrast, in-source CID of the MH(+) ions and QqTOF CID of the MH(+), [MH - NH(3)](+) and [MH <23 HN = C(NH(2))(2)](+) ions provide distinctly different spectra for the isomeric dipeptides, indicating that rearrangement to a common structure has not occurred to a significant extent under these conditions even near the threshold for fragmentation in the QqTOF instrument. Clearly, under normal operating conditions significantly different fragmentation behavior is observed in the ion trap and beam-type experiments. This different behavior probably can be attributed to the shorter observation times and concomitant higher excitation energies in the in-source and QqTOF experiments compared to the long observation times and lower excitation energies relevant to the ion trap experiments. Based largely on elemental compositions derived from accurate mass measurements in QqTOF studies fragmentation schemes are proposed for the MH(+), [MH - NH(3)](+), and [MH - (HN = C(NH(2))(2))](+) ions.     

用高分辨质谱研究二茂铁类化合物

研究了八种N－二茂铁甲基苯胺衍生物的电子电离质谱（EI－MS），并与快原子轰击质谱（FAB－MS）作了比较，讨论了这些化合物的质谱特征。通过精确质量测量，确认了M^ -2 OH离子，并采用碰撞诱导裂解（CID）探讨了化合物质谱中主要离子间的关系。     

Collision-induced fragmentation pathways including odd-electron ion formation from desorption electrospray ionisation generated protonated and deprotonated drugs derived from tandem accurate mass spectrometry

The rapid desorption electrospray ionisation (DESI) of some small molecules and their fragmentation using a triple-quadrupole and a hybrid quadrupole time-of-flight mass spectrometer (Q-ToF) have been investigated. Various scanning modes have been employed using the triple-quadrupole instrument to elucidate fragmentation pathways for the product ions observed in the collision-induced dissociation (CID) spectra. Together with accurate mass tandem mass spectrometry (MS/MS) measurements performed on the hybrid Q-ToF mass spectrometer, unequivocal product ion identification and fragmentation pathways were determined for deprotonated metoclopramide and protonated aspirin, caffeine and nicotine. Ion structures and fragmentation pathway mechanisms have been proposed and compared with previously published data. The necessity for elevated resolution for the differentiation of isobaric ions are discussed.     

Dissociation of deprotonated microcystin variants by collision-induced dissociation following electrospray ionization

Microcystins (MC) are a family of hepatotoxic cyclic heptapeptides produced by a number of different cyanobacterial species. Considering the recent advances in the characterization of deprotonated peptides by mass spectrometry, the fragmentation behavior of four structurally related microcystin compounds was investigated using collision-induced dissociation (CID) experiments on an orbitrap mass spectrometer. It is demonstrated in this study that significant structural information can be obtained from the CID spectra of deprotonated microcystins. A predominant ring-opening reaction at the isoMeAsp residue, as well as two major complementary fragmentation pathways, was observed, reducing the complexity of the product ion spectra in comparison with spectra observed from protonated species. This proposed fragmentation behavior was applied to characterize [Leu(1)]MC-LR from a cyanobacterial cell extract. In conclusion, CID spectra of microcystins in the negative ion mode provide rich structurally informative mass spectra which greatly enhance confidence in structural assignments, in particular when combined with complementary positive ion CID spectra.     

CID of singly charged antioxidants applied in lubricants by means of a 3D ion trap and a linear ion trap-Orbitrap mass spectrometer

The aim of this study was to investigate the fragmentation behavior induced by low‐energy collision‐induced dissociation (LE‐CID) of four selected antioxidants applied in lubricants, by two different types of ion trap mass spectrometers: a three‐dimensional ion trap (3D‐IT) and a linear IT (LIT) Orbitrap MS. Two sterically hindered phenols and two aromatic amines were selected as model compounds representing different antioxidant classes and were characterized by positive‐ion electrospray ionization (ESI) and LE‐CID. Various types of molecular ions (e.g. [M]^+?, [M + H]⁺, [M + NH₄]⁺ or [M + Na]⁺) were used as precursor ions generating a significant number of structurally relevant product ions. Furthermore, the phenolic compounds were analyzed by negative‐ion ESI. For both IT types applied for fragmentation, the antioxidants exhibited the same unusual LE‐CID behavior: (1) they formed stable radical product ions and (2) C? C bond cleavages of aliphatic substituents were observed and their respective cleavage sites depended on the precursor ion selected. This fragmentation provided information on the type of structural isomer usually not obtainable for branched aliphatic substituents utilizing LE‐CID. Comparing the two instruments, the main benefit of applying the LIT‐Orbitrap was direct access to elemental composition of product ions enabling unambiguous interpretation of fragmentation trees not obtainable by the 3D‐IT device (e.g. loss of isobaric neutrals). It should be emphasized that the types of product ions formed do not depend on the type of IT analyzer applied. For characterizing degradation products of antioxidants, the LIT‐Orbitrap hybrid system, allowing the determination of accurate m/z values for product ions, is the method of choice. Copyright © 2011 John Wiley & Sons, Ltd.     

Determination of linkage position and anomeric configuration in Hex-Fuc disaccharides using electrospray ionization tandem mass spectrometry

Fixed-energy sequential tandem mass spectrometry (MS(n)) capabilities offered by quadrupole ion trap instruments have been explored in a systematic study of six isomers of Gal-Fucalpha-OBenzyl disaccharides. Under collision-induced dissociation (CID), sodiated molecular species generated in the positive-ion electrospray ionization mode yield simple and predictable mass spectra. Information on interglycosidic linkages and configurations can be deduced from the relative intensities of the selected diagnostic fragments arising from the glycosidic bond cleavages and corroborated by the fragments arising from cross-ring cleavages. As the CID patterns are not dependent on the number of prior tandem mass spectrometric steps, structures can be unambiguously assigned by matching the spectra with a library. The rules governing the fragmentation behavior of this class of oligosaccharides were tested for a representative isomeric disaccharide, Glcbeta1,3Fucalpha-OAllyl. The findings establish a basis for using MS(n) with a quadrupole ion trap instrument to elucidate structures of hexose-fucose subunits from more complicated oligosaccharides. Energy-resolved mass spectra were also acquired by CID tandem triple-quadrupole mass spectrometry. The breakdown behavior of the molecular ions revealed patterns which could differentiate stereoisomers of Gal-Fuc disaccharides over a range of collision energy from 20 to 50 eV.