Investigations of taxoids by electrospray and atmospheric pressure chemical ionization with tandem mass spectrometry

The fragmentation behavior of taxoids was studied using electrospray (ESI) and atmospheric pressure chemical ionization (APCI) sources with multi-stage tandem mass spectrometry. In the positive ion mode taxoids gave prominent [M+Na]+ and [M+K]+ ions with the ESI source, and [M+NH4]+ or [M+H]+ ions with the APCI source. The MS/MS fragmentations of ions produced by APCI and ESI sources were very similar. For both sources, the presence of cinnamoyl or benzoyl groups could be characterized by initial losses of 148 or 122 u, respectively, from molecular adduct ions. However, the elimination of cinnamic acid was relatively difficult for the molecular adduct ions formed by APCI, and was comparable in importance to the loss of acetic acid. The other fragments involved losses of CH2CO, CO, and H2O. The 5/7/6 type taxoids underwent characteristic losses of 58 or 118 u from ions produced by both APCI and ESI sources. The fragmentation behavior was remarkably influenced by substitution locations. The elimination of the C-10 benzoyl group was usually the first fragmentation step, while that of the C-2 benzoyl group was relatively difficult. The acetoxyl group at C-7 was more active than those at C-2, C-9, and C-10, which in turn were more active than that at C-4. These fragmentation rules could facilitate the rapid screening and structural characterization of taxoids in plant extracts by high-performance liquid chromatography/mass spectrometry (HPLC/MS).     

Characteristic fragmentation behavior of some glucuronide-type triterpenoid saponins using electrospray ionization tandem mass spectrometry

The fragmentation behavior of some glucuronide-type triterpenoid saponins from Symplocos chinensis, and their analogues escin Ia and Ib, were investigated by positive ion electrospray ionization tandem mass spectrometry using a quadrupole linear ion trap mass spectrometer. The fragmentation patterns of these saponins significantly changed in accordance with structural variations in the glucuronyl residue of the oligosaccharide chain. It was found that the carboxyl group and hydroxyl group at the C-3' position of the glucuronyl residue were the key sites for determining the fragmentation behavior of these compounds. When the carboxyl group was esterified, only the C(2alpha) ion, and no B(2alpha) ion, and cationized aglycone were observed. When the hydroxyl group at C-3' was acylated, the inherent cross-ring cleavage was hindered. However, glycosidic cleavages always occurred, regardless of the crucial structural variations. The results of the present studies can benefit the determination of trace triterpenoid saponins of this type in crude plant extracts, and also provide background information to aid the structural investigations of similar glycoconjugates.     

Study of the characteristic fragmentation behavior of hydroquinone glycosides by electrospray ionization tandem mass spectrometry with optimization of collision energy

The fragmentation behavior of hydroquinone glycosides involving one or two sugar groups from Fraxinus sieboldiana and their analogue arbutin was investigated systematically by electrospray ionization tandem mass spectrometry in negative ion mode. The characteristic fragmentation reaction of these compounds was through the homolytic and heterolytic cleavage of the O‐glycosidic bond to produce radical aglycone ion ([Y₀ ? H]^??) and aglycone ion (Y₀^?), respectively. Unambiguous differentiation between the mono‐O‐glycoside isomers which differ in glycosylation position was achieved by comparing the relative abundance of [Y₀ ? H]^?? and Y₀^? ions with the optimized collision energy. In the fragmentation of 1, 4‐di‐O‐glycosides, only the Y₀^? ion was produced when the first glucosyl residue was expelled. However, both the [Y₀ ? H]^?? and Y₀^? ions were present when the second glucosyl residue was eliminated. In addition, an interesting [Y₀‐2H]^? ion was present in the product ion spectra of hydroquinone glycosides with methoxy group(s) substituted at C‐3 or/and C‐5 positions of the benzene ring. The results of this study can facilitate the rapid determination of hydroquinone glycosides in crude plant extracts and also reveal that the systematic investigation and optimization of collision energy play an important role in the differentiation of isomers which have subtle differences in structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid-phase synthesis of alpha-glucosamine sulfoforms with fragmentation analysis by tandem mass spectrometry

Sulfated epitopes of alpha-glucosamine (GlcN sulfoforms) were prepared by solid-phase synthesis as models of internal glucosamines within heparan sulfate. An orthogonally protected 2'-hydroxyethyl GlcN derivative was immobilized on a trityl resin support and subjected to regioselective deprotection and sulfonation conditions, which were optimized with the aid of on-resin infrared or Raman analysis. The sulfoforms were cleaved from the resin under mild Lewis acid conditions without affecting the O- or N-sulfate groups and purified by reversed-phase high-performance liquid chromatography (HPLC). The alpha-GlcN sulfoforms and their 4- O-benzyl ethers were examined by electrospray ionization tandem mass spectrometry (ESI-MS/MS), with product ion spectra produced by collision-induced dissociation (CID). ESI-MS/MS revealed significant differences in parent ion stabilities and fragmentation rates as a function of sulfate position. Ion fragmentation by CID resulted in characteristic mass losses with strong correlation to the positions of both free hydroxyl groups and sulfate ions. Most of these fragmentation patterns are consonant with elimination pathways, and suggest possible strategies for elucidating the structures of glucosamine-derived sulfoforms with identical m/ z ratios. In particular, fragmentation analysis can easily distinguish GlcN sulfoforms bearing the relatively rare 3- O-sulfate from isomers with the more common 6- O-sulfate.     

Characteristic fragmentation behavior of phosphoamino acid conjugates with 3'-azido-3'-deoxythymidine by electrospray ionization tandem mass spectrometry

The conjugates of phosphoamino acids with 3'-azido-3'-deoxythymidine were synthesized and their structures were determined by various spectral methods. In positive and negative ion electrospray mass spectrometry (ESI-MS), the fragmentation pathways were investigated in conjunction with tandem mass spectrometry (MS/MS). The results showed that there were very different characteristic fragment ions in the positive ion MS/MS spectra and the negative ion MS/MS spectra.     

A study of kynurenine fragmentation using electrospray tandem mass spectrometry

A combination of accurate mass measurement and tandem mass spectrometry (both product ion and precursor ion scans) have been used to characterize the major fragment ions observed in the ESI mass spectrum of kynurenine. Kynurenine is a metabolite of tryptophan found in the human lens and is thought to play a role in protecting the retina from UV-induced damage. Three major fragmentation pathways were evident, following initial elimination either of ammonia, H2O and CO or the imine form of glycine. The latter is proposed to occur via the formation of an ion-molecule complex. In the case of loss of H2O and CO from deaminated kynurenine, there is evidence for an acylium ion intermediate, which is not observed for the loss of H2O and CO directly from protonated kynurenine. Product ion scans of deuterated kynurenine enabled the elucidation of structural rearrangements that were not evident in the spectra of the native compound. Since UV filter compounds can often only be isolated in small quantities from the lens, this study forms the basis for the characterization of novel UV filter compounds using mass spectrometry. The approach presented here may also be useful for the characterization of related classes of small molecules.     

Identification and fragmentation of hydrolyzed aluminum species by electrospray ionization tandem mass spectrometry

Earlier characterization of some hydrolysis products of AlCl₃·6H₂O was confirmed by electrospray ionization tandem mass spectrometry with increasing collision energy of projectile ions. At lower collision energies, the aqua ligands were stripped off. At higher energies, two hydroxo groups formed a bridging oxo group with loss of one water molecule. Aluminum complexes could also capture aqua ligands in the collision chamber so long as the parent ion did not fragment, and the fragment ion spectra broadened toward higher m/z values. The chloro ligands were eliminated as hydrochloric acid. The aluminum cores remained highly intact. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of an arginine-selective tagging procedure on the fragmentation behavior of peptides studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS)

The presence of arginine as the naturally occurring amino acid with the highest gas-phase basicity strongly influences the fragmentation behavior of peptides undergoing collision-induced dissociation. Using a derivatization procedure recently developed in our group, based on a reversible reaction of the guanidino group with 2,3-butanedione and an arylboronic acid, we examined how this label affects the fragmentation patterns of labeled versus unlabeled peptides in MS/MS experiments. As part of this fundamental study, two groups of model peptides (angiotensins and bradykinins) as well as tryptic peptides were labeled according to our protocol and subjected to collision-induced dissociation (CID) in both a triple quadrupole and a quadrupole ion trap instrument. It was found that for angiotensins containing an AspArg sequence, C-terminal cleavage at Asp that occurs for native peptides was completely inhibited in Arg-labeled peptides. For bradykinins and peptides obtained from tryptic digests of standard proteins, some sample peptides were little affected by the tagging of arginine residues. Others, in contrast, exhibited an almost total loss of nonspecific backbone cleavage and their fragment ion spectra were dominated by loss of the arginine tag. These and other experimental results are discussed in view of the nature of the arginine tag and the concept of proton mobility.     

1-苯基3-甲基-吡唑啉酮-5电喷雾质谱裂解途径的研究

电喷雾质谱(ESI-MS)是近年来发展并完善起来的一种分析手段.作为一种新的软电离技术,其ESI源中化合物离子化行为不同于传统的电子轰击(EI)源,离子碎裂相对简单,但容易出现复杂的加合离子,同时其独有的CID-MSn技术可选择性地提供有用的碎片信息,研究这些碎裂和加合离子对有效确认化合物的准分子离子峰、研究化合物的结构和性质以及优化ESI条件有重要意义[1].     

Study of the mass spectrometric fragmentation of pseudouridine: comparison of fragmentation data obtained by matrix-assisted laser desorption/ionisation post-source decay, electrospray ion trap multistage mass spectrometry, and by a method utilising electrospray quadrupole time-of-flight tandem mass spectrometry and in-source fragmentation

Many nucleosides and their modified forms have been studied by mass spectrometry elaborating the detailed fragmentation pathways under MS2 and MS(n) conditions. Although the C-nucleoside pseudouridine has been fragmented and studied briefly, usually amongst many other nucleosides, it has not been investigated to the same extent as other nucleosides. In this report a number of different mass spectrometric techniques are applied to obtain a fuller picture of pseudouridine fragmentation. At the same time this study is used to compare different tandem mass spectrometric techniques, including a novel methodology utilising a quadrupole time-of-flight (Q-ToF) instrument for MS(n) analysis comparable with that available with an ion trap mass spectrometer.     

Study of atmospheric pressure chemical ionization of PETN by ion mobility spectrometry/tandem mass spectrometry

We present the results of studying the effects of temperature and humidity of the reaction medium and the intensity of ultraviolet radiation on the atmospheric pressure chemical ionization of Penthrite. The peculiarities of the ion mobility spectra of this compound obtained by ion mobility spectrometry-tandem mass spectrometry are analyzed.     

New fragmentation mechanisms in matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry of carbohydrates

The spectra recorded by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry (MALDI-TOF/TOF-MS/MS) of complex carbohydrates from human milk are presented. Besides ions originating from glycosidic cleavages and from sugar ring fragmentations, these spectra show intense peaks that may be assigned to ions produced by three new fragmentation pathways involving a six-atom rearrangement. These ions, together with the A fragments from sugar ring fragmentations, open the possibility of obtaining a complete mapping of the linkage positions present in the carbohydrates investigated by MALDI-TOF/TOF.     

High-throughput mass spectrometry: the mechanism of sudan azo dye fragmentation by ESI tandem mass spectrometry and extensive deuterium labeling experiments

The investigation of the gas-phase chemistry of azo compounds by mass spectrometry dates back to the introduction of this peculiar research field into the armory of physical organic chemistry tools. The mechanism of the fragmentation of the azo double bond from the protonated precursor is discussed with reference to the behavior of deuterium-labeled reference compounds. The investigated molecules belong to the sudan family and the results can be used for the detection of these potential carcinogenic compounds in different matrices by means of the isotope dilution method.     

Characteristic fragmentation behavior of the analogs of endomorphin-2 with phenylglycine in position 3 or 4 by ESI-FT tandem mass spectrometry

A series of analogs of endomorphin-2 （EM-2） with phenylglycine （Phg） in position 3 or 4 were synthesized. In electrospray ionization Fourier transform ion cyclotron resonance （ESI-Fr-ICR） MS/MS spectra of these compounds, some b, y, a, and internal ions were observed and slight mass differences between the calculated and observed results are obtained. Their sequences were derived successfully. However, the MS/MS patterns of these analogs with Dphg and Lphg were very similar. It is hard to distinguish them by MS/MS spectra. Moreover, if the third position was substituted by phenylglycine （L or D）, a rearrangement could occur in MS/MS experiment to lose proline residue.     

Analysis of MS/MS fragmentation of taxoids

The fragmentation pathways of seven types of taxoids were investigated by using a LC-MS/MS method, namely: (1) neutral taxoids with a C-4(20) double bond; (2) taxoids with a C-4(20) double bond and oxygenation at C-14; (3) 5-cinnamoyl taxoids with a C-4(20) double bond; (4) a basic taxoid with a C-4(20) double bond; (5) a taxoid with a C-4(20) epoxide; (6) taxoids with an oxetane ring; and (7) taxoids with an oxetane ring and a phenylisoserine C-13 side chain. Depending on the class of core structure and the substitution pattern, each taxoid gave either the molecular adduct ion [M+NH4]+ or [M+H]+. In the MS/MS, the molecular adduct ion gave characteristic product ions corresponding to the loss of water, acetic acid, benzoic acid, and cinnamic acid or the phenylisoserine group. These could reflect the difference of the substitutions and structural modifications and should be utilized for the structure elucidation oftaxoids by LC-MS.     

Study of the major fragmentation pathways of cypermethrin and related synthetic insecticides using tandem mass spectrometry

Fragmentation pathways of the synthetic pyrethroid cypermethrin and four structurally related insecticides were investigated using a tandem quadrupole mass spectrometer incorporating a hexapole collision cell under positive-ion electron impact ionization conditions. Conventional mass spectrometry using the first quadrupole analyser only and tandem mass spectrometry on selected precursor ions and product ions, and also constant neutral loss scan experiments, were used. Mechanisms and fragmentation pathways are proposed to explain the inherent stability of ions associated with the benzylphenoxy portion of this class of insecticide.     

Data requirements for protein identification using chemically-assisted fragmentation and tandem mass spectrometry

Many laboratories identify proteins by searching tandem mass spectrometry data against genomic or protein sequence databases. These database searches typically use the measured peptide masses or the derived peptide sequence and, in this paper, we focus on the latter. We study the minimum peptide sequence data requirements for definitive protein identification from protein sequence databases. Accurate mass measurements are not needed for definitive protein identification, even when a limited amount of sequence data is available for searching. This information has implications for the mass spectrometry performance (and cost), data base search strategies and proteomics research.