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.     

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.     

In-source fragmentation and accurate mass analysis of multiclass flavonoid conjugates by electrospray ionization time-of-flight mass spectrometry

In-source collision-induced dissociation (CID) fragmentation features of multiclass flavonoid glycoconjugates were examined using liquid chromatography electrospray time-of-flight mass spectrometry. Systematic experiments were performed to search for optimal conditions for in-source fragmentation in both positive and negative ion modes. The objective of the study was to attain uniformly appropriate conditions for a wide range of analytes independently of the aglycone, the attached sugar part and the type of bond between the aglycone and the glycan moieties (O- or C-glycosides). Studied substances included representatives of flavonols, flavones, flavanones and anthocyanins and, regarding their glycan parts, mono-, di- and triglycosides with varying distribution of carbohydrate moieties (di-O-glycosides, O-diglycosides, O,C-diglycosides). The breakdown properties of the analytes along with the abundances of the characteristic diagnostic ions required for structural elucidation of complex flavonoid derivatives were evaluated. An optimized value was found for the instrument parameter (fragmentor voltage) affecting the in-source CID fragmentation of the analytes [230 V (ESI+) and 330 V (ESI-)]. Thus, appropriate performance in terms of both highly sensitive full-scan acquisition and fragmentation information was obtained for all the investigated flavonoids. In addition, singularities in the abundance of selected diagnostic ions (e.g. Y(0), Y(1) and Y*) due to variations in the interglycosidic linkage (rutinoside-neohesperidoside) in the glycan part were found and are also evaluated and discussed in detail. The combination of in-source CID fragmentation with high mass accuracy MS detection establishes a working basis for the development of versatile and useful LC-MS methods for wide-scope screening, non-targeted detection and tentative identification of flavonoid derivatives.     

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.     

Serine-phosphoric acid cluster ions studied by electrospray ionization and tandem mass spectrometry

More than 310 kinds of cluster ions of S(m) P(n) H(k) (k+) are observed in a single ESI mass spectrum of a mixed solution of serine and phosphoric acid. Some typical cluster ions are selected, activated by collision in a FT ICR cell, and the dissociation pathways were deduced in detail. For large singly protonated ions, the collisions cause the ejection of subunits of serine or phosphoric acid subsequently producing the ions of S(2) P(4) H(1) (1+) , which can be further dissociated by the loss of phosphoric acid molecules in turn and form the protonated serine dimer and monomer. However, for the doubly protonated ions, the dissociation pathways change from the loss of a protonated serine dimer for the ions of S(7) P(9) H(2) (2+) to the neutral loss of H(3) PO(4) for the ions of S(7) P(12) H(2) (2+) or the neutral loss of serine or H(3) PO(4) for the larger clusters, indicating the effect of cluster sizes on the process of dissociation. The structure of S(2) P(4) H(1) (1+) is suggested based on B3LYP/6-31G(d,p) calculations. The diversity and structural orderliness of the hetero-cluster ions are mainly attributed to the network of hydrogen bonds inside the cluster ions and the extraordinary amphotericity of the components.     

Structural determination of sphingomyelin by tandem mass spectrometry with electrospray ionization

Alkaline metal adduct ions of sphingomyelin were formed by electrospray ionization in positive ion mode. Under low energy collisionally activated dissociation (CAD), the product ion spectra yield abundant fragment ions representative of both long chain base and fatty acid which permit unequivocal determination of the structure. Tandem spectra obtained by constant neutral loss scanning permit identification of sphingomyelin class and specific long chain base subclass in the mixture. The fragmentation pathways under CAD were proposed, and were further confirmed by source CAD tandem mass spectrometry. The total analysis of sphingomyelin mixtures from bovine brain, bovine erythrocytes, and chicken egg yolk is also presented.     

Diastereomeric differentiation of norbornene amino acid peptides by electrospray ionization tandem mass spectrometry

A new class of diastereomeric pairs of non‐natural amino acid peptides derived from butyloxycarbonyl (Boc‐)protected cis‐(2S,3R)‐ and trans‐(2S,3S)‐β‐norbornene amino acids including a monomeric pair have been investigated by electrospray ionization (ESI) tandem mass spectrometry using quadrupole time‐of‐flight (Q‐TOF) and ion‐trap mass spectrometers. The protonated cis‐BocN‐β‐nbaa (2S,3R) (1) (βnbaa = β‐norbornene amino acid) eliminates the Boc group to form [M+H–Boc+H]⁺, whereas an additional ion [M+H–C₄H₈]⁺ is formed from trans‐BocN‐β‐nbaa (2S,3S) (2). Similarly, it is observed that the peptide diastereomers (di‐, tri‐ and tetra‐), with cis‐BocN‐β‐nbaa (2S,3R)‐ at the N‐terminus, initially eliminate the Boc group to form [M+H–Boc+H]⁺ which undergo further fragmentation to give a set of product ions that are different for the peptides with trans‐BocN‐β‐nbaa (2S,3S)‐ at the N‐terminus. Thus the Boc group fragments differently depending on the configuration of the amino acid present at the N‐terminus. It is also observed that the peptide bond cleavage in these peptides is less favoured and most of the product ions are formed due to retro‐Diels‐Alder fragmentation. Interestingly, sodium‐cationized peptide diastereomers mainly yield a series of retro‐Diels‐Alder fragment ions which are different for each diastereomer as they are formed starting from [M+Na–Boc+H]⁺ in peptides with cis‐BocN‐β‐nbaa (2S,3R)‐ at the N‐terminus, and [M+Na–C₄H₈]⁺ in peptides with trans‐BocN‐β‐nbaa (2S,3S)‐ at the N‐terminus. All these results clearly indicate that these diastereomeric pairs of peptides yield characteristic product ions which help distinguish the isomers. Copyright © 2009 John Wiley & Sons, Ltd.     

Differentiation of diiodothyronines using electrospray ionization tandem mass spectrometry

Diiodothyronines 3,5-diiodothyronine (3,5-T2), 3',5'-diiodothyronine (3',5'-T2), and 3,3'-diiodothyronine (3,3'-T2) are important metabolites of 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3; reverse T3). In this paper, a novel and rapid method for identifying and quantifying 3,5-T2, 3',5'-T2 and 3,3'-T2 has been introduced using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Fragmentation patterns were proposed on the basis of our data obtained by ESI-MS/MS. MS2 spectra in either negative ionization mode or positive ionization mode can be used to differentiate 3,5-T2, 3',5'-T2 and 3,3'-T2. On the basis of the relative abundance of fragment ions in MS2 spectra under the positive ionization mode, quantification of the 3,5-T2, 3',5'-T2 and 3,3'-T2 isomers in mixtures is also achieved without prior separation.     

Differentiation of monoiodothyronines using electrospray ionization tandem mass spectrometry

In this work two monoiodothyronines, 3-T1 and 3'-T1, have been analyzed using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Fragmentation patterns were proposed based on our data obtained by ESI-MS/MS. MS2 spectra in either negative or positive ion mode can be used to differentiate 3-T1 and 3'-T1. Based on the relative abundance of fragment ions in MS2 spectra in the negative ion mode, quantification of the 3-T1 and 3'-T1 isomers in mixtures is achieved without prior separation. Solid-phase extraction in combination with ESI-MS/MS provides a practicable procedure that can be used to determine the molar ratio of 3-T1 and 3'-T1 in human serum with an error less than 3%. The detection limits for 3-T1 and 3'-T1 were 0.5 and 0.7 pg/microL, respectively.     

Key fragmentation patterns of aporphine alkaloids by electrospray ionization with multistage mass spectrometry

This work reports a detailed study of the fragmentations of aporphine alkaloids by electrospray ionization with multistage mass spectrometry (ESI-MS(n)) in positive mode. In a first step the loss of the amino group and its substituent is observed. Further steps display the loss of the peripheral groups. Losses of methanol and CO are observed if an OH is vicinal to an OCH(3) on the aromatic ring. Otherwise the spectra show radical losses of CH(3)* or CH(3)O* as the main fragmentations. If a methylenedioxy group is present losses of formaldehyde followed by CO are observed. These fragmentations yield important information on the structures of aporphines.     

Discrimination of cyclic peptide diastereomers by electrospray ionization tandem mass spectrometry

In this research, the characteristic ions' abundance ratio between two isomers A and B in MS/MS mass spectra was defined as a parameter for discriminating diastereomers. Through this ratio, the discrimination of four pairs of cyclic peptide (CP) diastereomers was successfully achieved. Furthermore, in the analysis of diastereomers' mixtures, both calibration curve and calculational methods were substantiated to have high precision and accuracy. The average absolute errors of the two methods were 2.0 and 2.5% in the 48 measurements of 16 samples, respectively. This research provided a promising approach for the analysis of the CP diastereomers in the fields of asymmetrical synthesis, chiral natural products and structural biology by ESI‐MS/MS. Copyright © 2009 John Wiley & Sons, Ltd.     

Gas-phase fragmentation study of novel synthetic 1,5-benzodiazepine derivatives using electrospray ionization tandem mass spectrometry

The fragmentation patterns of a series of three novel synthesized 3-hydroxy-4-phenyl-tetrahydro-1,5-benzodiazepin-2-ones (1-3), possessing the same backbone structure, were investigated using electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) techniques. A simple methodology, based on the use of ESI (positive ion mode) and by increasing the declustering potential in the atmospheric pressure/vacuum interface, collision-induced dissociation (CID), was used to enhance the formation of the fragment ions. In general, the novel synthetic 1,5-benzodiazepine derivatives afforded, in the gas phase, both protonated and sodiated molecules. This led to the confirmation of the molecular masses and chemical structures of the studied compounds. Exact accurate masses were measured using a high-resolution ESI-quadrupole orthogonal time-of-flight (QqToF)-MS/MS hybrid mass spectrometer instrument.The breakdown routes of the protonated molecules were rationalized by conducting low-energy collision CID-MS/MS analyses (product ion- and precursor ion scans) using a conventional quadrupole-hexapole-quadrupole (QhQ) tandem mass spectrometer. All the observed major fragmentations for the 1,5-benzodiazepines occurred in the saturated seven-membered ring containing the nitrogen atoms. These formed a multitude of product ions by different breakdown routes. All the major fragmentations involved cleavages of the N-1-C-2 and C-3-C-4 bonds. These occurred with concomitant eliminations of glyoxal, benzene and ethyl formate, forming the product ion at m/z 119, which was observed in all the studied compounds. In addition, an unique simultaneous CID-MS/MS fragmentation was noticed for the 1,5-benzodiazepines 1 and 3, which occurred by a pathway dictated by the substituent located on the N-1-position. It was evident that the aromatic ring portion of the 1,5-benzodiazepines was resistant to CID-MS/MS fragmentation. Re-confirmation of the various geneses of the product ions was achieved by conducting a series of precursor ion scans. ESI-MS and CID-MS/MS analyses have thus proven to be a specific and very sensitive method for the structural identification of these novel 1,5-benzodiazepine derivatives.     

Characterization of ageing products of ester-based synthetic lubricants by liquid chromatography with electrospray ionization mass spectrometry and by electrospray ionization (tandem) mass spectrometry

Ageing products of a commercial jet engine oil based on pentaerythritol tetraesters which were formed upon operation in an aviation turbine were detected by electrospray ionization mass spectrometry (ESI-MS) and characterized by LC-ESI-MS. The fatty acid composition of these ageing products was investigated by ESI-MS-MS analysis. The ammonium adducts of the newly formed pentaerythritol tetraester degradation products were found to be suitable parent ions for further structure elucidation work. ESI-MS, LC-ESI-MS and ESI-MS-MS proved to be versatile tools to study the chemical composition (distribution of homologues) as well as the mechanism of ageing of ester based lubricants on a molecular level. Due to its high sensitivity, ESI-MS can also be used to characterize and identify trace levels of ester-based lubricants.