Triple quadrupole tandem mass spectrometry of sesquiterpene lactones: a study of goyazensolide and its congeners

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to investigate the fragmentation pattern of ten sesquiterpene lactones of the goyazensolide type under low-energy collision-induced dissociation (CID) using a triple quadrupole mass spectrometer. The analysis revealed that loss of CO(2)[M + H - 44](+) is the predominant process for compounds that exhibit a hydroxyl at C-8. In contrast, compounds with different acyloxy groups at C-8 fragment by means of elimination of the corresponding carboxylic acids [M + H - (R(2)CO(2)H)](+) and consecutive losses of CO and H(2)O. Our results also demonstrate the influence of both the stereochemistry of the acyloxy group at C-8 on the relative abundances of product ions and the hydroxyl at C-15, which creates an additional pathway, resulting in highly diagnostic product ions. This work clearly demonstrates the utility of tandem quadrupole low-resolution mass spectrometry for studies on the rationalization of the fragmentation of a series of compounds with a highly conserved core structure, but differing in substituent groups.     

Analysis of triterpenoids in <Emphasis Type="Italic">Ganoderma lucidum</Emphasis> using liquid chromatography coupled with electrospray ionization mass spectrometry

Triterpenoids extracted from Ganoderma lucidum (Leyss. ex Fr.) Karst were separated and characterized using optimized reversed-phase liquid chromatography with diode array detection and electrospray ion trap tandem mass spectrometry (HPLC-DAD-ESI-MS(n)). They could be classified into five types depending on the fragmentation behavior. All triterpenoids gave [M - H](-) and [2M - H](-) ions by electrospray ionization monitored in the negative ion mode; in addition, compounds of types III and IV gave prominent [M - H - H(2)O](-) ions and the unsaturated bond at C-20, 22 would reduce the abundance of [M - H - H(2)O](-) ion. The key fragmentation information was cleavage at C- and D-rings despite the predominant losses of H(2)O and CO(2). Compounds with hydroxyls at C-7 and C-15 would produce a list of b, b - 1, b - 2, and b - 16 ions attributed to cleavage of D-ring; if the second alcohol at C-15 were oxidized to ketone, the prominent cleavage would occur at C-ring and produce a group of ions of a; if C-7 were oxidized to ketone, transference of two hydrogen atoms would occur during the cleavage of rings and a list of ions about a + 2 and/or b + 2 would appear instead. The above fragmentations and regularities in fragmentation pathways were reported for the first time, and were implemented for the analysis of triterpenoids in G. lucidum. The chloroform extract was separated on a Zorbax SB-C(18) column, eluting with an acetonitrile-0.2% acetic acid gradient. A total of 32 triterpenoids, including six new ones, were identified or tentatively characterized based on the tandem mass spectra of the HPLC peaks.     

A fragmentation study of podophyllotoxin and its 4'-demethyl-4beta-substituted derivatives by electrospray ionization ion-trap time-of-flight tandem mass spectrometry

High-resolution electrospray ionization multistage tandem mass spectrometry (MS(1-9)) was used to determine the accurate masses and the fragmentation pathways of protonated podophyllotoxin (1) and its corresponding 4'-demethyl-4beta-substituted derivatives (2-4). The protonated molecules, [M + H](+), of all the four compounds were observed in the conventional single-stage mass spectra. Two fragmentation pathways, that appear to be characteristic of the four compounds, are proposed on the basis of their multistage tandem mass spectrometric data. The characteristic elimination, from the precursor protonated ions, of the neutral groups 4-R(1)H, 1-ArH, CO, CH(2)O and C(4)H(4)O(2), in which R is located on C-4, is the common elimination, and the product ions at m/z 267, 239, 229, 181, 173, 153, 143 and 115 are the common diagnostic masses. The elimination of the R(1) group substituent located on the C-4 position of compounds 1-4 has a significant influence on the fragmentation pathway obtained in the conventional single-stage mass spectra. A large R(1) group would be unfavorable for this elimination, unless the collision energy is raised. Apart from the common fragmentations obtained for the protonated molecules 1-4, significant additional product ions were detected in the various multistage tandem mass spectrometric analyses, particularly in the case of the product ions derived initially from the phenolic hydroxyl group of 2-4, which are different from those of 1. Based on these additional formed product ions, several additional fragmentation pathways for 1 or 2-4 are also presented.     

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).     

Studies of iridoid glycosides using liquid chromatography/electrospray ionization tandem mass spectrometry

Fragmentation pathways of five iridoid glycosides have been studied by using electrospray ionization multi-stage tandem mass spectrometry (ESI-MS(n)). The first-stage MS data of the five iridoid glycosides were compared. The MS spectra showed that the adduct ions of iridoid glycosides and the formate anion were diagnostic ions to distinguish iridoid glycosides with a carboxyl group at the C-4 position or an ester group at the C-4 position. The MS fragmentation pathways of the five iridoid glycosides were also studied. Analyzing the product ion spectra of iridoid glycosides, some neutral losses were observed, such as H(2)O, CO(2) and glucose residues, which were very useful for the identification of the functional groups in the structures of iridoid glycosides. Furthermore, specific loss of one molecule of methyl 3-oxopropanoate or 3-oxopropanic acid was firstly discussed, which corresponded to the isomerization of the hemiacetal group in the structure of iridoid aglycone. According to the fragmentation mechanisms and HPLC/MS(n) data, the structures of five iridoid glycosides in a crude extract of Gardenia jasminoisdes fruit have been identified. Three compounds were compared with standards and the other two were identified as shanzhiside and genipin gentibioside by their MS(n) data without standard compounds. In order to further validate the veracity of the deduction, genipin gentiobioside was isolated from the extract of Gardenia jasminoisdes fruit using Purification Factory and was further identified by C- and H-NMR.     

Electrospray ionization tandem mass spectrometric analysis of ent‐6,7‐seco‐kaurane diterpenoids from the Isodon species

Electrospray ionization tandem mass spectrometry (ESI‐MSⁿ) using an ion trap instrument and accurate mass measurement on a quadrupole time‐of‐flight (Q‐TOF) mass spectrometer has aided the structural characterization and differentiation of the enmein and spiro‐lactone types of ent‐6,7‐seco‐kaurane diterpenoids from Isodon species. The mass spectral fragmentation data from both techniques was compared to obtain the mass spectrometric fragmentation pathways of the ent‐6,7‐seco‐kaurane diterpenoids with high confidence. The analysis revealed that losses of CH₂O and CO₂ are the predominant process for the enmein type of ent‐kauranes in negative ion mode, and the loss of CO₂ is typical for the spiro‐lactone type in positive ion mode. In addition, compounds of the spiro‐lactone type with a conserved core structure but different substituent groups, such as acetyl, hydroxyl, and aldehyde moiety, resulted in diagnostic product ions by means of successive losses of AcOH, H₂O, and CO, respectively. The fragmentation knowledge will facilitate the analysis and identification of the ent‐6,7‐seco‐kauranes in future plant research. Copyright © 2008 John Wiley & Sons, Ltd.     

Electrospray ionisation mass spectrometric study of degradation products of quercetin, quercetin-3-glucoside and quercetin-3-rhamnoglucoside, produced by in vitro fermentation with human faecal flora

Eight phenolic compounds, obtained by in vitro fermentation of quercetin, quercetin-3-glucoside and quercetin-3-rhamnoglucoside were analysed by electrospray ionisation mass spectrometry (ESI-MS). Low-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS) was performed on the [M - H]- precursor ions to obtain specific fragmentation. Typical fragmentation of the phenolic acids was loss of 44 (CO2) and 18 (H2O) u. Production of m/z 108 by loss of neutral radicals, e.g. HCO2, CH3 or HCO, was also favoured. Structures of the compounds, numbered 1-8, were suggested based on the fragmentation patterns.     

Electrospray ionization ion-trap time-of-flight tandem mass spectrometry of two furofurans: sesamin and gmelinol

High-resolution electrospray ionization multistage tandem mass spectrometry (MS(1-7)) in positive ion mode was used to determine the accurate masses and the fragmentation pathways of two furofurans, sesamin and gmelinol. The protonated molecules [M+H]+ were absent in the conventional mass spectra of both compounds, but two characteristic ions, [M+H-H(2)O]+ and [M+H-H(2)]+, were always observed. Successive losses of CH(2)O and CO are the common characteristic fragmentations. Based on the exact masses acquired from 21 different tandem mass spectra, two or three fragmentation pathways for each compound are proposed. The consecutive losses of two H(2)O molecules and one H(2) molecule readily take place from the furan rings for both sesamin and gmelinol, resulting in the absence of the protonated molecules in the single-stage experiments. HCHO loss is observed at least three times in the tandem mass spectra, mainly from methylenedioxy groups (OCH(2)O) for sesamin but only from tetrahydrofuran rings for gmelinol. Moreover, CO loss is found at least three times in the tandem mass spectra of both sesamin and gmelinol from the 3,4-methylenedioxyphenyl (ArOCH(2)O) moieties for sesamin and from both the dimethoxyphenyl and the tetrahydrofuran ring moieties for gmelinol. In addition, the disubstituted benzyl cation ArCH(2)+ at m/z 135 for sesamin and at m/z 151 for gmelinol was found in the MS(3) spectra of both sesamin and gmelinol, which is very helpful in the identification of the compositions of 3,4-disubstituted groups on the benzene rings of the furofurans.     

Charge-remote and charge-driven fragmentation processes in diacyl glycerophosphoethanolamine upon low-energy collisional activation: A mechanistic proposal

A mechanistic study of diacyl glycerophosphoethanolamine fragmentation under low energy collision-activated dissociation with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading to the formation of carboxylate anions (RxCO2-) (x = 1, 2) and the formation of the ions representing neutral loss of ketene ([M - H - Rx'CH=C=O]-) are charge-driven processes, which are governed by the gas-phase basicity and the steric configuration of the molecules. The fragmentation pathway for the formation of the [M - H - RxCO2H]- ions, reflecting neutral loss of fatty acid, is a charge-remote process, which involves the participation of the hydrogens at C-1 and C-2 of the glycerol, resulting in [M - H - R2CO2H]- > [M - H - R1CO2H]-. The preferential formations of R2CO2- > R1CO2-, and of [M - H - R2'CH=C=O]- > [M - H - R1'CH=C=O]- are attributed to the findings that charge-driven processes are sterically more favorable at sn-2. The observation of the abundance of [M - H - Rx'CH=C=O]- > [M - H - RxCO2H]- is attributed to the fact that the [M - H]- ions of GPE are basic precursor ions, which undergo preferential loss of ketene than loss of acid. The major pathway for the formation of RxCO2- ions arises from the nucleophilic attack of the anionic charge site of the phosphate on the C-1 or C-2 of the glycerol to render a charge transfer. The sterically more favorable attack on the C-2 than C-2 of the glycerol results in the abundance of R2CO2- > R1CO2-. These features of tandem spectra readily identify and locate the fatty acid substituents of GPE in the glycerol backbone.     

A fragmentation study of two compounds related to 4'-demethylepipodophyllotoxin in negative ion electrospray ionization by MSn ion-trap time-of-flight mass spectrometry

High-resolution electrospray ionization multistage tandem mass spectrometry (MS 1-7) in negative ion mode was used to determine the accurate masses and fragmentation pathways of two compounds, 4'-demethylepipodophyllotoxin and 4'-demethyl-4-azido-4-deoxyepipodophyllotoxin, which are key intermediate compounds for the preparation of podophyllotoxin-type anti-cancer drugs. The deprotonated molecules [M-H]* of both compounds were readily observed in the conventional single-stage mass spectra due to the presence of the phenolic hydroxyl group in the molecules. Abundant information on the product ions was obtained from tandem mass spectra (MS 2-7) in negative ion mode. Based on the exact masses acquired from 14 different tandem mass spectra, a similar MSn fragmentation pathway was proposed for both compounds. A characteristic product ion produced in the MS 2-4 product ion scan experiments is the cyclohexylenetrione anion [M-H-2Me-RH]* or [M-H-RH-2Me]* at m/z 351 (C19H11O7) formed by the consecutive losses of two CH3 radicals at the 3'- and 5'-positions and the neutral loss of RH, where R = a 4-substituted group (-OH or -N3), from the [M-H]* ion. This anion may be considered as diagnostic for the presence of this type of compound. The other common cleavages are the neutral losses of CO at least two times in the MS 6,7 product ion spectra. The results of this work could serve as an effective tool for the detection or determination of other derivatives of 4'-demethyl-4beta-substituted podophyllotoxin, which are widely used as intermediates for the preparation of anti-tumor drugs.     

Fragmentation pathways of oxygenated tetracyclic triterpenoids and their application in the qualitative analysis of Ganoderma lucidum by multistage tandem mass spectrometry

The fragmentation pathways of oxygenated tetracyclic triterpenoids from Ganoderma lucidum were systematically studied based on interpreting the mass spectra of 44 known triterpenoids using a combination of multistage tandem mass spectrometry (MS(n)) experiments and high-resolution mass spectrometry (HRMS) analysis. In negative ion mode, the fragmentation pathways of triterpenoid acids are rather characteristic. After the prominent loss of H(2) O or CO(2), cleavages take place on the A, B, C and D rings. Interestingly, the cleavage mode is highly dependent on the positions of the carbonyl groups and hydroxyl groups in the tetracyclic skeleton. Characteristic cleavage of ring A occurs in 7-oxo-11-H or 7-oxo-11-hydroxy derivatives; characteristic cleavage of ring B occurs in the 7-oxo-11-hydroxy derivatives; characteristic cleavage of ring C occurs in the 7-hydroxy-15-oxo derivatives; while the cleavage of ring D can be observed in the majority of the compounds investigated. The odd-electron species, which disobey the 'even-electron rule', are also observed and discussed in this paper. These phenomena provide an easy way to determine the tetracyclic skeleton and distinguish the isomers of the triterpenoids from each other. What is more, the fragmentation pathways of triterpenoid alcohols were also investigated in positive ion mode. The accurate masses of the product ions were determined using quadrupole orthogonal time-of-flight (QTOF) instruments. Finally, the fragmentation rules were applied to identify the components of G. lucidum. As a result, 73 triterpenoids including 11 new ones were identified. The triterpenoids were classified into six subclasses according to their different fragmentation behaviors. The application of tandem mass spectrometry was further explored.     

Analysis of homoisoflavonoids in <Emphasis Type="Italic">Ophiopogon japonicus</Emphasis> by HPLC-DAD-ESI-MS

The homoisoflavonoids in Ophiopogon japonicus (Thunb.) Ker-Gawler were analyzed by high-performance liquid chromatography-diode array detection-electrospray ion trap tandem mass spectrometry (HPLC-DAD-ESI-MSn). Homoisoflavonoids gave prominent [M - H]- ions by electrospray ionization monitored in the negative ion mode. They could be classified into two types depending on the fragmentation behavior of their [M - H]- ions in the ion trap mass analyzer. The [M - H]- ions of homoisoflavonoids with a saturated C2-3 bond underwent C3-9 bond cleavage to lose the B-ring, which was followed by the loss of a molecule of CO. The [M - H]- ions of homoisoflavonoids with a C2-3 double bond usually eliminated a CO molecule first, and then underwent the cleavage of C3-9 or C9-1' bonds. For homoisoflavonoids with a C-6 formyl group, however, the neutral loss of CO was the first fragmentation step; the presence of a methoxyl group at C-8 could lead to the cleavage of C-ring. No retro Diels-Alder (RDA) fragmentation characteristic for normal flavonoids was observed. The above fragmentation rules were reported for the first time, and were implemented for the analysis of homoisoflavonoids in O. japonicus. The CHCl3-MeOH extract was separated on a Zorbax Extend-C18 column, eluting with a acetonitrile-0.3% acetic acid gradient. A total of 18 homoisoflavonoids, including seven new minor constituents, were identified or tentatively characterized based on the UV spectra and tandem mass spectra of the HPLC peaks.     

Characterization of fifty-one flavonoids in a Chinese herbal prescription Longdan Xiegan Decoction by high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry and photodiode array detection

High-performance liquid chromatography coupled to electrospray ionization (ESI) tandem mass spectrometry and photodiode array detection (HPLC-DAD-ESI-MS(n)) was developed to identify and characterize the flavonoids in a Chinese formulated preparation, Longdan Xiegan Decoction (LXD). In total, fifty-one flavonoids (27 flavones, 10 flavanones, 7 chalcones, 5 flavonols and 2 isoflavones) were characterized. Eighteen compounds among them including a newly detected flavonoid, naringin, from the ingredient herbs, were unambiguously determined by comparing the retention times (t(R)), UV spectral data and mass fragmentation behaviors with those of the reference compounds. Another thirty-three compounds were tentatively identified by referencing to the reported data of their UV and MS spectra. The ESI-MS/MS fragmentation behavior of flavones (OMe-substituted, O-glycosides, C-glycosides), chalcones, flavonols and their appropriate characteristic pathways were proposed. In negative ion ESI-MS all the flavonoids yielded prominent [M--H](-) ions in the first order mass spectra. Fragmentation with a loss of mass of 15 Da (CH(3)), 18 Da (H(2)O), 28 Da (CO), 44 Da (CO(2)), 56 Da (2CO) and the residues of glucose and glucuronic acid observed in the MS/MS spectra were useful for aiding the structural identification of the flavonoids investigated.     

Fragmentation of dipyridamole and related dipyrimidines by electrospray ionization collisional activated decomposition mass spectrometry

The coronary vasodilator, co-activator of antitumor compounds and antioxidant drug dipyridamole and several of its derivatives were studied by electrospray ionization (ESI) combined with collisional activated decomposition (CAD) in both positive and negative modes. These compounds produce abundant monocharged ions ([M + H](+)) under ESI. Interpretation of the CAD spectra showed that fragmentation occurs preferentially in the ethanolamine groups attached at C-2, C-4, C-6 and C-8. 2-Methoxyethanol is eliminated when ethanolamines are in positions C-2/C-6 and 2-aziridinethanol is eliminated from C-4/C-8 ethanolamines. The proposed fragmentation schemes were supported by deuterium labeling experiments and tandem mass spectrometry.     

Tautomerization in gas-phase ion chemistry of isomeric C-8 deoxyguanosine adducts from phenol-induced DNA damage

Collision-induced dissociation (CID) of 8-(4'-hydroxyphenyl)-2'-deoxyguanosine and 8-(2'-hydroxyphenyl)-2'-deoxyguanosine was investigated using sequential tandem mass spectrometry. These adducts represent biomarkers of DNA damage linked to phenolic radicals and were investigated to gain insight into the effects of chemical structure of a C-8 modification on fragmentation pathways of modified 2'-deoxyguanosine (dG). CID in MS(2) of the deprotonated molecules of both the isomers generated the same product ion having the same m/z values. CID in MS(3) of the product ion at m/z 242 and CID in MS(4) experiments carried out on the selected product ions at m/z 225 and m/z 218 afford distinct fragmentation patterns. The conformational properties of isomeric product ions from CID showed that the ortho-isomers possess the unique ability to tautomerize through an intramolecular proton transfer between the phenolic OH group and the imine nitrogen (N7). Tautomerization of ortho-isomers to their keto-tautomers led to differences in their system of conjugated double bonds compared with either their enol-tautomer or the para-isomer. The charge redistribution through the N-7 site on the imidazole ring is a critical step in guanosine adduct fragmentation which is disrupted by the formation of the keto-tautomer. For this reason, different reaction pathways are observed for 8-(4'-hydroxyphenyl)-2'-deoxyguanosine and 8-(2'-hydroxyphenyl)-2'-deoxyguanosine. We present herein the dissociation and the gas-phase ion-molecule reactions for highly conjugated ions involved in the CID ion chemistry of the investigated adducts. These will be useful for those using tandem mass spectrometry for structural elucidation of C-8 modified dG adducts. This study demonstrates that the modification at the C-8 site of dG has the potential to significantly alter the reactivity of adducts. We also show the ability of tandem mass spectrometry to completely differentiate between the isomeric dG adducts investigated.     

In vivo biotransformation of 17 alpha-methyltestosterone in the horse revisited: identification of 17-hydroxymethyl metabolites in equine urine by capillary gas chromatography/mass spectrometry

The in vivo phase I biotransformation of 17 alpha-methyltestosterone in the horse leads to the formation of a complex mixture of regio- and stereoisomeric C(20)O(2), C(20)O(3) and C(20)O(4) metabolites, excreted in urine as glucuronide and sulphate phase II conjugates. The major pathways of in vivo metabolism are the reduction of the A-ring (di- and tetrahydro), epimerisation at C-17 and oxidations mainly at C-6 and C-16. Some phase I metabolites have been identified previously by positive ion electron ionisation capillary gas chromatography/mass spectrometry (GC/EI + MS) mainly from the characteristic fragmentation patterns of their methyloxime-trimethylsilyl ether (MO-TMS), enol-TMS or TMS ether derivatives. Following oral administration of 17 alpha-methyltestosterone to two castrated thoroughbred male horses, the glucuronic acid conjugates excreted in post-administration urine samples were selectively hydrolysed by E. coli beta-glucuronidase enzymes. Unconjugated metabolites and the steroid aglycones obtained after enzymatic deconjugation were isolated from urine by solid-phase extraction, derivatised as MO-TMS ethers and analysed by GC/EI + MS. In addition to some of the known metabolites previously identified from the characteristic mass spectral fragmentation patterns of 17 alpha-methyl steroids, some isobaric compounds exhibiting a diagnostic loss of 103 mass units from the molecular ions with subsequent losses of trimethylsilanol or methoxy groups and an absence of the classical D-ring fragment ion were detected. From an interpretation of their mass spectra, these compounds were identified as 17-hydroxymethyl metabolites, formed in vivo in the horse by oxidation of the 17-methyl moiety of 17 alpha-methyltestosterone. This study reports on the GC/EI + MS identification of these novel 17-hydroxymethyl C(20)O(3) and C(20)O(4) metabolites of 17 alpha-methyltestosterone excreted in thoroughbred horse urine.     

Characterization of compounds from the roots of Saposhnikovia divaricata by high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

In this study, four types of compounds including coumarins, chromones, furoylmethyl amino acid derivative and benzofuran glycoside were isolated from the roots of Saposhnikovia divaricata. The electrospray ionization (ESI) mass spectral fragmentation pathways of these compounds were proposed. In particular, the ESI-MS(n) fragmentation behavior of linear dihydrofurocoumarins, dihydrofuro- and dihydropyranochromones were deduced in detail. For the linear dihydrofurocoumarins, the fragmentation was triggered by the initial loss of the C-4' substituting group. Then, the characteristic ions were observed followed by the losses of 15, 18, 28 and 46 Da. It is noteworthy that the elimination of H(2)O (18 Da) from the cleavage of the dihydrofuran ring is reported for the first time. For the linear dihydrofurochromones, characteristic eliminations of 18, 48 and 72 Da were observed. The loss of 18 Da could arise from two different fragmentation pathways, and the observed ion was composed of a mixture of two different structural ions. For the linear dihydropyranochromones, it was found that the dihydropyran ring was converted into the pyran ring by the elimination of the C-3' substituting group. This fragmentation was followed by the diagnostic losses of 18, 28, 42 and 54 Da in tandem mass spectrometry. The above fragmentation rules were successfully applied for the analysis of the chemical constituents of the roots of Saposhnikovia divaricata. A total of 32 compounds were identified or tentatively characterized by HPLC/DAD/ESI-MS(n). Among them, eight compounds were new and seven compounds were reported from that genus for the first time.     

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.     

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.     

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.