A tandem mass spectrometric study of the N-oxides, quinoline N-oxide, carbadox, and olaquindox, carried out at high mass accuracy using electrospray ionization

A mass spectrometric study of three N-oxides, quinoline N-oxide, and the synthetic antibiotics carbadox and olaquindox, was carried out with a hybrid quadrupole/time-of-flight (TOF) mass spectrometer coupled with electrospray (ES) and atmospheric pressure chemical ionization (APCI) sources. The full scan mass spectra of the N-oxides obtained with ES are similar to those obtained with APCI, and the characteristic fragment ions corresponding to [M+H−O]⁺√ were observed in the full scan mass spectrum of each N-oxide examined. The protonated molecule of each N-oxide was subjected to collision-induced dissociation (CID) and accurate mass measurements were made of each fragment ion so as to determine its elemental composition. Fragment ions generated at enhanced cone voltages upstream of the first mass-resolving element were subjected to CID so as to identify the direct product ion–precursor ion relationship. Plausible structures have been proposed for most of the fragment ions observed. Elimination of OH√ radicals generated from the N→O functional group is a characteristic fragmentation pathway of the N-oxides. The expulsion of radicals and small stable molecules is accompanied by formation and subsequent contraction of heterocyclic rings.     

Two New Lycopodium Alkaloids from Lycopodium obscurum

Two new Lycopodium alkaloids, (+)‐cermizine D N‐oxide ( 1 ) and (8β)‐8‐(acetyloxy)obscurumine A ( 2 ), along with five known compounds, were isolated from the crude alkaloid portion of Lycopodium obscurum. Their structures were elucidated on the basis of spectroscopic data and chemical correlation. All of these alkaloids were tested in an assay for acetylcholine esterase (AChE) inhibitory activity.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen: XI—Einfluß von Substituenten auf die Bildung cyclischer Fragment-Ionen in den Massenspektren von N,N-Dimethyl-N′-2-chlorphenylformatmidinen und 2-Chlorformaniliden

Molecular ions of N,N-dimethyl-N′-2-chlorophenylformamidines (III) and 2-chloroformanilides (IV) lose a chlorine atom to give benzimidazolium and benzoxazolium ions, respectively. As with N,N-dimethyl-N′-phenylformamidines (I), a linear relationship exists between the Hammett σ-constants and the effect of substituents on the ionisation potentials of substituted III and IV. In contrast to this, the appearance potentials of the cyclic fragment ions of III and IV cannot be easily related to polar effects of substituents; these effects are similar for the cyclic fragment ions of I, III and IV however. Furthermore, the intensities of these ions are influenced in the same direction by substituents in the mass spectra of I, III and IV, and are strongly reduced by electron donating substituents in the para position. The formation of cyclic fragment ions in the mass spectra of I, III and IV therefore occurs by the same mechanism.     

Fawcettimine‐Type Lycopodium Alkaloids as a Driving Force for Discoveries in Organic Synthesis

Ever since the pioneering synthetic work reported by both Inubushi and Heathcock back in 1980s, the fawcettimine‐type Lycopodium alkaloids have continuously served as a driving force for discoveries in organic synthesis. In this personal account, we summarized our recent synthetic efforts towards the total synthesis of fawcettimine‐type Lycopodium alkaloids, along with a brief summary of relevant syntheses reported by others. Our discussions focus mainly on the key reactions applied during the synthesis of fawcettimine‐type Lycopodium alkaloids.     

Rapid structural determination of alkaloids in a crude extract of Stemona saxorum by high‐performance liquid chromatography/electrospray ionization coupled with tandem mass spectrometry

The electrospray ionization (ESI) mass spectrometric behavior of five Stemona alkaloids, stemokerrin, oxystemokerrin, oxystemokerrilactone, oxystemokerrin N‐oxide and stemokerrin N‐oxide, was studied using an ESI tandem mass technique (MSⁿ). These compounds, isolated from Stemona saxorum endemic in Vietnam, represent a class of alkaloids containing a pyrido[1,2‐a]azepine A,B‐ring core with a 1‐hydroxypropyl side chain attached to C‐4. Their fragmentation pathways were elucidated by ESI‐MSⁿ results and the elemental composition of the major product ions was confirmed by accurate mass measurement. In order to rationalize some fragmentation pathways, the relative Gibbs free energies of some product ions were estimated using the B3LYP/6‐31+G(d) method. Based on the ESI‐MSⁿ results of five reference compounds, a reversed‐phase high‐performance liquid chromatography with tandem mass spectrometry (RP‐HPLC/MSⁿ) method was developed for the characterization of Stemona alkaloids with a pyrido[1,2‐a]azepine A,B‐ring core from the extract of S. saxorum. A total of 41 components were rapidly identified or tentatively characterized, of which 12 compounds were identified as Stemona alkaloids with a pyrido[1,2‐a]azepine A,B‐ring core, including four new compounds. This method is convenient and sensitive, especially for minor components in complex natural product extracts. Copyright © 2009 John Wiley & Sons, Ltd.     

Elucidating the mass spectrum of the retronecine alkaloid using DFT calculations

Pyrrolizidine alkaloids are natural molecules playing important roles in different biochemical processes in nature and in humans. In this work, the electron ionization mass spectrum of retronecine, an alkaloid molecule found in plants, was investigated computationally. Its mass spectrum can be characterized by three main fragment ions having the following m/z ratios: 111, 94, and 80. In order to rationalize the mass spectrum, minima and transition state geometries were computed using density functional theory. It was showed that the dissociation process includes an aromatization of the originally five‐membered ring of retronecine converted into a six‐membered ring compound. A fragmentation pathway mechanism involving dissociation activation barriers that are easily overcome by the initial ionization energy was found. From the computed quantum chemical geometric, atomic charges, and energetic parameters, the abundance of each ion in the mass spectrum of retronecine was discussed.     

Structural characterization and identification of C19‐ and C20‐diterpenoid alkaloids in roots of Aconitum carmichaeli by rapid‐resolution liquid chromatography coupled with time‐of‐flight mass spectrometry

Aconite alkaloids from the roots of Aconitum carmichaeli (Fuzi, in Chinese) have been investigated by rapid‐resolution liquid chromatography coupled with time‐of‐flight mass spectrometry (TOFMS) in positive mode. With dynamic adjustment of the key role as fragmentor voltage in TOFMS, an efficient transmission of the ions was achieved to obtain the best sensitivity for providing the molecular formula for each analyte, and abundant fragment ions for structural information. Fifteen authentic standards isolated from Fuzi with various structures were first characterized by TOFMS, including diester‐diterpenoid alkaloids (DDAs), monoester‐diterpenoid alkaloids (MDAs), alkylol amine‐diterpenoid alkaloids (ADAs), veatchine‐type alkaloids and atisine‐type alkaloids. Fragmentation rules and key diagnostic fragment ions have been summarized, and possible pathways of fragmentation have been proposed. By accurate mass measurements within 5 ppm error for each ion, 30 C₁₉‐diterpenoid alkaloids including 10 DDAs, 3 MDAs, 9 ADAs and 8 other type alkaloids, and 8 C₂₀‐diterpenoid alkaloids including 4 veatchine‐type alkaloids and 4 atisine‐type alkaloids could be identified in a methanolic extract of Fuzi. Some isomers of aconite alkaloids were also differentiated. Based on the differences between their fragmentation pathways and special fragment ions, each type of aconite alkaloids was differentiated. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass spectra of isomeric monoalkylanisoles and monoalkylphenols with straight alkyl chains

An analysis of the mass spectra of ortho, meta and para isomers of monoalkylanisoles and monoalkylphenols, the alkyls being n-amyl, n-octyl and n-undecyl, has been performed. The regularities found allow determination of the structure of such compounds by inspection of the mass spectrum only. It was shown that Hammett's equation may be used for evaluating a ration of intensities of peaks originating formally from the simple β-cleavage and that accompanied by hydrogen migration of the resulting fragment ions.     

Lyconadins C and F, new Lycopodium alkaloids from Lycopodium complanatum

New Lycopodium alkaloids, lyconadins C (1) and F (2), were isolated from the club moss Lycopodium complanatum. Lyconadin C (1) is a new C₁₆N₂-type Lycopodium alkaloid possessing unique fused-tetracyclic ring system consisting of a cycloheptene ring fused to a decahydroquinoline and pyridone rings. Lyconadin F (2) possesses a primary amide moiety in its molecular, which is the first example of Lycopodium alkaloids. Biogenetically, lyconadins C (1) and F (2) might be related to lyconadins A (4) and B (5). The structures and relative stereochemistry of 1 and 2 were elucidated on the basis of spectroscopic data. The absolute stereochemistry of 2 was elucidated by chemical correlations with lyconadin B (5) through hemiaminal form of lyconadin F (3).     

Mass Spectra of Substituted N-Alkylpyrroles and Related Compounds

Under El conditions, substituents on N-alkylpyrroles affect fragmentation of the alkyl groups. The electron-withdrawing character of the cyano group retarded formation of a N-methylene cation, and a labile substituent, i.e., -CHO, -CH₂COOMe, competed with an N-alkyl group to be a initial fragmentation center, MNDO calculation of the heat of formation of possible fragment ions showed that protonated pyridine is the most stable among the isobaric ions.     

In situ N-phosphorylation of oligopeptides for fast atom bombardment mass spectrometry

Positive ion fast atom bombardment mass spectrometry (FABMS) of in situ N-phosphorylated oligopeptides showed intense quasi-molecular ions together with the successive alkene loss fragment ions, which afford multiple checks of the unequivocal reality of the relative molecular mass of the tested samples. More interesting, in a novel cleavage pattern only the N-phosphoryl fragment ions gave intense peaks, the C-terminal series ions being suppressed. For each of the N-terminal ions, losses of alkenes also occur to provide multiple checks for the existence of these ions. The FABMS of the in situ N- phosphorylated oligopeptides might provide an easily accessible routine method for peptide sequencing.     

Two Novel Lycopodium Alkaloids from Huperzia serrata

Huperzine Q ( 1 ) and N‐oxyhuperzine Q ( 2 ), two novel irregular fawcettimine‐type Lycopodium alkaloids were isolated from the CHCl₃ fraction of the basic material of the whole plant of the Chinese medicinal herb Huperzia serrata. Their structures were determined as 13‐epi‐13β,16‐epoxydihydrofawcettimine ( 1 ) and N‐oxy‐13‐epi‐13β,16‐epoxydihydrofawcettimine ( 2 ) by means of spectroscopic studies and X‐ray crystallographic analysis.     

Travelling‐wave ion mobility and negative ion fragmentation of high‐mannose N‐glycans

The isomeric structure of high‐mannose N‐glycans can significantly impact biological recognition events. Here, the utility of travelling‐wave ion mobility mass spectrometry for isomer separation of high‐mannose N‐glycans is investigated. Negative ion fragmentation using collision‐induced dissociation gave more informative spectra than positive ion spectra with mass‐different fragment ions characterizing many of the isomers. Isomer separation by ion mobility in both ionization modes was generally limited, with the arrival time distributions (ATD) often showing little sign of isomers. However, isomers could be partially resolved by plotting extracted fragment ATDs of the diagnostic fragment ions from the negative ion spectra, and the fragmentation spectra of the isomers could be extracted by using ions from limited areas of the ATD peak. In some cases, asymmetric ATDs were observed, but no isomers could be detected by fragmentation. In these cases, it was assumed that conformers or anomers were being separated. Collision cross sections of the isomers in positive and negative fragmentation mode were estimated from travelling‐wave ion mobility mass spectrometry data using dextran glycans as calibrant. More complete collision cross section data were achieved in negative ion mode by utilizing the diagnostic fragment ions. Examples of isomer separations are shown for N‐glycans released from the well‐characterized glycoproteins chicken ovalbumin, porcine thyroglobulin and gp120 from the human immunodeficiency virus. In addition to the cross‐sectional data, details of the negative ion collision‐induced dissociation spectra of all resolved isomers are discussed. Copyright © 2016 John Wiley & Sons, Ltd.     

Mass spectra of C-nucleosides II. An unusual fragmentation reaction of the heterocyclic moiety of pyrazomycin and some closely related compounds

The mass spectrum of pyrazomycin has been obtained and reveals a fragmentation pattern with several important peaks which are not normally found in the mass spectra of C-nucleosides. It has now been established, using model compounds, that these unusual fragment ions are a direct result of the juxtaposition of the exocyclic hydroxy and carboxamido groups of the aglycon. It appears that a facile elimination of ammonia and ethanol from o-hydroxycarboxamides and o-hydroxyethylesters, respectively, may be a general fragmentation reaction for aromatic heterocycles. A mass spectrum of the TMS derivative of pyrazomycin has also been obtained and factors which may result in exceptions to the empirical B+30 (M-103) rule for C-nucleosides are discussed.     

Structural analysis of rapamycin and related compounds using [M + Li]+ ions generated by liquid secondary ion mass spectrometry

Cationization of the macrocyclic immunosuppressant rapamycin with lithium ion upon liquid secondary ion mass spectrometric ionization yields a number of fragment ions, which are observable in the full-scan spectrum. These are clearly assigned using B/E linked scanning (fragment ion scanning), B²/E linked scanning (precursor ion scanning) and peak matching for accurate mass measurement. Many of the fragments are produced by processes that open the macrocyclic ring, and it is possible to observe several different pieces of the molecule as fragment ions. The diversity of fragments produced facilitates the elucidation of new rapamycin-like structures through mass spectrometry. Three structurally modified rapamycin analogues have been examined by this technique, and the modifications to the molecule may be located based on the nominal masses of their fragments.     

Characterization of N‐methylated amino acids by GC‐MS after ethyl chloroformate derivatization

Methylation is an essential metabolic process in the biological systems, and it is significant for several biological reactions in living organisms. Methylated compounds are known to be involved in most of the bodily functions, and some of them serve as biomarkers. Theoretically, all α‐amino acids can be methylated, and it is possible to encounter them in most animal/plant samples. But the analytical data, especially the mass spectral data, are available only for a few of the methylated amino acids. Thus, it is essential to generate mass spectral data and to develop mass spectrometry methods for the identification of all possible methylated amino acids for future metabolomic studies. In this study, all N‐methyl and N,N‐dimethyl amino acids were synthesized by the methylation of α‐amino acids and characterized by a GC‐MS method. The methylated amino acids were derivatized with ethyl chloroformate and analyzed by GC‐MS under EI and methane/CI conditions. The EI mass spectra of ethyl chloroformate derivatives of N‐methyl ( 1–18 ) and N,N‐dimethyl amino acids ( 19–35 ) showed abundant [M‐COOC₂H₅]⁺ ions. The fragment ions due to loss of C₂H₄, CO₂, (CO₂ + C₂H₄) from [M‐COOC₂H₅]⁺ were of structure indicative for 1–18 . The EI spectra of 19–35 showed less number of fragment ions when compared with those of 1–18 . The side chain group (R) caused specific fragment ions characteristic to its structure. The methane/CI spectra of the studied compounds showed [M + H]⁺ ions to substantiate their molecular weights. The detected EI fragment ions were characteristic of the structure that made easy identification of the studied compounds, including isomeric/isobaric compounds. Fragmentation patterns of the studied compounds ( 1–35 ) were confirmed by high‐resolution mass spectra data and further substantiated by the data obtained from ¹³C₂‐labeled glycines and N‐ethoxycarbonyl methoxy esters. The method was applied to human plasma samples for the identification of amino acids and methylated amino acids. Copyright © 2016 John Wiley & Sons, Ltd.     

Lycopodium Alkaloids from Lycopodium obscurum L.

Two new Lycopodium alkaloids, named obscurumines F and G ( 1 and 2 , resp.), together with eleven known alkaloids, were isolated from the club moss Lycopodium obscurum L. Their structures were elucidated on the basis of spectroscopic analyses.     

Specific rearrangement reactions of acetylated lysine containing peptide bn (n = 4–7) ion series

Characterization of ε‐N‐acetylated lysine containing peptides, one of the most prominent post‐translational modifications of proteins, is an important goal for tandem mass spectrometry experiments. A systematic study for the fragmentation reactions of b ions derived from ε‐N‐acetyllysine containing model octapeptides (K_AcYAGFLVG and YAK_AcGFLVG) has been examined in detail. Collision‐induced dissociation (CID) mass spectra of b_n (n = 4–7) fragments of ε‐N‐acetylated lysine containing peptides are compared with those of N‐terminal acetylated and doubly acetylated (both ε‐N and N‐terminal) peptides, as well as acetyl‐free peptides. Both direct and nondirect fragments are observed for acetyl‐free and singly acetylated (ε‐N or N‐terminal) peptides. In the case of ε‐N‐acetylated lysine containing peptides, however, specific fragment ions (m/z 309, 456, 569 and 668) are observed in CID mass spectra of b_n (n = 4–7) ions. The CID mass spectra of these four ions are shown to be identical to those of selected protonated C‐terminal amidated peptides. On this basis, a new type of rearrangement chemistry is proposed to account for the formation of these fragment ions, which are specific for ε‐N‐acetylated lysine containing peptides. Consistent with the observation of nondirect fragments, it is proposed that the b ions undergo head‐to‐tail macrocyclization followed by ring opening. The proposed reaction pathway assumes that b_n (n = 4–7) of ε‐N‐acetylated lysine containing peptides has a tendency to place the K_Ac residue at the C‐terminal position after macrocyclization/reopening mechanism. Then, following the loss of CO, it is proposed that the marker ions are the result of the loss of an acetyllysine imine as a neutral fragment. Copyright © 2014 John Wiley & Sons, Ltd.     

Three new Lycopodium alkaloids from Huperzia carinata and Huperzia squarrosa

Eleven Lycopodium alkaloids with lycodine-type, lycopodine-type, and fawcettimine-related skeletons were isolated from the whole plants of Huperzia carinata (Desv. Ex. Poir.) Trevis and Huperzia squarrosa (G. Forst) Trevis (Huperziaceae). Among them, 8,15-dihydrohuperzine A (2), lycocarinatine A (3), and lycoposerramine U N-oxide (11) are new compounds. The structures of these new alkaloids were elucidated on the basis of 2D NMR correlations. Some of these isolated alkaloids were assayed for acetylcholinesterase (AChE) inhibitory activity.     

Factors affecting immonium ion intensities in the high-energy collision-induced decomposition spectra of peptides

Each amino acid in a peptide has a characteristic immonium ion (H₂N⁺?CHR), the presence of which in a mass spectrum can indicate the presence of that amino acid. High-energy collision-induced decomposition studies on small peptide ions formed by fast atom bombardment showed the relative intensities of these immonium ions to be dependent on the relative positions of the amino acids in the peptide chain: C-terminal, N-terminal or in-chain. Evidence in favour of competition in the formation of immonium ions is presented.