Evidence for aryl participation in mass spectrometric fragmentation processes

While heterolysis of oxonium ions generated upon electron-impact in the mass spectrometer is a well documented phenomenon in the fragmentation of aliphatic ethers, little analogy exists for structurally comparable immonium ions. Exceptions are ions of the latter class in which dissociation is facilitated by the presence of proper substituents in position allowing satabilization of daughter ions. Two main types of subtitution were explored providing(1) resonance stabilization of the product ions by aryl substituents in benzylic,heterolysis,and (2) aryl neighboring group participation in homobenzylic heterolysis, possibly yielding phenonium ions (or isometric ions via further rearrangement) as stable products in the gas phase. In the discussion emphasis will be placed on Case 2.     

The mass spectrometric fragmentation of n-alkanes

The fragmentation of n-hexane, n-nonane and n-tetradecane under electron impact has been investigated, using ¹³C labelled compounds. The mechanism of the formation of the alkyl radical ions is quantitatively explained by using a method of calculation developed in an earlier publication for n-heptane. It is assumed that these ions are formed either by a direct C-C bond cleaveage or by a secondary olefin loss from an alkyl radical ion. In the latter case the probability for a particular carbon to be lost in the neutral fragment is assumed to be random. The probability for a direct cleavage to an alkyl ion is about 80% for an ion containing at least half of the number of carbon atoms of the molecular ion and 15% for the smaller ions. The [M? H]⁺ ion seems to be a special case not yet clearly understood. Former results about the loss of methyl from the molecular ion are confirmed.     

The mass spectrometric fragmentation of n-benzylacetamide

The low energy mass spectra of N-benzylacetamide have been recorded. The major fragmentations of the molecular ion are similar to those observed in the acetanilide spectrum. In addition, the secondary dissociation of the [C₆H₅CH₂NH]⁺ ion by loss of HCN is shown to occur with transfer of the -N–H hydrogen to the ring.     

The mass spectrometric fragmentation of n-heptane

The electron impact fragmentation of n-heptane has been investigated using ¹³C labelled derivatives. A mechanism is proposed for the loss of alkyl radicals where the cleavage of a C? C bond is coupled with the rearrangement of a hydrogen atom, thus yielding a secondary alkyl ion that eventually fragments further by a subsequent loss of olefin. For alkyl ions with less than six carbon atoms this consecutive pathway is in competition with formation directly from the molecular ion. The consecutive pathway contributes about 15% to the intensity of the alkyl ions with four and five carbon atoms and 80% for smaller ions. The electron energy dependence was studied.     

Electrospray ionization/tandem quadrupole mass spectrometric studies on phosphatidylcholines: the fragmentation processes

We applied low-energy collisionally activated dissociation (CAD) tandem quadrupole mass spectrometry to study the fragmentation pathways of the [M + H](+) and [M + Li](+) ions of phosphatidylcholine (PC), generated by electrospray ionization (ESI). It is revealed that the fragmentation pathways leading to loss of the polar head group and of the fatty acid substituents do not involve the hydrogens attached to the glycerol backbone as previously reported. The pathway for formation of the major ion of m/z 184 by loss of the polar head group from the [M + H](+) precursor of a diacyl PC involves the participation of the alpha-hydrogen of the fatty acyl substituents, whereas the H(+) participates in the loss of fatty acid moieties. The alpha-hydrogens of the fatty acid substituents also participate in the major fragmentation processes, including formation of [M + Li-R(x)CO(2)H](+) and [M + Li-59-R(x)CO(2)H](+) ions for the [M + Li](+) ions of diacyl PCs, when subjected to low-energy CAD. These fragmentation processes are deterred by substitution of the fatty acyl moieties with alkyl, alkenyl, or hydroxyl groups and consequentially, result in a distinct product-ion spectrum for various PC, including diacyl-, plasmanyl- plasmenyl-, and lyso-PC isomers. The alpha-hydrogens of the fatty acyl substituents at sn-2 are more labile than those at sn-1. This is reflected by the preferential loss of the R(1)CO(2)H over the R(2)CO(2)H observed for the [M + Li](+) ions of diacyl PCs. The spectrum features resulting from the preferential losses permit identification and assignment of the fatty acid moieties in the glycerol backbone. The new fragmentation pathways established by tandem and source CAD tandem mass spectra of various PC molecules, including deuterium-labeling analogs, were proposed. These pathways would clarify the mechanisms underlying the ion formations that lead to the structural characterization of PC molecules.     

The mass spectrometric fragmentation of selenophene and tellurophene

The mass spectra of selenophene and tellurophene have been measured and their modes of fragmentation compared with those of furan and thiophene.     

Electrospray ionization mass spectrometric fragmentation of hydroquinone derivatives

The fragmentation patterns of nine di-, tri- and tetracyclic hydroquinones with potential antitumor activity were rationalized by invoking competing mechanisms that included sterically accelerated homolytic cleavage, Meerwein-type rearrangements and dehydrations through elimination or intramolecular nucleophilic substitution.     

An unprecedented rearrangement in collision-induced mass spectrometric fragmentation of protonated benzylamines

The collision-induced dissociation (CID) mass spectra of several protonated benzylamines are described and mechanistically rationalized. Under collision-induced decomposition conditions, protonated dibenzylamine, for example, loses ammonia, thereby forming an ion of m/z 181. Deuterium labeling experiments confirmed that the additional proton transferred to the nitrogen atom during this loss of ammonia comes from the ortho positions of the phenyl rings and not from the benzylic methylene groups. A mechanism based on an initial elongation of a C--N bond at the charge center that eventually cleaves the C--N bond to form an ion/neutral complex of benzyl cation and benzylamine is proposed to rationalize the results. The complex then proceeds to dissociate in several different ways: (1) a direct dissociation to yield a benzyl cation observed at m/z 91; (2) an electrophilic attack by the benzyl cation within the complex on the phenyl ring of the benzylamine to remove a pair of electrons from the aromatic sextet to form an arenium ion, which either donates a ring proton (or deuteron when present) to the amino group forming a protonated amine, which undergoes a charge-driven heterolytic cleavage to eliminate ammonia (or benzylamine) forming a benzylbenzyl cation observed at m/z 181, or undergoes a charge-driven heterolytic cleavage to eliminate diphenylmethane and an immonium ion; and (3) a hydride abstraction from a methylene group of the neutral benzylamine to the benzylic cation to eliminate toluene and form a substituted immonium ion. Corresponding benzylamine and dibenzylamine losses observed in the spectra of protonated tribenzylamine and tetrabenzyl ammonium ion, respectively, indicate that the postulated mechanism can be widely applied. The postulated mechanisms enabled proper prediction of mass spectral fragments expected from protonated butenafine, an antifungal drug.     

Systematic identification of N-acetylheparosan oligosaccharides by tandem mass spectrometric fragmentation

Recently, a useful procedure for the preparation of both even- and odd-numbered series of N-acetylheparosan (NAH) oligosaccharides was established. The present report describes findings when these NAH oligosaccharides were subjected to comparative mass spectrometry (MS)/MS fragmentation analysis by matrix-assisted laser desorption/ionization (MALDI)-LIFT-time-of-flight (TOF)/TOF-MS/MS, and electrospray ionization (ESI) collision-induced dissociation (CID) MS/MS. The resultant fragment ions were systematically assigned to elucidate fragmentation characteristics. In the MALDI-LIFT-MS/MS experiments, all the NAH oligosaccharides underwent unique glycosidic cleavages that included B-Y ion cleavages (nomenclature system of Domon and Costello, Glycoconjugate J. 1988; 5: 397) at the C-1 side, and C-Z ion cleavages at the C-4 side, with respect to glucuronic acid (GlcA). In addition, (0,2)A and/or (0,2)X cross-ring cleavages were observed for relatively small oligosaccharides. The former observation clearly reflects the occurrence of a GlcA-N-acetylglucosamine (GlcNAc) alternating structure of NAH, while the latter feature implies the occurrence of the -beta-1-4-glucuronide linkage. Extensive glycosidic cleavages were also observed in the ESI-CID-MS/MS fragmentation, though cleavage specificity was less evident than in the case of MALDI-LIFT-TOF/TOF-MS/MS. The information obtained in this study should be valuable for understanding both biosynthetic and degradation processes of NAH and its derivatives including heparin and heparan sulfate, as well as artificially modified NAH oligosaccharides.     

The mass spectrometric fragmentation patterns of some biologically active 1,3,4-thiadiazoles

Mass spectra of some twelve derivatives of 1,3,4-thiadiazole are reported. The fragmentation scheme of the 1,3,4-thiadiazole ring is specific and indicative as to the structure. Derivatives of 5-phenyl-1,3,4-thiadiazoles show rearrangement to isothiocyanates.     

The mass spectrometric fragmentation of some diastereoisomeric 4-methyl-1,3,2-dioxaphosphorinanes

The mass spectra of nine diastereoisomeric pairs of 4-methyl-2-X-2-Y-1,3,2-dioxaphosphorinanes have been examined. The differences between the intensities of particular ions in the mass spectra of geometrical isomers are reported and rationalized in terms of the conformations adopted by compounds.     

Nitramine anion fragmentation: A mass spectrometric and Ab initio study

The fragment ion formation characteristics of the radical anions generated from hexahydro-1,3,5-trinitrotriazine (RDX) and its three nitroso metabolites were studied using GC/MS with negative chemical ionization (NCI) to understand the fragmentation mechanisms responsible for the formation of the most abundant ions observed in their NCI mass spectra. Ab initio and density functional theory calculations were used to calculate relative free energies for different fragment ion structures suggested by the m/z values of the most abundant ions observed in the NCI mass spectra. The NCI mass spectra of the four nitramines are dominated by ions formed by the cleavage of nitrogen-nitrogen and carbon-nitrogen bonds in the atrazine ring. The most abundant anions in the NCI mass spectra of these nitramines have the general formulas C(2)H(4)N(3)O (m/z 86) and C(2)H(4)N(3)O(2) (m/z 102). The analyses of isotope-labeled standards indicate that these two ions are formed by neutral losses that include two exocylic nitrogens and one atrazine ring nitrogen. Our calculations and observations of the nitramine mass spectra suggest that the m/z 86 and m/z 102 ions are formed from either the (M--NO)(-) or (M--NO(2))(-) fragment anions by a single fragmentation reaction producing neutral losses of CH(2)N(2)O or CH(2)N(2)O(2) rather than a set of sequential reactions involving neutral losses of HNO(2) or HNO and HCN.     

Identification of phosphorylcholine substituted peptides by their characteristic mass spectrometric fragmentation

Phosphorylcholine (PC) substituted biomolecules are wide-spread, highly relevant antigens of parasites, since this small hapten has been found to be a potent immunomodulatory component which allows the establishment of long lasting infections of the host. Structural data, especially of protein bound PC-substituents, are still rare due to the observation that mass spectrometric analyses are mostly hampered by this zwitterionic substituent resulting in low sensitivities and unusual but characteristic fragmentation patterns. Here we investigated the fragmentation behaviour of synthetic PC-substituted peptides by matrix-assisted laser desorption/ionization mass spectrometry and electrospray ionization ion trap mass spectrometry. We could show that the predominant neutral loss of a trimethylamine unit (Hoffmann elimination) leads to cyclic phosphate derivatives which prevent further fragmentation of the peptide backbone by stabilizing the positive charge at this particular side chain. Knowledge of this PC-specific fragmentation might help to identify PC-substituted biomolecules and facilitate their structural analysis.     

The mass spectrometric fragmentation of α-diazosulphones

The fragmentation of four α-diazosulphones under conditions of electron-impact was studied with the aid of high resolution mass measurements and the metastable defocusing technique. An important difference from the spectra of the related α-diazoketones is the absence of peaks for [M ? N₂]^+˙. Metastables show, however, that these fragments probably exist as intermediates.     

Competing fragmentation processes in tandem mass spectra of heparin-like glycosaminoglycans

Heparin-like glycosaminoglycans (HLGAGs) are highly sulfated, linear carbohydrates attached to proteoglycan core proteins and expressed on cell surfaces and in basement membranes. These carbohydrates bind several families of growth factors and growth factor receptors and act as coreceptors for these molecules. Tandem mass spectrometry has the potential to increase our understanding of the biological significance of HLGAG expression by providing a facile means for sequencing these molecules without the need for time-consuming total purification. The challenge for tandem mass spectrometric analysis of HLGAGs is to produce abundant ions derived via glycosidic bond cleavages while minimizing the abundances of ions produced from elimination of the fragile sulfate groups. This work describes the competing fragmentation pathways that result from dissociation of high negative charge state ions generated from HLGAGs. Glycosidic bond cleavage ion formation competes with losses of equivalents of H2SO4, resulting in complex ion patterns. For the most highly sulfated structure examined, an octasulfated tetramer, an unusual loss of charge from the precursor ion was observed, accompanied by low abundance ions originating from subsequent backbone cleavages. These results demonstrate that fragmentation processes competing with glycosidic bond cleavages are more favored for highly sulfated HLGAG ions. In conclusion, reduction of charge-charge repulsions, such as is achieved by pairing the HLGAG ions with metal cations, is necessary in order to minimize the abundances of ions derived via fragmentation processes that compete with glycosidic bond cleavages.     

Mass spectrometry of nitrogen heterocycles. 2. Correlation of mass spectrometric fragmentation processes and chemical aromatization reactions of dihydroazines

The results of mass spectrometric studies of aromatization processes of dihydroazines have been compared with experimental data relative to their chemical oxidation. The results of electron impact induced ionization of dihydroazines can be correlated directly with the direction of chemical oxidation.For Communication 1, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1083–1087, August, 1989.     

Similarities between thermolysis and mass spectrometric fragmentation of tetrazoles (review)

Both thermolysis and dissociative ionization of tetrazole and its derivatives form products of the same elemental composition. This similarity permits mass spectrometry to be used to predict thermal conversions and the compositions of thermolysis products.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 723–741, June, 1985.