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.     

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.     

Ortho effects in mass spectrometric fragmentation processes

The mass spectra of the phenylhydrazones and 2,4-dinitrophenylhydrazones of ortho substituted benzaldehydes and acetophenones (X = I, Br, Cl, OCH₃, OH) show characteristic [M ? X]⁺ ions which allow the ortho derivatives to be distinguished from their meta and para isomers.     

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.     

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.     

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.     

Ring contraction upon electron-impact as a function of ring size: The mass spectrometric fragmentation of acylated cyclic amines

The structural prerequisites of skeletal rearrangement by ring contraction, previously observed in N-acetylmorpholine, were explored by extending the study to N-acetylpyrrolidine, -piperidine and -hexamethyleneimine. Fragmentation via ring contraction was found to be absent in the five-membered ring, detectable in the six-membered ring, and significant in the seven-membered homologue. Ring contraction in these model compounds, i.e. in absence of oxygen as a second heteroatom in the cyclic moiety, is discussed in view of its implications and its support for an initially proposed radical-induced rearrangement mechanism.     

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.     

Cyclization of molecular ions: The mass spectrometric fragmentation of allene-carboxylic esters

A series of six 2,4,4-substituted allenecarboxylic esters R₁R₂C?C?C(R₃)COOAlk has been found to exhibit O-Alkyl cleavage as the most prominent feature of both their 70 and 12 eV mass spectra. Loss of linearity of the allene system upon ionization, followed by conversion of the molecular ion to a 5-membered cyclic isomer, is thought to induce this unusual mode of bond rupture. Oxygen-migration from the ester function to C-4 as is indicated by the subsequent decomposition of the [M — Alk] ions to [R₁? C?O]+ and [R₂? C?O]⁺ fragments can be taken as direct evidence in favour of this rationale.     

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.     

Ring contraction upon electron-impact as a function of ring size: The mass spectrometric fragmentation of 2,3,4,5-tetrahydrobenzoxepin and lower homologues

The origin of a prominent [M – CH₃] ion in the mass spectrum of 2,3,4,5-tetrahydrobenzoxepin is investigated with the aid of extensive deuterium labelling of the saturated heterocyclic portion of the molecule, identifying the C-2 methylene group as the chief source of the methyl radical expelled. Ring contraction of the seven-membered cyclic molecular ion to a six-membered intermediate is proposed in accordance with interpretations of similar findings on related acylated cyclic amines.     

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.     

Mass spectrometric fragmentation of the tautomers of 1,3-diketones. A gas chromatographic/mass spectrometric study

The diketo and ketoenol tautomers of aliphatic 1,3-diketones can be easily separated by gas chromatography. The mass spectra of tautomers of pentane-2,4-diones substituted at C(l) and C(3), separated in this way, have been obtained. The fragmentation mechanisms are discussed. The mass spectra of the tautomers are quite different, and the main fragmentation pathways can be easily linked to the structures of the (non-interconverting!) tautomeric molecular ions. Furthermore, isomers differing by the position of the substituent can also be identified by their mass spectra.     

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.     

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.