Solvation in Electrospray Mass Spectrometry: Effects on the Reaction Kinetics of Fragmentation Mediated by Ion-Neutral Complexes

Differentiation of isomeric heptylamines by in-source collision-induced dissociation of [M + H]+ ions

RATIONALE

1,3-Dimethylamylamine (1,3-DMAA), 1,4-dimethylamylamine (1,4-DMAA) and 2-aminoheptane are isomeric heptylamines commonly used as stimulants. The fragmentations of their [M + H]⁺ ions (m/z 116) are almost identical and consist of the formation of a fragment ion C₄ H₉ ⁺ at m/z 57. It is interesting to check if analysis of electrospray ionization (ESI) mass spectra of these three amines enables their differentiation. In order to better understand the fragmentation of isomeric heptylamines, 1-, 3- and 4-aminoheptane were also included in the study.

METHODS

ESI in-source collision-induced dissociation (CID) mass spectra were obtained with a Waters/Micromass ZQ2000 mass spectrometer (single quadrupole-type instrument) at a cone voltage of 12-34 V. The influence of three commonly used solvents on the fragmentation of [M + H]⁺ ions was tested, namely methanol, methanol/water (1/1) and acetonitrile/water (1/1).

RESULTS

Only one quantitative difference was observed in the mass spectra of 1,3-DMAA, 1,4-DMAA and 2-aminoheptane, namely formation of a product ion at m/z 57 from the [M + H]⁺ ion of 1,3-DMAA is more efficient than the formation of this ion from [M + H]⁺ ions of 2-aminoheptane and 1,4-DMAA. In-source CID of 3- and 4-aminoheptane consists of (besides the formation of ion C₄ H₉ ⁺ ) the formation of a low-abundance, but clearly seen, fragment ion at m/z 114 (loss of H₂ ).

CONCLUSIONS

Differentiation of 1,3-DMAA from 1,4-DMAA and 2-aminoheptane seems to be possible by in-source CID of their [M + H]⁺ ions. However, comparison of the mass spectrum obtained for the analysed sample with that of a standard seems to be necessary. Only 3- and 4-aminoheptane may be easily differentiated from the other heptylamines by the presence of fragment ions at m/z 114.

Gas-phase fragmentation of the protonated benzyl ester of proline: intramolecular electrophilic substitution versus hydride transfer

In this study, the gas phase chemistry of the protonated benzyl esters of proline has been investigated by electrospray ionization mass spectrometry and theoretical calculation. Upon collisional activation, the protonated molecules undergo fragmentation reactions via three primary channels: (1) direct decomposition to the benzyl cation (m/z 91), (2) formation of an ion-neutral complex of [benzyl cation + proline](+), followed by a hydride transfer to generate the protonated 4,5-dihydro-3H-pyrrole-2-carboxylic acid (m/z 114), and (3) electrophilic attack at the amino by the transferring benzyl cation, and the subsequent migration of the activated amino proton leading to the simultaneous loss of (H2O + CO). Interestingly, no hydrogen/deuterium exchange for the fragment ion m/z 114 occurs in the d-labeling experiments, indicating that the transferring hydride in path-b comes from the methenyl hydrogen rather than the amino hydrogen. For para-substituted benzyl esters, the presence of electron-donating substituents significantly promotes the direct decomposition (path-a), whereas the presence of electron-withdrawing ones distinctively inhibits that channel. For the competing channels of path-b and path-c, the presence of electron-donating substituents favors path-b rather than path-c, whereas the presence of electron-withdrawing ones favors path-c rather than path-b.

An unusual intramolecular transfer of the fluorobenzyl cation between two remote amidic nitrogen atoms induced by collision in the gas phase

A highly unusual rearrangement in collision-induced dissociation mass spectrometry is reported that involves intramolecular transfer of the fluorobenzyl cation between two remote amidic nitrogen atoms separated by five chemical bonds. The same intramolecular transfer was also observed for two related analogs. It is postulated that the ionic reactions are initiated by protonation of the first amidic nitrogen, resulting in formation of the fluorobenzyl cation and a neutral partner that are maintained together in the gas phase by electrostatic interactions as an intermediate ion-neutral complex. In the ion-neutral complex, the nascent fluorobenzyl cation approaches geometrically to the second amidic nitrogen atom on the neutral partner, and subsequently forms a new C-N bond and an isomeric precursor ion as the charge is retained on the amidic nitrogen. The newly formed isomeric precursor ion eventually undergoes the final fragmentation by amide bond cleavage. Alternatively, the ionic reactions proceed through a direct intramolecular transfer mechanism by which the molecular ion adopts to a ring-like configuration in the gas phase, so that both the donor and recipient nitrogens are geometrically close to each other within a bonding distance to permit a direct transfer between two sites even though they are separated by multiple chemical bonds.

Intramolecular transacylation: fragmentation of protonated molecules via ion-neutral complexes in mass spectrometry

An intramolecular transacylation reaction was observed in the mass spectrometry of molecules containing both benzoyl and carboxymethyl groups on an aromatic heterocyclic core. The reaction is triggered by a dissociative protonation on the heterocyclic ring at the atom (carbon or nitrogen) that bonds to the benzoyl group, leading to an intermediate ion-neutral complex. The incipient benzoyl cation in the complex migrates to attack the carboxyl group of the neutral partner at the carbonyl or hydroxyl oxygen under thermodynamic or kinetic control, respectively. Elimination of benzoic acid followed by loss of carbon monoxide takes place as a result of the transacylation.

Electrospray tandem mass spectrometry of alendronate analogues: fingerprints for characterization of new potential prodrugs

1-hydroxymethylene-1,1-bisphosphonic acids (HMBPs) are important drugs for the treatment of a variety of bone diseases. Since these compounds have no chromophore, their detection is challenging and mass spectrometry (MS) appears to be an appropriate sensitive tool. Our work deals with the analysis by electrospray ionization tandem mass spectrometry (ESI-MSn) of the well-known nitrogen-containing HMBP alendronate and of three analogues, considered as potential prodrugs. These four molecules share a common structure with different protecting groups on the phosphonic acid and on the amine functions. We describe the dissociation mechanisms of nitrogen-containing HMBPs in positive ion mode and we compare, in negative ion mode, our results with literature data. In both modes, the dissociations are essentially losses of ROH, and of phosphorus-containing species (HPO2, ROP(OH)2 and ROPO(OH)2), where R=H, C6H5, or CH3OC6H5. These fingerprints will be of great value for differentiating alendronate from its potential prodrugs in complex biological mixtures.

Fragmentation patterns of new esterified and unesterified aromatic 1-hydroxymethylene-1, 1-bisphosphonic acids by ESI-MSn

1-Hydroxymethylene-1,1-bisphosphonic acids (HMBPs) are compounds that have interesting pharmacological applications. Unfortunately few studies exist on their analyses by mass spectrometry (MS). In this work, we have analyzed new aromatic HMBPs and their prodrugs with electrospray tandem mass spectrometry (ESI-MS(n)). We describe, for the first time, a complete study of fragmentation patterns, in both positive and negative-ion modes. In positive mode, the cation dissociations are mainly elimination of water and phosphorus fragments. In negative mode, losses of ROH (R==H, C(6)H(5), CH(3)OC(6)H(5)) and HPO(2) were observed. The results have revealed specific structural fingerprints for the screening of these compounds in complex biological mixtures.