L. Nizigiyimana

Mechanistic Aspects of Charge-remote Fragmentation in Saturated and Mono-unsaturated Fatty Acid Derivatives. Evidence for Homolytic Cleavage

The high-energy collision-induced dissociation (CID) of [M+Li]+ ions of n-butyl ester derivatives of palmitic acid and oleic acid as well as 9,9-2H2-palmitic acid and 11,11-2H2-oleic acid has been studied in order to obtain information on the charge-remote fragmentation mechanism of saturated and mono-unsaturated fatty acid ions containing a stable charge centre. The results obtained in the present study indicate that homolytic cleavage reactions, involving C—H cleavage as an initial rate-determining step, operate during the charge-remote fragmentation observed for high-energy CID of [M+Li]+ ions of n-butyl palmitate and correspond to a major fragmentation route. With respect to the charge-remote fragmentation of n-butyl oleate, our 2H-labelling results point to the same mechanism, involving an initial C—H cleavage at allylic positions, for the formations of ions corresponding to a formal homo-allylic cleavage, and are also consistent with a direct allylic C—C cleavage for the formation of ions due to a formal allylic C—C cleavage. These results, however, do not exclude the possibility of other minor homolytic fragmentation pathways for the formation of ions involving formal allylic and homo-allylic cleavages.

Charge‐remote and charge‐proximate fragmentation processes in alkali‐cationized fatty acid esters upon high‐energy collisional activation. A new mechanistic proposal

The effect of the metal ion on the high-energy collision-induced dissociation (CID) of alkali metal-cationized n-butyl and methyl ester derivatives of palmitic and oleic acid was examined. The results show that the alkali metal ion has a pronounced effect and does not act as a mere ‘spectator’ ion with respect to the fragmentation process. While C–H cleavage is a dominant process for [M+Li]+ as well as [M+Na]+ precursor ions, C–C cleavage is also significant for the [M+Na]+ ions. Homolytic mechanisms involving the formation of a transient biradical cation are proposed which enable us to rationalize in a straightforward manner all product ions formed by both charge-remote and charge-proximate fragmentations. The mechanistic proposal is discussed in view of available knowledge on electron impact, CID and related processes. In order to predict how the alkali metal ion could affect the reactivity of the postulated biradical state formed following electronic excitation of the alkali metal-cationized molecules, quantum chemical calculations were performed on methyl and n-butyl acetate as model substances. The decreased spin density at the carbonyl oxygen atom in the biradical state may provide an explanation for the greater tendency towards C–C cleavage reactions of the sodium-cationized fatty acid esters relative to the corresponding lithium complexes.

CHARGE-REMOTE MOLECULAR HYDROGEN REMOVAL IN PROTONATED AND ALKALI-CATIONIZED LONG-CHAIN FATTY ACID ESTERS UPON CESIUM ION BOMBARDMENT

Abstract The mass spectral behavior of cationized saturated fatty acid derivatives has been studied in order to gain insight into the loss of molecular hydrogen during cesium ion bombardment. It is shown that molecular hydrogen and hydrogen radical loss occurs for protonated and alkali (Li+/Na+)-cationized fatty acid methyl esters as well as for protonated acylcarnitines and that molecular hydrogen loss is dependent upon the acyl chain length. Investigation of ion structures by collision-induced dissociation tandem mass spectrometry indicates that in the case of alkali-cationization a hydrogen radical is lost from all positions of the saturated acyl chain, whereas in the case of protonated molecules a hydrogen radical is predominantly eliminated from the protonated ester group. For the primary reaction of the dehydrogenation chemistry, an ion-beam-induced excitation of a cationized molecule is proposed, yielding a diradical species that gives rise to an intermolecular reaction with a neutral analyte molecule. Subsequent cationization of the neutral [M–H2] species formed in this primary reaction leads to the formation of the satellite ion at m/z values 2 u lower compared to the cationized molecule.

Comparison of Low- and High-energy Collision-induced Dissociation Tandem Mass Spectrometry in the Analysis of Ricinoleic and Ricinelaidic Acid

Ricinoleic acid and itstransisomer ricinelaidic acid, and their methyl esters, were analyzed by low- and high-energy collision-induced dissociation (CID) tandem mass spectrometry. It is shown that a stable charge centre is required to observe the rearrangement, which occurs in the β-hydroxyalkene part and results in the loss of heptaldehyde, and that this rearrangement corresponds to a low-energy CID reaction. Differences between the CID behaviour of ricinoleic acid and that of itstransisomer are related to the loss of water, which can also be regarded as a low-energy rearrangement reaction. Comparison of low- and high-energy CID spectra further revealed that high-energy CID gives rise to loss of a H•radical and H2 together with low-energy fragmentation. Examination of different molecule ions, including [M-H]-, [M+Li]+ and [M-H+2Li]+ ions of free fatty acids and [M+Li]+ ions of the methyl esters shows that charge-remote homolytic fragmentation is most pronounced for the [M+Li]+ ions of the methyl esters.