Isomer differentiation via collision‐induced dissociation: The case of protonated α‐, β²‐ and β³‐phenylalanines and their derivatives

Abstract

A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments is used to examine the gas-phase fragmentation reactions of the three isomeric phenylalanine derivatives, alpha-phenylalanine, beta(2)-phenylalanine and beta(3)-phenylalanine. Under collision-induced dissociation (CID) conditions, each of the protonated phenylalanine isomers fragmented differently, allowing for differentiation. For example, protonated beta(3)-phenylalanine fragments almost exclusively via the loss of NH(3), only beta(2)-phenylalanine via the loss of H(2)O, while alpha- and beta(2)-phenylalanine fragment mainly via the combined losses of H(2)O + CO. Density functional theory (DFT) calculations were performed to examine the competition between NH(3) loss and the combined losses of H(2)O and CO for each of the protonated phenylalanine isomers. Three potential NH(3) loss pathways were studied: (i) an aryl-assisted neighbouring group; (ii) 1,2 hydride migration; and (iii) neighbouring group participation by the carboxyl group. Finally, we have shown that isomer differentiation is also possible when CID is performed on the protonated methyl ester and methyl amide derivatives of alpha-, beta(2)- and beta(3)-phenylalanines.

Infrared spectroscopic confirmation of α‐lactone formation in the dissociation of a gaseous amino acid

“…α‐Lactones are highly reactive molecules that have been proposed as short‐lived intermediates in a variety of chemical reactions, often involving α‐substituted carboxylates or α,β‐unsaturated acids, in the photolysis of cyclic peroxides and α‐halocarboxylic acids, in the oxidation of ketenes, or in the gas‐phase dissociation of α‐substituted carboxylic acids or in the decomposition of amino acids, both in solution and in the gas phase . They have also been proposed as intermediates in atmospheric oxidation reactions in mass spectrometry and as products of the reaction of carbenes with CO 2 . There has long been discussion regarding the electronic structure of α‐lactones, and whether they are best considered to be cyclic, canonical structures or zwitterionic, although those discussions generally relate to the structure in solution.…”

Mass spectrometric study of the decomposition pathways of canonical amino acids and α‐lactones in the gas phase

“…α-Lactones are highly reactive molecules that have been proposed as short-lived intermediates in a variety of chemical reactions, often involving α-substituted carboxylates [50,[54][55][56][57][58][59][60] or α,β-unsaturated acids, [66,67] in the photolysis of cyclic peroxides [47,[68][69][70][71] and α-halocarboxylic acids, [72] in the oxidation of ketenes, [73] or in the gas-phase dissociation of α-substituted carboxylic [24][25][26][27][28][29] or in the decomposition of amino acids. [23,74] They have also been proposed as intermediates in atmospheric oxidation reactions [75,76] in mass spectrometry, [77][78][79][80] and as products of the reaction of carbenes with CO 2 . [81][82][83][84][85] Despite being highly reactive, α-lactones have been investigated spectroscopically by using matrix isolation, [82][83][84] gas phase, [86] and solution-phase time-resolved IR.…”

Quantum chemical mass spectrometry:ab initioprediction of electron ionization mass spectra and identification of new fragmentation pathways

“…Fragmentation pathways were elucidated by proposing several possibilities that may give rise to a certain peak: the calculated energy of the different intermediates or the activation energy associated with the pathway was then used to predict which pathway is most likely to occur. Density functional theory (DFT) has also been used to differentiate between isomers by studying their fragmentation behaviour …”