Département de chimie moléculaire

The aims of this study were to highlight the impact of minor structural differences (e.g. an amino acid side chain enlargement by one methylene group), on ion dissocn. under collision-induced dissocn. conditions, and to det. the underlying chem. mechanisms. Therefore, we compared fragmentations of deprotonated aspartic and glutamic acids generated in neg. electrospray ionization. Energy-resolved mass spectrometry breakdown curves were recorded and MS3 expts. performed on an Orbitrap Fusion for high-resoln. and high-mass accuracy measurements. Activated fragmentations were performed using both the resonant and non-resonant excitation modes (i.e., CID and HCD, resp.) in order to get complementary information on the competitive and consecutive dissociative pathways. These expts. showed a specific loss of ammonia from the activated aspartate but not from the activated glutamate. We mainly focused on this specific obsd. loss from aspartate. Two different mechanisms based on intramol. reactions (similar to those occurring in org. chem.) were proposed, such as intramol. elimination (i.e. Ei-like) and nucleophilic substitution (i.e. S_Ni-like) reactions, resp., yielding anions as fumarate and alpha-lactone from a particular conformation with the lowest steric hindrance (i.e. with antiperiplanar carboxyl groups). The detected deaminated aspartate anion can then release CO₂ as obsd. in the MS3 exptl. spectra. However, quantum calcns. did not indicate the formation of such a deaminated aspartate product ion without loss of carbon dioxide. Actually, calcns. displayed the double neutral (NH₃+CO₂) loss as a concomitant pathway (from a particular conformation) with relative high activation energy instead of a consecutive process. This disagreement is apparent since the concomitant pathway may be changed into consecutive dissocns. according to the collision energy i.e., at higher collision energy and at lower excitation conditions, resp. The latter takes place by stabilization of the deaminated aspartate solvated with two residual mols. of water (present in the collision cell). This desolvated anion formed is an alpha-lactone substituted by a methylene carboxylate group. The vibrational excitation acquired by [(D-H)-NH₃]-during its isolation is enough to allow its prompt decarboxylation with a barrier lower than 8.4 kJ/mol. In addn., study of glutamic acid-like diastereomers constituted by a cyclopropane, hindering any side chain rotation, confirms the impact of the three-dimensional geometry on fragmentation pathways. A significant specific loss of water is only obsd. for one of these diastereomers. Other expts., such as stable isotope labeling, need to be performed to elucidate all the obsd. losses from activated aspartate and glutamate anions. These first mechanistic interpretations enhance understanding of this dissociative pathway and underline the necessity of studying fragmentation of a large no. of various compds. to implement properly new algorithms for de novo elucidation of unknown metabolites.