Atypical cleavage of protonated N-fatty acyl amino acids derived from aspartic acid evidenced by sequential MS3 experiments
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Lipidomics calls for information on detected lipids and conjugates whose structural elucidation by mass spectrometry requires to rationalization of their gas phase dissociations toward collision-induced dissociation (CID) processes. This study focused on activated dissociations of two lipoamino acid (LAA) systems composed of N-palmitoyl acyl coupled with aspartic and glutamic acid mono ethyl esters (as LAA(*D) and LAA(*E)). Although in MS/MS, their CID spectra show similar trends, e.g., release of water and ethanol, the [(LAA(*D/*E)+H)–C2H5OH]+ product ions dissociate via distinct pathways in sequential MS3 experiments. The formation of all the product ions is rationalized by charge-promoted cleavages often involving stepwise processes with ion isomerization into ion–dipole prior to dissociation. The latter explains the maleic anhydride or ketene neutral losses from N-palmitoyl acyl aspartate and glutamate anhydride fragment ions, respectively. Consequently, protonated palmitoyl acid amide is generated from LAA(*D), whereas LAA(*E) leads to the [*E+H–H2O]+ anhydride. The former releases ammonia to provide acylium, which gives the C n H(2n−1) and C n H(2n−3) carbenium series. This should offer structural information, e.g., to locate either unsaturation(s) or alkyl group branching present on the various fatty acyl moieties of lipo-aspartic acid in further studies based on MS n experiments.
KeywordsN-fatty-acyl amino-acid Regioselectivity Ion–dipole ESI/MSn
Thanks to the University Hassiba Benbouali, Chlef, Algeria and the head of the Faculty of Science (Professor Abdelkader Ali Benamara) for financing the internship and Dr. B. Floriant (INSA, Toulouse, France) for NMR analyses. Thanks to Corinne Aubert for allowing T. Taalabi to come to IPCM of the University Pierre and Marie Curie.
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The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest, or non-financial interest in the subject matter or materials discussed in this manuscript.
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