Résumé
Permanent charge N-terminal peptide derivatization prior analysis by mass spectrometry has been primarily investigated to optimize detection sensitivity and sequencing efficiency. In particular, the simplification of the recorded MS/MS spectra for straightforward ion assignment is of utmost interest when de novo sequencing of peptides is required or extensive protein sequence coverage needed. Additionally, such N-terminal insertion, provided that isotope labels are used, is also intended to afford comparative quantitation data. Depending on the N-terminal chemical modification, the fixed charge is either triggering skeletal peptide bond cleavage, as expected for sequence assignment, or producing fragment ions that are only representing the charged group, which is completely inadequate for sequencing purposes but highly valuable for detection/quantitation purposes. Among all fixed charge reagents that have been investigated so far, we focused our interest on well-documented phosphonium and far less investigated pyridinium moieties as derivatization chemicals for mass spectrometry analyses. According to the structural peptide modification, we aimed at inducing specific predictable charge remote fragmentation (CRF) pathways upon low energy activation conditions, such as collision-induced and/or metastable dissociations available with any conventional ESI-QqTof and MALDI-Tof/Tof instruments, to be used for either efficient sequencing or sensitive peptide monitoring (detection/quantitation). All MS/MS behaviors recorded for the prepared N-terminal modified model peptides presenting a permanent positive phosphonium or pyridinium charge will be presented and discussed in the attempt to govern the dissociation pathways. Acetylation with commercially available tris(trimethoxyphenyl)phosphonium (TMPP) is known to trigger peptide backbone ruptures leading to abundant a-type sequence ions. Although it has been reported as a valuable proteogenomic method for systematic identification of N-termini of proteins in large shotgun proteomic surveys, we observed peculiar dissociation events that prompted us to investigate the influence of aliphatic residues on the MALDI mass spectrometry fragmentation pattern. We also demonstrated that the chemical regioselectivity was not perfectly mastered leading to isobaric molecular ions that were evidenced through ion mobility experiments and represented potent pitfalls for correct sequence assignment. Regarding N-terminal peptide functionalization with a pyridinium moiety, we designed and synthesized various reagents by introducing different organic function to modulate the anchoring chemistry and by allowing substituents on the aromatic ring. As expected, such pyridinium substituted peptides were found to merely producing fragment ions related to the charged pyridine entity. The electronic effect of the aromatic substituent greatly influenced the observed dissociation pathways.