Competitive Gas Phase Fragmentation Reactions Of Sodiated Peptides LADA And LAAX

Protein as a kind of biological macromolecules that presents most abundantand is very important in living organisms. It is a polymer constructed by thecombination of20common amino acids through dehydration condensation reactions.The polypeptide is the intermediate product of protein hydrolyzed. Electrosprayionization is the most soft ionization method that can retain non-covalent interactionsof macrobiomolecules forming in solution in the gas phase of mass spectrometry, andhas been widely used for the identification of peptides/proteins. Typically, protonatedor sodiated peptides are selected under collision induced dissociation (CID) toproduce fragment ions for peptide sequence. The fragmentation pathways ofprotonated peptides have been well studied and “charge directed” and “chargeremote” modes have been proposed to describe the cleavage of the protonatedpeptides. In contrast, the dissociation mechanism of metal associated peptides is lessunderstood. In this study, two series of peptides LADA and LAAX are designed andused to study their fragmentation pathways by using ESI-Q-TOF tandem massspectrometry。Firstly, the peptide LADA is used to optimize instrumental conditions and to findthe best collision energy (CE) to produce fragment ions. Previous work shows thatsodiated peptides produce bn-1+Na+OH and bn+Na–H ions respectively throughnucleophillic attack of the carboxylic oxygen in the C-terminus or in the Asp sidechain to the carbonyl carbon of amide bond. In this work, the carboxyl groups in theC-terminus and in the Asp side chain are amidated and methylated respectively. Bycomparing the MS/MS spectra of the sodiated-peptides under same CE, the influenceof C–terminal carboxyl oxygen, carboxyl oxygen in the aspartic acid side chain andcarbonyl oxygen on the peptide fragmentation pathways are studied. The results showthat the activity of competitive fragmentation reactions follows this order: C–terminalcarboxyl oxygen> carboxyl oxygen in aspartic acid side chain> carbonyl oxygen.Secondly, fragmentation pathways of the peptide LADA are studied (Xrepresents amino acids C, E, G, H, K, N, Q, R, S and T, respectively). Through these specific amino acids, the competitive reactions occurred in the C-terminus of thepeptide and the influence of the C-terminal residues on the reactions are described.