Effects Of Sodium Cation On Peptide Fragmentation Under Low-energy Collision

Sodium cation can be inevitably detected in the electrospray mass spectrometry and has obvious effects on peptide fragmentation.Differing from the "mobile proton" model,sodiated peptides fragment in a "charge-remote" way that charge is fixed in a local area and dissociation is not always initiated by the flow of a charge carrier.In this thesis,the effects of protonation and sodium cationization on the peptide fragmentation were studied by tandem mass spectrometry and quantum chemistry calculation.The details are shown in the following three parts:(1)As a natural and labile amino acid,glutamine plays an important role in many biological processes.Herein,glutamine C-terminated peptides were studied,and the loss of NH3 and CO was observed.Compared with the protonated species,C-terminated glutamine is more reactive under sodium cationization,and can be more easily attacked by the terminal-COOH to lose neutral molecules.To investigate the effect of sodium,quantum chemistry calculations were carried out to illustrate the fragmentation pathways.(2)The fragmentation of protonated and sodiated peptides LAXA(X= G,A,V,L,I or F)was investigated.The differences in intensity ratios of y2/b2 of [LAXA+H]+ were observed.Density functional theory(DFT)calculations demonstrated that a linear fit was observed between the intensity ratios and the proton affinities of y2+ ions.By contrast,obvious cleavage of the second amino bond of sodiated species was observed,generating [H-Xxx-OH+Na]+.DFT calculations were carried out to illustrate the interactions between the Xxx residue and Na~+.(3)Peptides N-terminated by Lys were studied.Since the N-terminus was a basic site to capture a proton and alkali metal ions could interact with the C-terminus,the double charged Lys-N peptide ions [M+H+Cat]2+ were investigated,presenting cationized yn ions that were barely generated in single protonation.Quantum calculations were carried out to illustrate different interactions of different cations(Li,Na and K).