Fast side-chain losses in keV ion-induced dissociation of protonated peptides

In this paper, we compare ionization and dissociation of a series of singly and doubly protonated peptides, namely leucine enkephalin, bradykinin, LHRH and substance P as induced by collisions with keV H+, He+ and He2+. For all peptides under study, the fragmentation pattern depends strongly on the electronic structure of the projectile ions. Immonium ions, side-chains and their fragments dominate the spectrum whereas fragments due to peptide backbone cleavage are weak or even almost absent for He+. Here, resonant electron capture from the peptide is ruled out and only interaction channels accompanied by much higher excitation contribute. Cleavage of the side-chain linkage appears to be a process alternative to backbone fragmentation occurring after internal vibrational redistribution of excitation energy. Depending on the peptide, this process can lead to the loss of a side-chain cation (leucine enkephalin, LHRH) or a neutral side-chain (substance P).

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (74 K)Download as PowerPoint slideResearch highlights▶ Experimental study on keV ion-induced dissociation protonated peptides leucine enkephalin, bradykinin, LHRH and substance P. ▶ Ion-peptide interaction governed by resonant electron capture from the peptide and excitation due to electronic stopping. ▶ Immonium ions and side-chain fragments dominate the mass spectrum. ▶ KID seems to induce prompt cleavage of a side-chain linkage. ▶ Neutral side-chain cations are observed.