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 key H, He and He2+. For all peptides under study, the fragmentation pattern depends strongly on the electronic structure of the projectile ions. lmmonium 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). (C) 2010 Elsevier B.V. All rights reserved.