Finding and using Diagnostic Ions in Collisional Induced Crosslinked Peptide Fragmentation Spectra

Crosslinking mass spectrometry (XL-MS) has emerged as a powerful tool in its own right for the investigation of protein structures and interactions. Utilizing standard shotgun MS mass spectrometry equipment and specialized database search software, crosslinked peptide-pairs can be identified and directly translated into distance constraints for protein structure and protein-protein interaction investigations. Whereas the gas-phase dissociation behavior of linear peptides is well understood, less is however known about the gas-phase dissociation behavior of crosslinked peptides. In this work, we set out to expose the behavior of commonly used crosslinking reagents, both non- as well as gas-phase cleavable, using synthetic peptides to establish mechanistic insights. We describe that crosslinked peptide pairs generate specific fragmentation patterns under HCD and CID fragmentation conditions, distinct from mono-linked peptide and non-modified peptides. We discuss in detail the resulting diagnostic ions that can help distinguishing linear peptides from mono-linked and crosslinked peptide pairs and how that may be used to further increase the efficiency of XL-MS analysis.