Distributions of ion series in ETD and CID spectra: making a comparison.

Databases which capture proteomic data for subsequent interrogation can be extremely useful for our understanding of peptide ion behaviour in the mass spectrometer, leading to novel hypotheses and mechanistic understanding of the underlying mechanisms determining peptide fragmentation behaviour. These, in turn, can be used to improve database searching algorithms for use in automated and unbiased interpretation of peptide product ion spectra. Here, we examine a previously published dataset using our established methods, in order to discover differences in the observation of product ions of different types, following ion activation and unimolecular dissociation either by collisional dissociation or the ion/ion reaction, electron transfer dissociation. Using a target-decoy database searching strategy, a large data set of precursor ions, were confidently predicted as peptide sequence matches (PSMs) at either a 1% or 5% peptide false discovery rate, as reported in our previous study. Using these high quality PSMs, we have conducted a more detailed and novel analysis of the global trends in observed product ions present/absent in these spectra, examining both CID and ETD data. We uncovered underlying trends for an increased propensity for the observation of higher members of the ion series in ETD product ion spectra in comparison to their CID counterparts. Such data-mining efforts will prove useful in the generation of new database searching algorithms which are well suited to the analysis of ETD product ion spectra.