A major strength of electron capture dissociation (ECD) and electron transfer dissociation (ETD) is their ability to retain labile post-translational modifications (PTMs) while cleaving the peptide/protein backbone to yield extensive sequence information, including PTM sites.
Electron Capture Dissociation (ECD)
[M + nH]n+ + e–slow → [M + nH](n-1)+• → fragments
Electron Transfer Dissociation (ETD)
[M + nH]n+ + A-• → [M + nH](n-1)+• + A– → fragments
However, a majority of PTM-containing peptides are acidic, thus exacerbating the problem associated with the requirement for multiply positively charged precursor ions. One solution is to enrich for acidic peptides prior to analysis. Kweon and Håkansson pioneered the use of zirconium dioxide for phosphopeptide enrichment and subsequently demonstrated that such enrichment improves phosphopeptide ECD analysis. For sulfopeptides, the Håkansson group has shown that metal adduction is required for sulfate retention in ECD. However; sulfate groups are much more stable in negative ion mode and our more recent work has included exploration of anion activation techniques. niECD by far outperforms the other strategies and appears highly promising for sulfopeptide analysis.