Stepwise collision energy-resolved tandem mass spectrometric experiments for the improved identification of citrullinated peptides

The use of tandem mass spectrometry (MS/MS) is a fundamental prerequisite of reliable protein identification and quantification in mass-spectrometry-based proteomics. In bottom-up and middle-down proteomics, proteins are identified by the characteristic fragments of their constituting peptides. Posttranslational modifications (PTMs) often further complicate proteome analyses. Citrullination is an increasingly studied PTM converting arginines to citrullines (Cit, X) and is implicated in several autoimmune and neurological diseases as well as different types of cancer. Confirmation of citrullination is known to be very challenging since it results in the same molecular mass change as Asn/Gln deamidation. In this study, we explore which MS/MS characteristics can be used for the reliable identification of citrullination. We synthesized several peptides incorporating Cit residues that model enzymatic cleavages of different proteins with verified or putative citrullination. Collision-induced dissociation (CID) was used to investigate the energy dependence of Byonic, Mascot scores and confirmed sequence coverage (CSC) alongside with the neutral loss of HNCO characteristic to citrulline side chains. We found that although the recommended values (19-45 V) for ramped collision energy settings cover the optimal Mascot, Byonic or %CSC scores effectively, the diagnostic HNCO loss from precursors and fragments may reach their maximum intensities at lower and higher collision energies, respectively. Therefore, we suggest broadening the ramp range to ~5-60V to get more favorable identification rates for citrullinated peptides. We also found that Byonic was more successful in correctly identifying citrullinated peptides with deamidated residues than Mascot