A novel approach for quantitative peptides analysis by selected electron transfer reaction monitoring

Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) with selective reaction monitoring (SRM) is a selective and sensitive method for quantitation of peptides. SRM is achieved via MS/MS utilizing collision-induced dissociation (CID) while monitoring unique precursor-product ion transitions. Low-energy CID tandem mass spectrometry has been, by far, the most common method used to dissociate peptide ions for sequence analysis. However, collisional scattering of product ions in CID results in decreased intensity of the primary product ion. The lower intensity of the targeted product ion can lead to a reduction in the sensitivity of a quantitative method that uses SRM. Electron transfer dissociation (ETD) is a fragmentation method that is complementary to CID. During the ETD reaction for doubly protonated peptides ([M+2H](2+)), there is a significant shift toward nondissociative electron transfer (ET) product species ([M+2H](+)). We utilized that particular defect in ETD to develop a new quantitative method for monitoring the transition of unique precursors ([M+2H](2+)) to charge-reduced ions ([M+2H](+)). We refer to this method as selective electron transfer reaction monitoring (SETRM). In ESI-MS, trypsin-digested peptides tend to generate doubly protonated peptide precursors. We found that SETRM was more suitable than SRM for these doubly charged tryptic peptides with nano-LC-MS/MS. The quantitative capabilities of SETRM provide a more sensitive way of performing quantitative experiments using the same instrument, thereby improving the application of electron transfer dissociation in proteomics.