Ion Mobility-Assisted Free Radical-Initiated Peptide Sequencing.

Free radical-initiated peptide sequencing (FRIPS) is a tandem mass spectrometry technique (MS/MS) that enables radical-based dissociation on instruments only capable of collisional activation. In FRIPS, peptides are chemically-derivatized with a compound that undergoes homolytic cleavage and generates radicals upon collisional activation. These radicals then propagate through the peptide backbone enabling the sequencing of peptide ions. This MS/MS technique has shown promise in sequencing post-translationally modified peptides, but it is typically performed in an MS(3) workflow and single-step MS/MS approaches result in the generation of both collisional- and radical-driven dissociation products and highly complex spectra. Recently, our group developed a method to dissociate peptide ions prior to ion mobility analysis within a trapped-ion mobility spectrometry (TIMS) device. In this work, we examine if this "CIDtims" technique can initiate the homolytic cleavage of the FRIPS precursor. We then examine if the resultant ion mobility separation results in additional assignments of product ions and improved sequence coverage. We demonstrate that activation within the TIMS device does indeed promote robust radical initiation and fragmentation of peptide cations and that the generated product ions are mobility separated enabling facile assignment and increased sequence coverage.