One-Step Peptide Backbone Dissociations in Negative-Ion Free Radical Initiated Peptide Sequencing Mass Spectrometry

Peptide dissociation behavior in TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-based FRIPS (free radical initiated peptide sequencing) mass spectrometry was analyzed in both positive- and negative-ion modes for a number of peptides including angiotensin II, kinetensin, glycoprotein IIb fragment (296–306), des-Pro2-bradykinin, and ubiquitin tryptic fragment (43–48). In the positive mode, the ·Bz-C(O)–peptide radical species was produced exclusively at the initial collisional activation of o-TEMPO-Bz-C(O)–peptides, and two consecutive applications of collisional activation were needed to observe peptide backbone fragments. In contrast, in the negative-ion mode, a single application of collisional activation to o-TEMPO-Bz-C(O)–peptides produced extensive peptide backbone fragmentations as well as ·Bz-C(O)–peptide radical species. This result indicates that the duty cycle in the TEMPO-based FRIPS mass spectrometry can be reduced by one-half in the negative-ion mode. In addition, the fragment ions observed in t...