Gas-phase peptide sequencing by TEMPO-mediated radical generation

Collisional activation of 2-[(2,2,6,6-tetramethylpiperidin-1-yloxy)methyl]benzoic acid (TEMPO-Bz)-conjugated peptide cations, prepared by attaching a TEMPO-derived precursor 1 to an N-terminal amino group or a lysine side chain, resulted in the formation of radical species. The subsequent tandem mass spectrometry on the radical cations exhibited odd-electron peptide backbone dissociations in the same manner as that observed by electron capture dissociation (ECD) or electron transfer dissociation (ETD). For example, a-, x-, or z-types of ions were major fragments and the disulfide bond was readily cleaved. The TEMPO-FRIPS (free radical initiated peptide sequencing) was also applicable to characterizing even singly protonated peptides, in contrast to ECD or ETD in which only doubly or highly protonated cations are responsive. The TEMPO-FRIPS approach also has universality in that it can be used in any type of a tandem mass spectrometer.