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Multi-stage collisionally-activated decomposition in an ion trap for identification of
sequences, structures and bn→bn–1 fragmentation pathways of protonated cyclic peptides Chenxi Jia, Wei Qi,*
Zhimin He and Bin Qiao
Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, People's Republic
of China. E-mail: qiwei@tju.edu.cn
ABSTRACT:
Cyclic penta-, hexa- and heptapeptides have been designed, synthesized and their fragmentations induced by multistage tandem mass
spectrometry have been studied. Under low-energy collisionally activated decomposition (CAD), the protonated cyclic peptides mainly dissociate via ring opening pathways and the
corresponding bn→bn–1 pathways to form several sets of b ions as oxazolone rings (and
b1 ions as aziridinone rings). Through repeated observation of these b ions in multistep CAD experiments, accurate sequencing and head-to-tail ring
structure of cyclic peptides can be determined. The mistaken assignments of these b ions can be avoided by this sequencing method. Semiempirical molecular orbital
calculations have been utilized to provide insight into the proposed dissociation mechanism. In addition, for cyclic peptides that include an Asn residue, the nitrogen of the Asn side
chain is observed to be preferentially protonated, which can induce a unique ring-opening pathway with a loss of ammonia that competes with the conventional ring opening
pathway
Keywords:
tandem mass spectrometry, cyclic peptides, fragmentation pathways, collisionally activated decomposition, sequencing
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