Multistep tandem mass spectrometry for sequencing cyclic peptides in an ion-trap mass spectrometer

L. C.M. Ngoka, M. L. Gross

Research output: Contribution to journalArticlepeer-review

64 Scopus citations


Collisionally activated decomposition (CAD) of a protonated cyclic peptide produces a superposition spectrum consisting of fragments produced following random ring opening of the cyclic peptide to give a set of acylium ions (or isomeric equivalents) of the same m/z. Assignment of the correct sequence is often difficult owing to lack of selectivity in the ring opening. A method is presented that utilizes multiple stages of CAD experiments in an electrospray ion-trap mass spectrometer to sequence cyclic peptides. A primary acylium ion is selected from the primary product-ion spectrum and subjected to several stages of CAD. Amino-acid residues are sequentially removed, one at each stage of the CAD, from the C-terminus, until a b2 ion is reached. Results are presented for seven cyclic peptides, ranging in sizes from four to eight amino-acid residues. This method of sequencing cyclic peptides eliminates ambiguities encountered with other MS/MS approaches. The power of the strategy lies in the capability to execute several stages of CAD upon a precursor ion and its decomposition products, allowing the cyclic peptide to be sequenced in an unambiguous, stepwise manner.

Original languageEnglish
Pages (from-to)732-746
Number of pages15
JournalJournal of the American Society for Mass Spectrometry
Issue number8
StatePublished - 1999


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