A strategy to sequence lysine-containing cyclic peptides by MSn is presented. Doubly protonated cyclic peptides ions are transformed into gold (I) cationized peptide ions via cation switching ion/ion reaction. Gold(I) cationization facilitates the oxidation of neutral lysine residues in the gas phase, weakening the adjacent amide bond. Upon activation, facile cleavage N-terminal to the oxidized lysine residue provides a site-specific ring opening pathway that converts cyclic peptides into acyclic analogs. The ensuing ion contains a cyclic imine as the new N-terminus and an oxazolone, or structural equivalent, as the new C-terminus. Product ions are formed from subsequent fragmentation events of the linearized peptide ion. Such an approach simplifies MS/MS data interpretation as a series of fragment ions with common N- and C-termini are generated. Results are presented for two cyclic peptides, sunflower trypsin inhibitor and the model cyclic peptide, β-Loop. The power of this strategy lies in the ability to generate the oxidized peptide, which is easily identified via the loss of HAuNH3 from [M + Au]+. While some competitive processes are observed, the site of ring opening can be pinpointed to the lysine residue upon MS4 enabling the unambiguous sequencing of cyclic peptides.
Keywords: Cyclic peptides; Gold cationization; Ion/ion reactions; Sunflower trypsin inhibitor.