Sequencing Orbitides by Acid-Mediated Ring Cleavage Followed by Tandem Mass Spectrometry

Homodetic cyclic peptides have aroused interest because of their pharmacological potential. Sequencing cyclic peptides is difficult—Edman degradation is not possible as there is no N-terminus, NMR requires quantities that are hard to gather from native samples, and tandem mass spectrometry data are difficult to interpret due to the peptide ring opening at multiple points. Sequencing can be simplified by cleaving the peptide ring at a specific peptide bond. Partial acid hydrolysis is a possible solution, but to date sequencing by this method has only been demonstrated for linear peptides and cyclotides, which are larger cyclic peptides (∼30 amino acids) with three disulfide bonds. This study tests whether partial acid hydrolysis could be used to aid sequencing of Cys-less cyclic peptides with fewer than ten amino acid residues. We show that, with the right combination of temperature and acid, ring cleavage occurs and offers relatively simple MS/MS spectra amenable to sequencing. We describe how this method was used in our recent study in which we sequenced annomuricatin D (cyclo-GHSIFPPIP) from seeds of the soursop, Annona muricata. We found that orbitides can be linearized for MS/MS sequencing by incubation with 1.2 M HCl at 90 °C for 15–20 min. This fast, economical sequencing method will be useful to those studying small cyclic peptides lacking disulfide bonds, which are commonly found in many organisms, especially plants.