Figure 3.

Evaluation of post-translational modifications in cyclotides identified by MS/MS sequencing and database analysis. Database searching of MS/MS spectra of cyclotides revealed a number of peptides with posttranslational modifications. The most abundant ones have been illustrated exemplarily: (A) Aspartate methylation in vitri peptide 94b; (B) tryptophan degradation product kynurenine in varv peptide E; (C) tryptophan oxidation to 3-hydroxy-tryptophan in kalata B1; and (D) glutamate ethylation and glutamine or asparagine deamidation in vitri peptide 42a. (E) The MS/MS fragment spectrum of a C-terminal fragment of endoproteinase GluC processed vitri peptide 22a has been shown to confirm the presence of acyclic cyclotides in V. tricolor. The major b- and y-ion series have been labeled in all spectra. Furthermore, peptide mass, cleavage protease, mass error, and Mascot score of the illustrated peptides have been provided.