Exploring the Diversity of Cysteine-Rich Natural Product Peptides via MS/MS Fingerprint Ions.

Natural product extracts present inherently complex matrices in which the identification of novel bioactive peptide species is challenged by low-abundance masses and significant structural and sequence diversity. Additionally, discovery efforts often result in the re-identification of known compounds, where modifications derived in vivo or during sample handling may obscure true sequence identity. Herein, we identify mass spectral (MS(2)) "fingerprint" ions characteristic of cyclotides, a diverse and biologically active family of botanical cysteine-rich peptides, based on regions of high sequence homology. We couple mass shift analysis with MS(2) spectral fingerprint ions cross referenced with CyBase-a cyclotide database-to discern unique mass species in Viola communis extracts from mass species that are likely already characterized and those with common modifications. The approach is extended to a related class of cysteine-rich peptides, the trypsin inhibitors, using the characterized botanical species Lagenaria siceraria. Coupling the observation of highly abundant MS(2) ions with mass shift analysis, we identify a new set of small, highly disulfide-bound cysteine-rich L. siceraria peptides.