Antimicrobial peptides from the skin of the Asian frog, Odorrana jingdongensis: De novo sequencing and analysis of tandem mass spectrometry data

Eight intact antimicrobial peptides were identified from the skin of Odorrana jingdongensis by de novo sequencing following low energy ESI CID Q-TOF MS/MS in positive-mode with the help of Edman degradation and structural similarity analysis. We devised exact mass measurements to discriminate the K/Q amino acid residue in the peptides between 2.0kDa to 3.8kDa. Moreover, the cleavage at the CS bond at the side chain of Met was observed in all the spectra of the peptides containing Met residue. And we found unusual cleavages within the intramolecular disulfide loop with high frequency. Our data revealed that the cleavage pathways are significantly different from those reported previously which are similar to the cycle peptide cleavage mode followed by the secondary cleavage at the CS bond on oxidized Cys. Thus, our results highly suggest that ion series generated from the cleavages within the intramolecular disulfide loop should be considered in both the top-down sequencing and the disulfide bridge location with the presence of a relatively high intensity of MH(+)-28 ion marker. Furthermore, our activity data implied that different AMPs may use different strategies to kill microbes.