Identification of toxic cyclopeptides based on mass spectral library matching

• The new library of 263 tandem mass spectra of 59 toxic cyclopeptides was built.
• Mass spectra of microcystins and amatoxins predominate in the library.
• The principal performance of library searches is 88–91% true positive results.
• Electrospray tandem spectra appeared to be close to MALDI fragment spectra.
• Identification of extract components of two cyanobacterial strains is outlined.

To gain perspective on the use of tandem mass spectral libraries for identification of toxic cyclic peptides, the new library was built from 263 mass spectra (mainly MS2 spectra) of 59 compounds of that group, such as microcystins, amatoxins, and some related compounds. Mass spectra were extracted from the literature or specially acquired on ESI-Q-ToF and MALDI-ToF/ToF tandem instruments. ESI-MS2 product-ion mass spectra appeared to be rather close to MALDI-ToF/ToF fragment spectra which are uncommon for mass spectral libraries. Testing of the library was based on searches where reference spectra were in turn cross-compared. The percentage of 1st rank correct identifications (true positives) was 70% in a general case and 88–91% without including knowingly defective (‘one-dimension’) spectra as test ones. The percentage of 88–91% is the principal estimate for the overall performance of this library that can be used in a method of choice for identification of individual cyclopeptides and also for group recognition of individual classes of such peptides. The approach to identification of cyclopeptides based on mass spectral library matching proved to be the most effective for abundant toxins. That was confirmed by analysis of extracts from two cyanobacterial strains.

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