An approach to the mass spectrometry identification of cyanobacterial peptides. The case of demethylmicrocystin-LR

s A general approach to the identification of cyanobacterial peptides is considered. It is based on a combination of liquid chromatography high resolution tandem mass spectrometry with the prior data. The algorithm of identification involves a comparison of the experimental spectra and theoretical ones generated both by the Mass Frontier software for the prediction of mass spectra and using various rules of cyclopeptide fragmentation. As an example, the demethylated variant of the most toxic microcystin was taken, namely demethylmicrocystin-LR, present in phytoplankton of the lake Sestroretskii Razliv. The substance was identified as [D-Asp 3 ]microcystin-LR, an abundant demethylmicrocystin, or, less likely, as the later described [D-Asp 3 , Dhb 7 ]microcystin-LR. Among the several types of tandem mass specta, the most useful for identification appeared to be the spectrum of fragments formed from the precursor ion [M+H] + in the mode of dissociation induced by high energy collisions (HCD). The Mass Frontier software predicted most of the observed fragment ions.