To obtain further structural information, we acquired MS/MS data (for the twelve new microginins Fig. ). To establish a comparative framework, we looked in detail to the MS/MS data for the monochlorinated representative of each of the four different groups (compounds 2, 5, 8 and 11, Fig. ). Extensive analysis of microginin fragmentation under ESI conditions is available in the literature and was used as reference. Notably, the MS/MS spectra of 2, 5, 8 and 11 contained a m/z 162 ion corresponding to fragmentation of a chlorinated 3-amino decanoic acid-derived moiety. The three larger metabolites (2, 5 and 8) were composed of a C-terminal Pro-Tyr moiety, giving rise to a characteristic m/z 279 fragment ion. The smaller compound (microginin 592) did not show this ion, and instead showed a m/z 116 fragment ion corresponding to a C-terminal Pro. From this observation, and because the mass difference between 2 and 11 is consistent with a Tyr residue, we propose that 10, 11 and 12 are truncated versions of 1, 2 and 3 lacking the C-terminal Tyr.