Isolation and identification of 12 microcystins from four strains and two bloom samples of Microcystis spp.: structure of a new hepatotoxin.

Sixteen microcystins, cyclic heptapeptide hepatotoxins, were isolated and purified by high performance liquid chromatography (HPLC) and thin layer chromatography (TLC) from four hepatotoxic strains and two Microcystis spp. bloom samples originating from five different lakes in Finland. The structures of a new [Dha7]MCYST-FR and 11 known microcystins MCYST-LR, [D-Asp3]MCYST-LR, [Dha7]MCYST-LR, [D-Asp3, Dha7] MCYST-LR, MCYST-RR, [D-Asp3]MCYST-RR, [Dha7]MCYST-RR, [D-Asp3,Dha7]MCYST-RR, [L-Ser7]MCYST-RR, MCYST-YR and [Dha7] MCYST-YR were assigned based on amino acid analysis, fast atom bombardment mass spectrometry (FABMS) and tandem FABMS. Four other new compounds allowed only determination of their molecular formulas and amino acid components because of inadequate amounts obtained. [Dha7]MCYST-RR was found most frequently in these samples as the main toxin.