Structures of three new homotyrosine-containing microcystins and a new homophenylalanine variant from Anabaena sp. strain 66.

A hepatotoxic strain of cyanobacterium Anabaena sp. 66 was isolated from a hepatotoxic water bloom sample in Lake Kiikkara, Finland. Four cyclic heptapeptide hepatotoxins were isolated and purified by HPLC from cultured cells of this organism. The structures of three new homotyrosine (Hty) containing toxins, [Dha7]microcystin-HtyR (Dha = dehydroalanine) (1), [D-Asp3,Dha7]microcystin-HtyR (2), and [L-Ser7]microcystin-HtyR (3), were assigned, based upon amino acid analyses using both a Waters Pico Tag HPLC system and chiral capillary GC, 1H NMR, fast atom bombardment mass spectrometry (FABMS), and collisionally induced tandem FABMS. A new homophenylalanine (Hph) variant of 1, [Dha7]microcystin-HphR (4), was also obtained as a minor component. Compound 3 is most likely a biosynthetic precursor of 1. The four new toxins did not have an N-methyl group at the dehydroamino acid or its precursor unit.