Cyanopeptolin 963A, a chymotrypsin inhibitor of Microcystis PCC 7806

A new depsipeptide, cyanopeptolin 963 A (1), was isolated from an axenic strain of the toxic freshwater cyanobacterium Microcystis PCC 7806. The structure of this compound was elucidated by chemical and spectroscopic analyses, including high-resolution ESI-FTICR-MS, 2-D NMR, and GC-MS of the hydrolysate. The major structural difference compared to previously characterized cyanopeptolins of this strain is the replacement of the basic amino acid in position 3 by L-tyrosine. Compound 1 displayed inhibitory activity against chymotrypsin with an IC50 value of 0.9 muM.