Cyanopeptolin 954, a Chlorine-Containing Chymotrypsin Inhibitor of Microcystis aeruginosa NIVA Cya 43

A new depsipeptide, cyanopeptolin 954 (1), was isolated from the freshwater cyanobacterium Microcystis aeruginosa NIVA Cya 43. The structure of the compound was elucidated by chemical and spectroscopic analyses, including 2D NMR and GC-MS of the hydrolysate. The major structural differences compared to previously characterized heptadepsipeptides of Microcystis are the replacement of the basic amino acid in position 4 by L-leucine, the presence of L-phenylalanine in position 6, and the uncommon residue 3'-chloro-N-Me-L-tyrosine in position 7. Cyanopeptolin 954 inhibited chymotrypsin with an IC50 value of 45 nM. Nostopeptin BN920, formerly isolated from the cyanobacterium Nostoc,(1) was isolated from the same strain of Microcystis, and a cis amide bond between Phe (6) and N-Me-Tyr (7) was shown. Nostopeptin BN920 inhibited chymotrypsin with an IC50 value of 31 nM.