Modified peptides from a water bloom of the cyanobacterium Nostoc sp.

Six new metabilites, nostopeptin BN920 (), nostoginin BN741 (), nostoginin BN578 (), banyascyclamide A () banyascyclamide B () and banyascyclamide C (), were isolated from the hydrophilic extract of a Nostoc sp. The planar structure of compounds was determined by homonuclear and inverse-heteronuclear 2D NMR techniques as well as high-resolution mass spectrometry. The absolute configuration of the asymmetric centers was studied using Marfeyʹs method for HPLC. Compound inhibits the serine-protease chymotrypsin while compound inhibits amino-proteases.