Seco[D-Asp3]microcystin-RR and [D-Asp3,D-Glu(OMe)6]microcystin-RR, two new microcystins from a toxic water bloom of the cyanobacterium Planktothrix rubescens

Two novel microcystins, seco[D-Asp(3)]microcystin-RR (1) and [D-Asp(3),D-Glu(OMe)(6)]-microcystin-RR (2), along with the known [D-Asp(3)]microcystin-RR (3), were isolated from a Planktothrix rubescens toxic bloom collected in Lake Bled, Slovenia. The structures were deduced using one- and two-dimensional NMR techniques, ESIMS/CID/MS analysis, and Marfey's method for determining the amino acids' absolute stereochemistry. Compounds I and 3 exhibit weak PP1 inhibitory activity. The NMR data of compound 3 are reported here for the first time.