Two New Cyclic Peptides from Marine Cyanobacteria

Two new cyclic peptides portobelomide A (1) and B (1) were isolated from the extract of marine cyanobacteria collected from Portobelo-Panama. The cyanobacterial biomass was extracted with 2:1 CH₂Cl₂/CH₃OH. A portion of the extract was fractionated by silica gel VLC using a stepwise gradient solvent system of increasing polarity starting from 100% hexanes to 100% MeOH (nine fractions, A-I). The fraction eluting with 75% EtOAc in MeOH (fraction H) was separated further using RP SPE [500 mg SPE, stepwise gradient solvent system of decreasing polarity starting with 25% CH₃CN in H₂O to 100% CH₃CN, and 100% CH₃OH to produce four fractions (1-4). Further fractionation of 3 by RP-HPLC using a Phenomenex 5 μm Phenylhexyl analytical column, with a gradient from 50% CH₃CN/H₂O to 100% CH₃CN over 30 min, yielded pure cyclic peptides 1 and 2. Their planar structures were elucidated through a combination of 2D-NMR and MS₂/MS₃ techniques. Absolute stereochemistries were determined by Marfey’s analyses.