Two novel cyclic hexapeptides, designated NW-G08 (1) and NW-G09 (2), were isolated from the fermentation broth of Streptomyces alboflavus 313. Their structures were elucidated on the basis of extensive spectroscopic analysis, MS experiments, and chemical analysis. Their antibacterial activities against several strains of bacteria were evaluated by micro-broth dilution method. NW-G08 (1) and NW-G09 (2) were highly antibacterial against Gram-positive but not active against Gram-negative bactaria. The minimum inhibitory concentrations (MICs) of 1 against Bacillus cereus, Bacillus subtilis, and Staphylococcus aureus were 6.25, 6.25, and 1.56 μg/ml, respectively. It is worth noting that 1 and 2 exhibited much higher activities against methicillin-resistant Staphylococcus aureus (MRSA) than ampicillin, which implies that they might be potential candidates for the development of novel drugs against resistant pathogenic bacteria.

To investigate the chemical biodiversity of biologically active cyclic hexapeptides in the metabolites from microorganisms, the fermentation broth of Streptomyces alboflavus 313 was analyzed using HPLC, equipped with a diode array detector (DAD), coupled with ESI tandem MS (HPLC-DAD-ESI-MS/MS). In the mass spectra of cyclic hexapeptides, predominant ions [M+H](+), as well as [M-18+H](+), [M-28+H](+) and [M+Na](+), were observed and used to determine the molecular masses, while fragmentation reactions of [M+H](+) were recorded to provide information on the contents of amino acids and their linkage sequence. Based on the fragmentation patterns and comparison with standards, 15 pyrroloindoline-containing natural cyclic hexapeptides, cp01-15, were identified from the microorganism and six of these are reported for the first time.