Abstract
Cyanobacterial blooms are a significant environmental concern due to their production of toxic metabolites with potential impacts on ecosystem and human health. Microcystin-LR and microcystin congeners are a major concern with respect to human exposure and intoxication, but there are hundreds of characterized cyanobacterial peptides and metabolites that are of interest for their environmental impact in a variety of classes such as cyanopeptolins/micropeptins, microviridins, microginins, and anabaenopeptins. In this study, we report the isolation and characterization of new micropeptins (1-4), a new ferintoic acid (5) a new microginin (6), and three new microviridins (7-9) from a cyanobacterial bloom sample. The new micropeptins, in particular, exhibited unprecedented modifications in their amino acid composition and configuration, which differentiate them from previously known variants. Certain alterations significantly influenced their biological activity with respect to chymotrypsin inhibition and human neutrophil elastase inhibition, highlighting the potential ecological and biomedical relevance of these compounds. We report d-amino acid incorporation into the micropeptins for the first time providing new insights into the chemical diversity of cyanobacterial natural products. These results have important implications for understanding biosynthetic flexibility, the development of bioactive agents for therapeutic applications, and highlight the need for reference materials for mass spectrometry-based metabolite annotation.