Cellulamides: A New Family of Marine-Sourced Linear Peptides from the Underexplored Genus.

Bioprospecting the secondary metabolism of underexplored Actinomycetota taxa is a prolific route to uncover novel chemistry. In this work, we report the isolation, structure elucidation, and bioactivity screening of cellulamides A and B ( and ), two novel linear peptides obtained from the culture of the macroalga-associated CT-R177. The host of this microorganism, the Chlorophyta , was collected in the northern Portuguese coast and, in the scope of a bioprospecting study focused on its associated actinobacterial community, strain CT-R177 was isolated, taxonomically identified, and screened for the production of antimicrobial and anticancer compounds. Dereplication of a crude extract of this strain using LC-HRMS(/MS) analysis unveiled a putative novel natural product, cellulamide A (), that was isolated following mass spectrometry-guided fractionation. An additional analog, cellulamide B () was obtained during the chromatographic process and chemically characterized. The chemical structures of the novel linear peptides, including their absolute configurations, were elucidated using a combination of HRMS, 1D/2D NMR spectroscopy, and Marfey's analysis. Cellulamide A () was subjected to a set of bioactivity screenings, but no significant biological activity was observed. The cellulamides represent the first family of natural products reported from the Actinomycetota genus , showcasing not only the potential of less-explored taxa but also of host-associated marine strains for novel chemistry discovery.