Single atom substitutions in backbone aromatic groups alter membrane permeability and biological activity of cyclic peptides

Cyclic peptides hold significant potential as disruptors of challenging targets such as protein-protein interactions, but their poor membrane permeability often limits their use for intracellular targets. Inspired by nature, we explored the introduction of aromatic groups in the peptide backbone to address this limitation. Starting from model peptide Sanguinamide A, we synthesized a series of analogues in which the native thiazole moiety was replaced with various simple aromatic groups. This approach allowed us to systematically investigate how minimal changes in the nature of the aromatic group and the flexibility of the peptide backbone influence membrane permeability. Significant differences in permeability were observed among the analogues, and for those that were permeable, NMR spectroscopy revealed conformational changes in response to environment polarity. In addition to studying permeability, we assessed the impact of these chemical modifications on biological activity using a target-agnostic cell painting assay. While permeability generally correlated with biological activity, distinct differences in activity were observed depending on the specific aromatic group incorporated. These findings highlight the importance of backbone-embedded aromatic groups in modulating both membrane permeability and biological activity, offering valuable insights for the development of cyclic peptides as therapeutic agents.