Lipophilic mono-guanidines versus mono-(thio)ureas: anion transport, lipid binding, and antibacterial activity

Synthetic anion transporters (anionophores) can collapse ion gradients and have been widely explored as candidate therapeutics for channelopathies and cancer, yet their potential as antibacterial agents remains comparatively underdevelopped. Here, we systematically compare lipophilic mono-guanidines with isosteric mono-ureas and mono-thioureas, evaluating anion binding, membrane transport, lipid association and antibacterial activity. Chloride transport experiments in POPC show that CF₃-substituted guanidines generally outperform unfluorinated analogues, but remain less potent than their CF₃-substituted urea and thiourea counterparts. Vesicle-based mechanistic studies suggest that the guanidines function as mobile carriers and can facilitate both electroneutral and electrogenic chloride transport. Interestingly, guanidine-mediated chloride transport increases with increasing pH, suggesting that transport is dominated by the neutral guanidine form and not the protonated guanidinium form-presumably due to the high polarity and limited membrane partitioning of the cationic guanidiniums. In addition, all compounds preferentially partition into anionic POPG membranes that mimic bacterial membranes. Several compounds therefore also display antibacterial activity against a variety of Gram-positive bacteria, with minimum inhibitory concentrations in the nanomolar range for the most potent compound and no measurable hemolytic activity. Bacterial mechanistic studies confirmed that the antibacterial activity primarily arises from ion-transport-driven membrane depolarization rather than nonspecific membrane lysis.