Antibacterial activity and mechanistic insights into para-substituted triphenylphosphonium salts

Dodecyl- and tetradecyl-substituted triphenylphosphonium (TPP) derivatives (1d and 1e) were previously identified by our group as potent antimicrobials, although their mammalian cytotoxicity and hemolytic activity limited their therapeutic potential. To mitigate these drawbacks, six novel para-substituted TPP derivatives bearing dodecyl or tetradecyl chains were synthesized by introducing electron-donating (8d, 8e) or electron-withdrawing (9d, 9e, 10d, 10e) substituents on the phenyl rings. The 4-CF₃ derivatives (10d and 10e) retained potent activity against the tested Staphylococcus aureus strains (MICs of 1-2 μg/mL) while exhibiting substantially reduced mammalian cytotoxicity compared with the parent compounds. Their biological profiles also revealed differences between mammalian cytotoxicity, hemolytic activity, and antibacterial activity across bacterial species. Mechanistic studies revealed preferential accumulation in bacterial membrane fractions, alongside effects on membrane potential and bacterial respiratory and metabolic activity. Notably, the potent anti-S. aureus activity of 10d and 10e, despite their weaker effects on membrane depolarization and bioenergetics, points to a more complex relationship between these effects and antibacterial potency. Overall, these findings highlight electronic modification of the TPP moiety as a promising strategy for modulating the biological properties of TPP-based antimicrobials and reducing mammalian cytotoxicity while retaining activity against selected bacterial pathogens.