Improvement of bacterial cell selectivity of melittin by a single Trp mutation with a peptoid residue

To design melittin (ME) analogues that are not cytotoxic against mammalian cells but which possessing potent antimicrobial activity, we synthesized a ME analogue (ME-w) in which the Trp-19 residue of ME was replaced by a Trp-peptoid residue (Nhtrp). ME-w exhibited similar antimicrobial activity compared to ME against the tested six bacteria and C. albicans. However, it was much less cytotoxic against the hRBCs and HeLa and NIH-3T3 cells than ME. Tryptophan fluorescence and CD spectra revealed that the Trp-19 --> Nhtrp substitution in ME contributed to a much lower helical assembly in an aqueous environment and structural flexibility and exterior localization to zwitterionic membrane which modulates its selectivity toward bacterial cells.