Reverse Engineering Truncations of an Antimicrobial Peptide Dimer to Identify the Origins of Potency and Broad Spectrum of Action

Antimicrobial peptides hold promise against antibiotic resistant pathogens. Here, to find the physicochemical origins of potency and broad spectrum antimicrobial action, we report the structure activity relationships of synthetic intermediates (peptides A D) of a potent lysine branched dimeric antibacterial peptide Delta Fd. Our studies show that a tetracationic character in a weak helical fold (peptide C) elicits potent but narrow spectrum antimicrobial activity [Minimum inhibitory concentrations (MICs) E. coli 10 mu M, S. aureus > 100 mu M]. In contrast, a hexacationic character in a strong, amphipathic helix (Delta Fd) confers potent and broad spectrum action [MICs E. coli 2.5 mu M, S. aureus. 5 mu M]. While Delta Fd caused rapid and potent permeabilization of the E. coli membranes, the less helical intermediates (peptides A D) showed slow and weak to no responses. Two seminal findings that may aid future drug design are (a) at identical helicity, increasing charge enhanced outer membrane permeabilization, and (b) at identical charge, increasing helicity stimulated rate of outer membrane permeabilization and kill kinetics besides enhancing potency leading to broad spectrum action.