Self-Assembly, Antimicrobial Activity, and Membrane Interactions of Arginine-Capped Peptide Bola-Amphiphiles

The self-assembly and antimicrobial activity of two novel arginine-capped bola-amphiphile peptides, namely RA(6)R and RA(9)R (R, arginine; A, alanine) are investigated. RA(6)R does not self-assemble in water due to its high solubility, but RA(9)R self-assembles above a critical aggregation concentration into ordered nanofibers due to the high hydrophobicity of the A(9)block. The structure of the RA(9)R nanofibers is studied by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Circular dichroism spectroscopy shows that both RA(6)R and RA(9)R adopt coil conformations in water at low concentration, but only RA(9)R adopts a beta-sheet conformation at high concentration. SAXS and differential scanning calorimetry are used to study RA(6)R and RA(9)R interactions with a mixed lipid membrane that models a bacterial cell wall, consisting of multilamellar 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol/1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine vesicles. Cytotoxicity studies show that RA(6)R is more cytocompatible than RA(9)R. RA(6)R has enhanced activity against the Gram-negative pathogen P. aeruginosa at a concentration where viability of mammalian cells is retained. RA(9)R has little antimicrobial activity, independently of concentration. Our results highlight the influence of the interplay between relative charge and hydrophobicity on the self-assembly, cytocompatibility, and bioactivity of peptide bola-amphiphiles.