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 RA₆R and RA₉R (R, arginine; A, alanine) are investigated. RA₆R does not self-assemble in water due to its high solubility, but RA₉R self-assembles above a critical aggregation concentration into ordered nanofibers due to the high hydrophobicity of the A₉block. The structure of the RA₉R nanofibers is studied by cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Circular dichroism spectroscopy shows that both RA₆R and RA₉R adopt coil conformations in water at low concentration, but only RA₉R adopts a β-sheet conformation at high concentration. SAXS and differential scanning calorimetry are used to study RA₆R and RA₉R 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 RA₆R is more cytocompatible than RA₉R. RA₆R has enhanced activity against the Gram-negative pathogen P. aeruginosa at a concentration where viability of mammalian cells is retained. RA₉R 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.