Self-Assembly of Branching and Linear Synthetic Polycationic Peptides in Phosphate Buffer and Its Impact on Antimicrobial Activity

Among therapeutic peptides, antimicrobial peptides (AMPs) have gained significant attention for their potential to combat antimicrobial resistance. Their efficacy often relies on the ability to adopt organized structures, such as alpha-helices or beta-strands. However, the formation of supramolecular aggregates can hinder their antimicrobial effectiveness. This study explores the correlation between supramolecular organization in phosphate buffer (PB) and biological activity in three cationic peptides with identical amino acid compositions, nine Leu-Lys pairs, but differing architectures: linear and branched, using lysine or a triazine as the branching unit. Structural conformation and self-assembly behaviors in water and PB were analyzed using Circular Dichroism (CD), Dynamic Light Scattering, and Atomic Force Microscopy (AFM). Results show that the linear peptide, largely unstructured or randomly coiled in water at neutral pH, adopts a beta-sheet conformation in PB. AFM imaging revealed that at low peptide and phosphate concentrations, the linear peptide showed small helical rods self-assembled via beta-sheet interactions. This structural transition is driven by electrostatic interactions between phosphate ions and the amine group. The linear peptide, which lacks antibacterial activity, shows a strong tendency to form large beta-strand aggregates. In contrast, branched peptides were less prone to aggregation and showed enhanced antibacterial activity, particularly the triazine-branched peptide.