Chain length effect on the structure and stability of antimicrobial peptides of the (RW)_n series

Three peptides containing (RW)_n-NH₂ units (where n=4, 6, and 8) have been chosen to study the effect of the chain length on the structure and stability of the peptide using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) techniques. Their interactions with Escherichia coli (E. coli) membrane mimetic vesicles are discussed. Infrared results indicate that addition of (RW)_n-NH₂ units increases intermolecular H bonds with antiparallel orientation. TGA and DSC results reveal that (RW)₆-NH₂ shows the optimal chain length in terms of stability and all three peptides show a preferential interaction with one of the anionic lipids in E. coli membranes. SEM images of (RW)₄-NH₂ present large aggregates while those of (RW)₆-NH₂ and (RW)₈-NH₂ present layers of sheet-like structure. In the presence of model membranes, (RW)_n-NH₂ show fibrillar peptide superstructures. This study suggests that repeating structures of (RW)_n-NH₂ promotes lateral assembly.