How Unnatural Amino\nAcids in Antimicrobial Peptides\nChange Interactions with Lipid Model Membranes

This study investigates\nthe potential of antimicrobial peptides\n(AMPs) as alternatives to combat antibiotic resistance, with a focus\non two AMPs containing unnatural amino acids (UAAs), E2-53R (16 AAs)\nand LE-54R (14 AAs). In both peptides, valine is replaced by norvaline\n(Nva), and tryptophan is replaced by 1,2,3,4-tetrahydroisoquinoline-3-carboxylic\nacid (Tic). Microbiological studies reveal their potent activity against\nboth Gram-negative (G(−)) and Gram-positive (G(+)) bacteria\nwithout any toxicity to eukaryotic cells at test concentrations up\nto 32 μM. Circular dichroism (CD) spectroscopy indicates that\nthese peptides maintain α-helical structures when interacting\nwith G(−) and G(+) lipid model membranes (LMMs), a feature\nlinked to their efficacy. X-ray diffuse scattering (XDS) demonstrates\na softening of G(−), G(+) and eukaryotic (Euk33) LMMs and a\nnonmonotonic decrease in chain order as a potential determinant for\nbacterial membrane destabilization. Additionally, XDS finds a significant\nlink between both peptides’ interfacial location in G(−)\nand G(+) LMMs and their efficacy. Neutron reflectometry (NR) confirms\nthe AMP locations determined using XDS. Lack of toxicity in eukaryotic\ncells may be related to their loss of α-helicity and their hydrocarbon\nlocation in Euk33 LMMs. Both AMPs with UAAs offer a novel strategy\nto wipe out antibiotic-resistant strains while maintaining human cells.\nThese findings are compared with previously published data on E2-35,\nwhich consists of the natural amino acids arginine, tryptophan, and\nvaline.