Cellular uptake and membrane-destabilising properties of α-peptide/β-peptoid chimeras: lessons for the design of new cell-penetrating peptides

Novel peptidomimetic backbone designs with stability towards proteases are of interest for several pharmaceutical applications including intracellular delivery. The present study concerns the cellular uptake and membrane-destabilising effects of various cationic chimeras comprised of alternating N-alkylated β-alanine and α-amino acid residues. For comparison, homomeric peptides displaying octacationic functionalities as well as the Tat47–57 sequence were included as reference compounds. Cellular uptake studies with fluorescently labelled compounds showed that guanidinylated chimeras were taken up four times more efficiently than Tat47–57. After internalisation, the chimeras were localised primarily in vesicular compartments and diffusively in the cytoplasm. In murine NIH3T3 fibroblasts, the chimeras showed immediate plasma membrane permeabilising properties, which proved highly dependent on the chimera chain length, and were remarkably different from the effects induced by Tat47–57. Finally, biophysical studies on model membranes showed that the chimeras in general increase the permeability of fluid phase and gel phase phosphatidylcholine (PC) vesicles without affecting membrane acyl chain packing, which suggests that they restrict lateral diffusion of the membrane lipids by interaction with phospholipid head groups. The α-peptide/β-peptoid chimeras described herein exhibit promising cellular uptake properties, and thus represent proteolytically stable alternatives to currently known cell-penetrating peptides.