Peptide interaction with mixed lipid bilayers alters packing and hydrocarbon chain conformations.

Cell-penetrating peptides (CPPs) with a cationic-hydrophobic character are recognized as carriers for delivering various therapeutics and diagnostic agents across cell membranes and into the cells. Among the most studied CPPs, nona-arginine (R9) exhibits superior penetration compared to its equally charged counterpart, nona-lysine (K9). This indicates that penetration ability relies not only on charge but also on the structure, distribution, and concentration of peptides, as well as the composition of lipid membranes. However, interactions of heptapeptides composed of arginine (R), lysine (K), and phenylalanine (F) residues with membranes remain poorly explored. This study sheds light on the interaction of R5F2/K5F2 on lipid membranes containing a zwitterionic lipid (phosphatidylcholine; PC) and an anionic lipid (either phosphatidylglycerol, PG or phosphatidylserine, PS) in the 90:10 molar ratio. Using differential scanning calorimetry (DSC) and temperature-dependent UV-Vis spectroscopy, we observed peptide interaction-induced changes that stabilize a particular phase of lipid bilayers, as well as their effect on the melting of the latter in terms of cooperative unit size (CUS). The distinct interaction of R5F2/K5F2 on DPPC+DPPG and DPPC+DPPS lipid bilayers revealed that the changes in lipid packing and hydrocarbon chain conformations are peptide-specific features. The peptide-induced formation of vacancies in the non-polar bilayer part is consistent with partial membrane leakage observed in giant unilamellar vesicles. This study provides new insights into the peptide-lipid interactions underlying the functionality of CPPs.