A Re-evaluation of the Free Energy Profiles for Cell-Penetrating Peptides Across DOPC Membranes

Cell penetrating peptides (CPPs) hold immense potential for the transport of therapeutic agents to their active targets, due to their low cytotoxicity and high transduction efficiency. Adoption of CPPs as delivery systems, and development of novel peptides has been hampered by the variety of mechanisms and complexity of factors involved in cellular uptake. Quantitatively analyzing these systems is further hindered by the inability to compare data among reports due to varying experimental conditions. In this study we investigate the translocation of seventeen CPPs, representing cationic, amphipathic and hydrophobic physicochemical classes, through a DOPC membrane using molecular dynamics simulations. Free energy profiles for individual peptides inserting into the lipid bilayer were generated giving insight into both the approach and adsorption of different CPPs to cell membranes, and the feasibility of their direct translocation. The control of physical conditions and composition allows objective comparison of different peptides and analyze the effect of sequence on the adsorption and translocation energetics. Our results indicate that positively charged residues impart repulsion, while hydrophobic residues increase bilayer interaction. This study is the first step in fully understanding the processes and energetics involved in the passive translocation mechanisms of CPPs, which usually involve multiple peptides.