(125c) Interaction Mechanisms Between a Homologous Series of Tripodal Cationic Peptides and Lipid Bilayer Membranes

In the past few years, the ability to traverse the plasma membrane of mammalian cells has been investigated with a wide range of cell permeating peptides. An understanding of the underlying non-endocytic mechanisms of these interactions may lead to the design of efficient peptide based therapeutic delivery systems. While the exact mechanism is poorly understood, peptides are thought to adsorb at the lipid/membrane water interface, disrupt headgroup interactions and produce local effects within the membrane that permit entry into the cell. In this study, we examine water-soluble tripodal cationic peptides synthesized as Arg-C_n-Arg-C_n-Lys, where C_n represents the alkyl linkage separating the cationic residues (n = 4 to 10) as potential drug delivery platforms. Our goal is to study the interactions of these peptides with zwitterionic (DPPC) and mixed zwitterionic/anionic (DPPC/DPPS and DPPC/DPPG) lipid bilayers as a function of peptide concentration and lipid composition. Our preliminary differential scanning calorimetry studies show that electrostatic interactions, coupled with the hydrophobic effect with increasing alkyl chain length, dominate peptide membrane interactions. Melting temperature and cooperativity of DPPC reduces with increased C_n and peptide concentration. In the case of anionic lipids, a more pronounced effect was observed as the peptides strongly adsorb to the negatively charged head groups and induce the formation of domains rich in anionic lipids. The longer alkyl linkage (C₁₀) was then able to penetrate into the bilayer and lead to significant disruption. Our results correlate with in vitro studies conducted on human breast carcinoma BT20 cells that show a significant uptake of fluorescein-labeled C₁₀ peptide indicating its functionality of as a delivery tool. We are also investigating additional peptides withy varying linkage lengths and peptide residues. In situ cryogenic transmission electron microscopy and atomic force microscopy studies are also being employed to characterize mechanisms of membrane permeation.