Design and characterization of anchoring amphiphilic peptides and their interactions with lipid vesicles.

In an effort to develop a polymer/peptide assembly for the immobilization of lipid vesicles, we have made and characterized four water-soluble amphiphilic peptides designed to associate spontaneously and strongly with lipid vesicles without causing significant leakage from anchored vesicles. These peptides have a primary amphiphilic structure with the following sequences: AAAAAAAAAAAAWKKKKKK, AALLLAAAAAAAAAAAAAAAAAAAWKKKKKK, and KKAALLLAAAAAAAAAAAAAAAAAAAWKKKKKK and its reversed homologue KKKKKKWAAAAA AAAAAAAAAAAAAALLLAAKK. Two of the four peptides have their hydrophobic segments capped at both termini with basic residues to stabilize the transmembrane orientation and to increase the affinity for negatively charged vesicles. We have studied the secondary structure and the membrane affinity of the peptides as well as the effect of the different peptides on the membrane permeability. The influence of the hydrophobic length and the role of lysine residues were clearly established. First, a hydrophobic segment of 24 amino acids, corresponding approximately to the thickness of a lipid bilayer, improves considerably the affinity to zwitterionic lipids compared to the shorter one of 12 amino acids. The shorter peptide has a low membrane affinity since it may not be long enough to adopt a stable conformation. Second, the presence of lysine residues is essential since the binding is dominated by electrostatic interactions, as illustrated by the enhanced binding with anionic lipids. The charges at both ends, however, prevent the peptide from inserting spontaneously in the bilayer since it would involve the translocation of a charged end through the apolar core of the bilayer. The direction of the amino acid sequence of the peptide has no significant influence on its behavior. None of these peptides perturbs membrane permeability even at an incubation lipid to peptide molar ratio of 0.5. Among the four peptides, AALLLAAAAAAAAAAAAAAAAAAAWKKKKKK is identified as the most suitable anchor for the immobilization of lipid vesicles.