The interaction of five poly[L-lysine]-derived branched chain polypeptides of poly[Lys(X-i)] (XiK) or poly[Lys(X-i-DL-Ala(m))] (XAK) with lipid bilayers (DPPC and DPPC/PG, 8:2) was studied by fluorescence polarization techniques. Two fluorescent probes, DPH and TMA-DPH, were utilized to monitor changes of motion in the internal and/or in the polar head regions, respectively. Results indicate that the interaction of polypeptides with neutral (DPPC) bilayers is mainly dependent on the polarity and electrical charge of side chains. The amphoteric EiK shows the highest level of interaction. Polycationic polypeptides (HiK, PiK, TAK) have a relatively small effect on the transition temperature of the lipids, while the polyanionic Succ-EAK has no effect at the alkyl chain region of the bilayer. Data with TMA-DPH indicate the lack of pronounced interaction between the polypeptides and the outer surface of the liposome. Similar tendency was documented for DPPC/PG vesicles. Polypeptides, HiK, and PiK induce significant changes in the transition temperature, thus indicating their insertion into the hydrophobic core of the bilayer without marked effect on the polar head region. Results suggest that these polypeptides (except EiK) have no destabilizing effect on liposomes Studied. These properties are considered as beneficial for their use as safe carriers for bioactive molecules. (C) 2003 Elsevier Inc. All rights reserved.