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ABSTRACT
Interactions between the graft copolymer poly(L-lysine)-g(-poly(ethylene glycol), PLL-g-PEG, and two kinds of surface-supported lipidic systems (supported phospholipid bilayers and supported vesicular layers) were investigated by a combination of microscopic and spectroscopic techniques. It was found that the application of the copolymer to zwitterionic or negatively charged supported bilayers in a buffer of low ionic strength led to their decomposition, with the resulting formation of free copolymer-lipid complexes. The same copolymer had no destructive effect on a supported vesicular layer made up of vesicles of identical composition. A comparison between poly(L-lysine), which did not induce decomposition of supported bilayers, and PLL-g-PEG copolymers with various amounts of PEG side chains per backbone lysine unit, suggested that steric repulsion between the PEG chains that developed upon adsorption of the polymer to the nearly planar surface of a supported phospholipid bilayer (SPB) was one of the factors responsible for the destruction of the SPBs by the copolymer. Other factors included the ionic strength of the buffer used and the quality of the bilayers, pointing toward the important role defects present in the SPBs play in the decomposition process.
INTRODUCTION
Interactions between lipid membranes and charged macromolecules-such as DNA and proteins-play important roles in many cellular and biotechnological processes. An example of the former is the binding of peripheral proteins to the surface of membranes that can induce lipid demixing, alter phase behavior, and cause other effects (such as alterations in membrane curvature) (Wang et al., 1993; Heimburg and Marsh, 1995; Macdonald et al., 1998; Saurel et al., 1998; Sweitzer and Hinshaw, 1998; Cézanne et al., 1999; Heimburg et al., 1999).
Linear polypeptides composed of basic amino acids (especially polylysines) have been widely used as model systems for the interactions between peripheral proteins and lipid bilayers (Chapman et al., 1974; Hartmann and Galla, 1978; de Kruijff et al., 1985; Carrier and Pézolet, 1986; Laroche et al., 1988; Kim et al., 1991; Kleinschmidt and Marsh, 1997; Diederich et al., 1998; Yaroslavov et al., 1998; Huster et al., 2000; Sagrrstá et al., 2000; Franzin and Macdonald, 2001 ; Yaroslavov et al., 2003). They were found to induce domain and pore formation, enhance the flip-flop rates of lipid molecules, alter the phase behavior of the lipids, influence bilayer stability, etc. These...