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Prof. Avinoam Ben-Shaul
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Conference abstract - invited talk:
STRUCTURAL AND THERMODYNAMIC CONSEQUENCES OF THE INTERACTION BETWEEN MACROMOLECULES AND MIXED-SOFT LIPID MEMBRANES*
Being a (two-dimensional) fluid mixture a binary, or multicomponent, lipid membrane can respond to the interaction with an approaching macromolecule, such as DNA or a peripheral protein, by a local modulation of its lipid composition. Furthermore, flexible membranes can also change their local curvature (and sometimes thickness). Quite generally the compositional and curvature changes are coupled each other. If strong enough, the interactions between macromolecules and mixed and/or soft lipid bilayers can induce global changes in membrane structure and/or composition, i.e., in a thermodynamic phase transition; for instance protein aggregation or vesicle budding. Dramatic effects of this kind can also be induced by integral macromolecules, e.g., the peptide mediated transition of a (peptide poor) lipid bilayer to a (peptide rich) inverse hexagonal lipid phase. Here, however, our discussion will focus on the interaction between peripheral, more specifically charged, macromolecules (namely DNA and proteins) with mixed, oppositely charged membranes. Using Poisson-Boltzmann theory for the electrostatic interaction between the macromolecules and the membrane we shall consider two types of systems: a) lipoplexes, i.e., the complexes formed between DNA and mixed, cationic/neutral lipids, b) the adsorption of basic proteins on membranes containing acidic lipids. It will be shown that the ability of the charged lipids to migrate toward or away from the interaction zone plays a major role in determining the structural and thermodynamic aspects of the membrane-macromolecule assembly. In lipoplex systems this compositional degree of freedom is strongly coupled to membrane lasticity, resulting sometimes in rather complex phase behaviors of the lipid/DNA mixture. We shall show, for example, that certain lipid membranes combine with DNA to form lamellar ("sandwich") complexes, while others favor the formation of hexagonal ("honeycomb") complexes. In many cases transitions between the two structures take place upon varying the overall lipid/DNA ratio. Our treatment of protein adsorption on mixed membranes shows that the diffusion of charged lipids into the interaction zone can greatly enhance the adsorption; the saturation concentration of proteins on the membrane is dictated by the ratio between the charge densities on the membrane and protein surfaces, by lateral interactions between the adsorbed proteins and, of course, by salt concentration. We shall also discuss the conditions for global lateral phase separation in the lipid protein system.For relevant literature see for examples the following articles in the Biophysical Journal: 73,2427(1997);75,159(1998),76,751(1999);78,1681(2000);79,1747(2000)
* In collaboration with Sylvio May (Fredrich-Schiller University, Jena), and Daniel Harries (Hebrew University, Jerusalem)