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978-3-86853-847-2, Reihe Physik
Coarse-grained simulations of membranes under tension and lipid-protein interactions
142 Seiten, Dissertation Universität Konstanz (2011), Softcover, A5
The first focus of this work is on the description of biological membranes under the influence of an additional lateral tension by Monte Carlo simulations of a generic coarse-grained model for lipid bilayers. A comprising overview of the behavior in the low-temperature regime (tilted gel), in the high-temperature regime (fluid), and in the asymmetric ripple state at intermediate temperatures is given. In the ripple state a suppression of the ripple under tension and a transition of the lateral compressibility from soft fluid-like to hard gel-like behavior is revealed, in agreement with experimental results. The biologically prevalent fluid phase is also significantly affected by lateral tension. Partial interdigitation of the opposing monolayers, which are discernibly separated in the stressfree state, is induced. Concerning the fluctuation spectra of fluid bilayers we establish that a suitably extended elastic theory for membranes is still applicable in the range of moderate tension, but fails to describe the spectra correctly for long wavelength fluctuations at higher tension.
Besides lipids, proteins are the major components of biological membranes and their properties and functionality can not be understood without taking them into account. Therefore, the second part deals with the interaction of bilayers and transmembrane inclusions. Three different types of rigid models are employed: long cylindrical proteins, capped tiltable cylindrical proteins, and proteins constructed from stacked disks of beads. The model named last is newly developed and tested for the use in the framework of our lipid model. Both the influence of the hydrophobic length of the proteins and the strength of the hydrophobic interaction between proteins and lipids on properties of the surrounding bilayer and the orientation of the protein are studied systematically. Here, the comparison between results obtained for capped cylindrical and bead-composed proteins is of particular interest. Both models lead to similar results concerning the radial profiles of the bilayer thickness or the lipid director fields. The same holds for the average tilt of the proteins.
The profiles around two fixed bead-proteins at medium distance provide an indication of induced lipid nucleation by corrugated proteins with negative hydrophobic mismatch and strong hydrophobicity, which is not observed for proteins with smooth surface. We show that this effect enables the formation of, at least metastable, protein-lipid complexes on the scale of several protein diameters. By serving as nucleation centers for lipid phase transitions, proteins may promote the formation of interdigitated rafts and stabilize them.