Minimal models for lipid membranes: local modifications around fusion objects
Giovanni Marelli
Doctoral thesis
Date of Examination:
2013-01-21
Date of issue:
2013-02-01
Advisor:
Müller, Marcus Prof. Dr.
Referee:
Müller, Marcus Prof. Dr.
Referee:
Groot, Bert de Prof. Dr.
Persistent Address: http://hdl.handle.net/11858/00-1735-0000-000D-F28E-0
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Abstract
English
Solvent-free soft coarse-grained models are particularly appropriate to investigate collective phenomena in lipid membranes. In this work we exploit such a model to show how modifying a few model parameters we can control the bending rigidity of the membrane, the hydration repulsion, and the macroscopic phases of self-assembled structures. Further, we investigate the lipid mediated interactions between fusion objects: transmembrane proteins, pores and stalks. The presence of such defects induces a perturbation in the shape of the membrane and in the conformations of lipids. The modi cations induced by single defects superimpose and for some defect interaction (peptide-peptide, pore-peptide) we identify the equilibrium distance between these objects. The prediction of the results of the simulations are compared with the numerical solution of a continuum model parametrized from the analysis of the simulation snapshots. The presence of transmembrane proteins with a large hydrophobic mismatch weakens the membrane over the direct interaction range decreasing slightly the membrane thickness. This involves the lowering of the line tension of the pore and for a particular number and spatial arrangement of proteins the line tension of the pore is negative and the pore is stable in tensionless membranes. Another agent that in uences the line tension of the pore is oil (short hydrophobic chains). The oil has a con guration space larger than the one of the lipids and partitions to relax the frustration of the lipids at the interfaces and increases the line tension of the pore (the membrane is more resistant under lateral tension). The model parameters have a large in uence on the equilibrium properties of a stalk and we study the characteristic sizes of stalks depending on the hydration between two opposed bilayers and compare the results to other simulation models and experimental data. We show how hydration and lateral tension in uence bilayer repulsion and how the combined e ect of both contributions leads to membrane fusion.
Keywords: Lipid membranes; Molecular Dynamics; Coarse-grained; Monte Carlo; Membrane fusion
