We examine complex fluid systems where both translational and conformational degrees of freedom are present and focus on systems in which the interplay between the two sets of degrees of freedom is manifested in the macroscopic phase behaviour. We develop an efficient random lattice algorithm describing the translational degrees of freedom and analyze a series of microscopic models defined on a two dimensional fluid surface. Different degrees of complexity in the description of the microscopic coupling between the translational and conformational degrees of freedom allow us to study a variety of models related to pure lipid membrane and lipid-sterol membrane systems. / The phase equilibrium described by the models is calculated by use of Monte Carlo simulation techniques. The different models are shown to exhibit a rich phase behaviour. Depending on the specific model parameters, the phase transition associated with the conformational degrees of freedom is found to be either coupled to, or uncoupled from, that associated with the conformational degrees of freedom. / Specifically, the order-disorder transition of an Ising model defined on a fluid surface is shown to be of first order, when the two sets of degrees of freedom are strongly coupled. In contrast, the transition falls in the universality class of the two-dimensional Ising model when the two sets of degrees of freedom are weakly coupled. / We next analyze a model for pure lipid bilayers which is shown to exhibit a phase behaviour with different types of macroscopic coupling between the two sets of degrees of freedom. Depending on the strength of the microscopic interactions the lipid chain melting transition and the lattice melting transition may be either macroscopically coupled or uncoupled. / A related model for lipid-sterol mixtures is shown to provide a consistent interpretation of the various phases of lipid-cholesterol and lipid-lanosterol binary mixtures based on the microscopic dual action of the sterol molecule on the lipid-chain degrees of freedom. We discuss the results for the systems in the context of membrane evolution and suggest that evolution has tended to optimize the lipid-sterol interaction so as to stabilize optimally the mechanical properties of the membrane. Furthermore, a specific small-scale structure is identified and characterized in the liquid-ordered phase in lipid-cholesterol mixtures. This structure is found to be absent in lipid-lanosterol mixtures. / Finally, a model for membrane lysis gives evidence for the high mechanical stabilizing effect of cholesterol on the membrane. The inclusion of cholesterol is shown to inhibit lysis whereas lanosterol only has little stabilizing effect.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.35924 |
| Date | January 1998 |
| Creators | Nielsen, Morten. |
| Contributors | Zuckermann, Martin J. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Physics.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001657670, proquestno: NQ50230, Theses scanned by UMI/ProQuest. |
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