Theoretical and experimental description of permeability of peptide-containing membranes

Abstract

In this dissertation combined experimental and theoretical studies on membrane permeability in the presence of peptides are presented. We study the permeability of lipid membranes as a function of temperature, and how it is influenced by peptides and proteins. The theoretical approach consists in the application of statistical thermodynamics methods including Monte Carlo simulations. Monte Carlo methods are stochastic computer simulation methods for systems in thermodynamic equilibrium. We use Ising-like model on a hexagonal matrix of changeable size to simulate the spontaneous pore formation process. Experimental studies include calorimetry and fluorescence correlation spectroscopy (FCS). FCS is a fluctuation correlation method for measuring the dynamics of molecular processes via the observation of microscopic fluctuations in molecular position, number density, and orientation. We use FCS to monitor relative populations of fluorescence markers inside and outside of lipid vesicles. The time dependent ratio between the two label populations can be used to determine membrane permeability. We found that the permeability is maximum at the phase transition temperature of the lipid membrane. This is in agreement with previous studies by other authors. We have shown that peptides influence the permeability indirectly by influencing the fluctuations of the local lipid environment. An experimental FCS method to measure this effect was developed. Changes of the permeability as a function of temperature caused by peptide insertion were observed experimentally. A theoretical model combined with simulations based on local lipid fluctuations was developed. It was shown how changes of the local organization of molecules, caused by the formation of peptide aggregates, and the related changes in local lipid fluctuations influence the membrane permeability. The experimentally observed changes of the permeability caused by peptide insertion are explained by the theory.