The properties of biological interfaces play a role in all processes where organisms interact with their environment. The mucus barrier encountered in higher organisms is a prominent example of such an interface and regulates passage of nutrients and pathogens. On a molecular level, cellular membranes composed of lipid bilayers represent another fundamental biological interface, which is almost always in contact with solutions containing ions or other solutes. This thesis studies the properties of these two interfaces and the interactions with their environment.
First, the barrier properties of an uncharged mucus analogous hydrogel are analyzed based on non-normalized experimental concentration profiles of penetrating tracer molecules. For this, a numerical model of the diffusion process is developed that allows for the extraction of diffusion constants of the tracer particles in the bulk solution and in the hydrogel, as well as free energy differences from which partition coefficients are computed. The computational extraction method is validated by comparison of the obtained diffusion constants with results from experiments and with scaling laws from polymer theory. Based on the extracted partition coefficients a free volume model is developed, which takes into account the tracer and hydrogel flexibility. The model suggests a broad pore size distribution of the unordered hydrogel, in which the larger pores are found to predominantly determine the partitioning process, a phenomenon which might be general to unordered biological hydrogels like mucus.
The second part of this thesis covers the interactions of lipid bilayers in contact with solutions containing different co-solutes or ions, which are analyzed using atomistic molecular dynamics simulations. The hydration repulsion of lipid bilayers, commonly observed for nanometer separations, is found to be universally increased by the presence of co-solutes. This effect is quantitatively reproduced from experiments, thus validating the modeling approach. The added repulsion is in a next step modeled as an osmotic pressure afforded by the co-solutes and further augmented by the incorporation of solute-solute and solute-lipid interactions. Finally, ionic adsorption to the lipid interface is investigated in detail by computation of the surface potential obtained from a combination of equilibrium and non-equilibrium simulations in the presence of an electric field. By developing an electrostatic continuum model, which additionally incorporates the presence of minute amounts of negatively charged surface active impurities, initially counterintuitive experimental data is unified for the first time with simulation results. The assumption of contaminations existing in experiments has previously explained a range of other puzzling surface properties and is found to also allow for in detail modeling of electrophoresis experiments on lipid bilayers in ionic solutions.
