Langmuir monolayers to study interactions at model membrane surfaces

Langmuir monolayers at the liquid–air interface are well-defined interfacial systems and, therefore, excellent model systems to learn about interactions at interfaces beyond the classical DLVO description. Many parameters can be independently varied over a broad range and the structure can be analyzed with Å precision. In the first part of the paper, the rich polymorphism in monolayers composed of amphiphilic molecules is demonstrated. Using homologues series generic phase diagrams can be derived. The delicate interplay of interactions causes a richness of phases which in turn can be used to measure fine variations in these interactions. Based on the understanding of the polymorphism in pure or mixed lipid monolayers, one can study the interaction of molecules dissolved in the subphase with monolayers. Samples presented are chemical reactions catalyzed by enzymes and coupling of polyelectrolytes to oppositely charged monolayers. To relate structure and reactivity, the activity of enzymes at the interface can be studied, predominantly combining X-ray diffraction and FTIR-spectroscopy. It is shown that the activity depends on monolayer structure. In one case, the reaction product leads to structural changes in the monolayer and stops the reaction, hence, indicating a subtle case of product inhibition via the membrane. On the other hand it has become possible to manipulate the organization of polyelectrolytes at interfaces via lipid charge density and ionic strength. In the most important case of DNA interacting with a membrane surface we show that DNA arranges at the interface in a lamellar manner, and the intermolecular distances, measured by Synchrotron X-ray diffraction can be varied by the lipid density.