Thermodynamic Coupling in Asymmetric Lipid Membranes

Most, if not all natural membranes are asymmetrically composed with respect to their lipids. However, the knowledge on the influence of the two leaflets on each other is rather limited. It is known that the fluidity of a cell membrane is important for its function and a fluid membrane is favourable.The possibility to prepare asymmetric vesicles gives a new approach to study the interaction of the two monolayers in membranes. To get certain results we used three different reconstitution systems for asymmetric bilayers - solid supported LB-films, planar lipid bilayers, and asymmetric vesicles - and applied various biophysical techniques to investigate the phase behaviour of the mono- and bilayers.Because the most prominent example for an asymmetric membrane is the outer membrane of Gram-negative bacteria which outer leaflet consists of lipopolysacchrides (LPS) and the inner leaflet of phospholipids, these components were in the focus of this study. But there were also different types of phospholipids used to confirm the findings of the experiments with LPS. Surprisingly, LPS have phase transition temperatures between 35°C and 50°C depending also on the buffer. In asymmetric bilayer the phase transition temperature is shifted to lower tmperatures which makes it more fluidic. This shift takes place only in asymmetric and not in symmetric bilayer. We demonstrate that the two leaflets strongly influences the phase behaviour of each other and thus a bilayer is not just the sum of the two monolayers.