Lipids as model membranes

Extrinsic proteins can be separated by treating membranes with aqueous salt solutions of high ionic strength whereas intrinsic proteins can only be isolated by solubilizing the membranes in micellar surfactant solutions. Recently, this model has been modified taking into account that the lipid matrix is more complicated and that the lipid transverse and also the lateral distribution might not be uniform, but essentially it is still widely accepted [2-5]. 

The composition of a vesicle membrane can be manipulated in a wide range using different lipid classes and different lipid chain lengths. A prerequisite is that the lipids form well hydrated lamellar phases and that the vesicles are stable against aggregation and ftision. When these prerequisites are fulfilled, even membrane proteins can be incorporated into the synthetic membranes using appropriate procedures, such as detergent dialysis. Therefore, most physico-chemical studies of membrane properties of bilayers of defined composition have been performed with liposomes of the multilamellar or unilamellar type. 

In this contribution, there will be described the principal type of calorimetric experiments which can be perfonned to elucidate the behavior of lipid model membranes. Most of the examples come from work performed in the author s laboratory. Numerous other groups are involved with calorimetric studies on tliese model membrane systems and there exists a vast amount of literature, because calorimetry has become a standard analytical method in the last ten to twenty years. It is our goal to present the principles of the method and not an extensive review over the current literature. Therefore, this chapter is necessarily biased towards work from the author s laboratory and apologies are extended to all those whose work is not adequately represented here. 

Plasmalogens are phospholipids in which the chain in the. vn-1 position is linked to the glycerol via an a-P-unsaturated ether linkage. The plasmalogens have similar headgroups as the diester phospholipids and their physico-chemical behavior is also closely related to their normal counterparts with diacyl chains. 

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