Phase transitions of lipid mixtures

It has been known for quite a while that certain types of membrane located enzymes show a specificity for the binding of certain lipids but it still is unclear whether lateral domain fonnation is mainly caused by proteins or whether pure lipid mixtures can show this phenomenon by themselves [54-60]. 

Biological membranes and the lipid mixtures extracted from them are far too complex to gain suitable infonnation on their mixing behavior by calorimetric methods. Therefore, the systematic investigation of mixtures of two or more lipids, preferentially synthetic ones, in aqueous dispersion is the method which has led to important conclusions about lipid miscibility in these quasi-two-dimensional lamellar phases. The analysis of the phase diagrams obtained from the investigation of the thermotropic transitions of suspensions with different lipid compositions in excess water in tenns of the curvature and the location of the phase boundaries provides the necessary information about non-ideality parameters. 

Cholesterol is abundant in many membranes of eukaryotic cells, the total percentage reaching up to 50% of the total lipid content. The behavior of lipid-cholesterol mixtures has therefore attracted much attention Despite the vast amount of publications on the thennotropic behavior of phospholipid-cholesterol mixtures, the macroscopic description of lipid-cholesterol systems using phase diagrams is still very much debated, because different analytical methods such as NMR-, ESR-, fluorescence- and FT-IR-spectroscopy monitor changes in physicochemical behavior of lipid-cholesterol mixtures as a function of composition and temperature which cannot easily reconciled with observations using DSC [61-72]. 

DSPC/cIioIesterol as a function of cholesterol content (adapted from reference [67]). 

The effect of cholesterol on other phospholipid mixtures is qualitatively similar. For PCs with saturated chains, it is observed that the mid point temperature of the broad transition is higher than that of the sharp transition when the PC carries shorter chains, such as in DMPC. For DAPC with its C20 chains, the broad transition is shifted to a lower temperature compared to the sharp transition [67,68], 

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