2D-ELDOR of the Liquid Ordered Phase in Multilamellar Vesicle Membranes

In biological membranes, heterogeneity frequently appears. That is, there is lateral phase separation, characterized by the formation of microdomains, which have been termed lipid rafts. Cholesterol, or its related sterol derivatives, is mainly found in plasma membranes of eucaryotic cells, where it can constitute as high as 50 mol% of the lipid composition. It has been appreciated that its presence leads to liquid-ordered domains. These domains, or rafts in biological membranes, have been shown to be involved in transmembrane signal transduction. We have shown that 2D-ELDOR spectroscopy may employed to study the dynamic structure of the liquid ordered (Lo) phase versus that of the liquid crystalline (Lc) phase in multibilayer phospholipid vesicles without (Lc) and with (Lo) cholesterol. The spectra are in most cases found to be dramatically different for these two phases. Thus, visual inspection of the 2D-ELDOR spectra provides a convenient way to distinguish the two phases in membranes as shown in the figure below. Detailed analysis shows these observations are due to increased ordering in the Lo phase and modified reorientation rates. In the Lo phase, acyl chains undergo a faster rotational diffusion and higher ordering than in the Lc phase, whereas CSL exhibits slower rotational diffusion and higher ordering. On the other hand, the choline headgroup in the Lo phase exhibits faster motion and reduced but realigned ordering vs. the Lc phase. The microscopic translational diffusion rates in the Lo phase are significantly reduced in the presence of cholesterol. These results may be interpreted in terms of a consistent model of the differences in the dynamic structure of the Lo and Lc phases. Thus one may expect 2D-ELDOR to be very useful in characterizing the dynamic structure of biological membranes.