Lipid-protein interactions in model membranes from bovine brain white matter. An ESR spin label and electron microscopy study

Lipid-protein model membranes, prepared from bovine brain white matter and containing all the lipids and Folch-Lees proteolipids, have been studied in macroscopically oriented multibilayers. To examine the lipid environment the membranes were spin labeled with the cholestane spin label ($\text{3-spiro(2′-(N-}\text{oxyl-4′,4′-dimethyl-oxazolidine))5α-cholestane}$) and a fatty acid spin label ($\text{4′-,4′-dimethyloxazolidine-N-}\text{oxyl}$ derivative of 5-ketostearic acid). The ESR spectra exhibit two components arising from fairly well oriented and completely unoriented lipids. Up to a temperature of 55°C the amount of oriented lipids is almost constant, being about 35%. At higher temperatures this percentage drops rapidly to zero. It is shown that the presence of unoriented lipids arises mainly from disrupted areas in the lipid bilayer structure. This is confirmed by electron microscopy and from an analysis of the temperature dependence of the order parameters of the spin labels. The presence of locally disrupted lipid parts in the bilayer is discussed in relation to the interaction of the brain white matter lipids with Folch-Lees protein.