Double Electron–Electron Resonance of Spin-Labeled Cholestane in Model Membranes: Evidence for Substructures inside the Lipid Rafts

Plasma membranes are assumed to be highly compartmentalized, which is believed to be important for the membrane protein functionality. The liquid ordered-disordered phase segregation in the membranes results in nanoscale liquid-ordered assemblies-lipid rafts. Double electron-electron resonance spectroscopy (DEER, also known as PELDOR) is sensitive to spin-spin dipolar interactions between spin labels at the nanoscale range of distances. Here, DEER is applied to spin-labeled cholestane, 3β-doxyl-5α-cholestane (DChl), diluted in bilayers composed of an equimolar mixture of dioleoyl-glycero-phosphocholine (DOPC) and dipalmitoyl-glycero-phosphocholine (DPPC) phospholipids, with cholesterol (Chol) added. The DEER data allowed us to detect clustering of the DChl molecules. Their lateral distribution in the clusters in the absence of Chol was found to be random, while in the presence of Chol it became quasi-regular. DEER time traces are fairly well simulated within a simple square superlattice model. For the 20 mol % Chol content, for which at physiological temperatures, the lipid rafts are formed, the found superlattice parameter was 3.7 nm. Assuming that lipid rafts are captioned upon shock freezing at the temperature of investigation (80 K), the found regularity of DChl lateral distribution was interpreted by raft substructuring, with the DChl molecules embedded between the substructures.