Physical Properties of High-Cholesterol Containing Membranes

Electron paramagnetic resonance spin-labeling methods were used to investigate the physical properties of Chol/POPC (cholesterol/1-palmitoyl-2-oleoylphosphatidylcholine) membranes. The Chol/POPC mixing ratio was changed from 0 to 3. The membrane samples were prepared using the rapid solvent exchange method to preserve the compositional homogeneity throughout the membrane suspension. Recently, it was shown that cholesterol bilayer domains (CBDs) start to form at Chol/POPC mixing ratio of 1. Here we have investigated the physical properties of the membranes with increasing Chol content using cholesterol analogue spin labels CSL and ASL. Physical properties include maximum splitting, mobility parameter, and hydrophobicity. The profiles of maximum splitting increase with an increase of Chol content and saturate at about 50 mol% Chol, showing that CBDs and phospholipid cholesterol domains (PCDs) have similar order. The profiles of mobility parameter decrease with an increase of Chol content and start to increase again after the formation of CBDs demonstrating that CBDs have higher dynamics in comparison to PCDs. The hydrophobicity in the interior of the membrane does not change with an increase of cholesterol content whereas near the membrane surface, hydrophobicity decreases with an increase of Chol content. This indicates the increase of water accessibility near polar headgroup region of phospholipids.