Nanoscale dynamics in model phospholipid biomembranes probed by muon spin resonance spectroscopy: The effects of membrane composition and temperature on acyl chain and cholesterol motion

The physical properties of lipid bilayers are known to depend on their composition, but there has recently been controversy about whether cholesterol (chol) does or does not stiffen biomembranes containing unsaturated phospholipids. Herein, avoided level crossing muon spin resonance (ALC-$\mu$SR) spectroscopy has been used to probe the local dynamics in model biomembranes composed of the saturated phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the unsaturated phospholipids 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and the sterol chol. The presence of chol significantly stiffens the acyl chains in lipid mixtures as evident from the reduction of the amplitude of restricted reorientational motion in the acyl chain at the C${}_9$-C₁₀ position and the increase of the torsional barrier for rotation about the bonds in the acyl chain. Swapping POPC for DOPC slightly increases the amplitude of restricted reorientational motion and decreases the torsional barrier of the acyl chains, but the magnitude of the effect is much smaller than the inclusion of chol.