Probing Nanoscale Acyl Chain and Cholesterol Dynamics in Phospholipid Membranes Using Muon Spin Resonance Spectroscopy: The Effect of Temperature and Composition on Dynamic

Molecular-scale dynamics in biomembranes depend on its composition and impact the macroscopic properties and function. Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy was used to study 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) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and cholesterol (chol). The acyl chains at the C₉-C₁₀ position are labelled in situ by the reaction of muonium with unsaturated C=C bonds in POPC and DOPC, and chol is labelled by Mu addition to the C=C bond. The resulting muoniated radicals are sensitive probes of dynamics on the ns to μs timescales, bridging that accessible by NMR and neutron scattering. The presence of chol significantly reduces the amplitude of restricted reorientation motion in the acyl chain, increases the torsional barrier for rotation about the bonds in the acyl chain, and increases the activation energy for bending or thickness fluctuations. Swapping POPC for DOPC has very little effect on the amplitude of restricted reorientational motion, the torsional barrier of the acyl chains, or the activation energy.