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Synopsis

Understanding how molecules move and interact in soft materials is crucial for biology, yet these dynamic processes are often elusive. Muon spin resonance (μSR) offers a unique window into this problem: unlike neutron scattering or NMR, μSR probes the intermediate timescales(∼10⁵−10⁹ Hz) where many biologically relevant motions occur. In this talk, I will show how μSR can unravel the nanoscale dynamics of lipid membranes and their interactions with small molecules. By forming spin labels through Mu addition to unsaturated bonds in cholesterol, POPC, and DOPC, we can directly probe the local motion of cholesterol and phospholipid acyl chains. Our results reveal that cholesterol markedly stiffens lipid bilayers—reducing the amplitude of acyl chain reorientations and raising torsional barriers—while replacing POPC with DOPC produces much subtler effects. Going beyond model membranes, I also use μSR to investigate how drug-like molecules interact with membranes—a notoriously difficult problem due to their low concentrations. Remarkably, μSR detects both the location and the mobility of cosurfactants within bilayers. For example, ortho- and para-benzoate anions localize at the oil–water interface, whereas neutral molecules such as 2-phenylethanol penetrate deeply into the bilayer interior. Together, these studies demonstrate the power of μSR to bridge a critical gap in our understanding of soft-matter dynamics, from fundamental membrane physics to drug–membrane interactions.