Depth-Resolved Oxygen Accessibility and Chain Dynamics in Monoolein/Phospholipid Cubic-like Nanoparticles Probed by Spin-Label EPR

Abstract

Monoolein (MO)-based bicontinuous cubic phases are widely used as soft nanomaterials, yet the relationships among curvature, chain dynamics, and oxygen transport remain poorly quantified. Here, we use continuous-wave spin-label EPR and power-saturation measurements to map local mobility and oxygen accessibility in POPC, DOPC, and DOPE large unilamellar vesicles (LUVs) and in analogous polymer-free MO/phospholipid cubic-like nanoparticles (85 wt % water, 17:3 MO/phospholipid) doped with TEMPO-PC, 5-doxyl-PC, or 10-doxyl-PC (1 mol %). In vesicles, the oxygen-accessibility parameter,ΔP1/2(oxy), increases from TEMPO-PC to 5- to 10-doxyl, with headgroup-dependent magnitudes, consistent with classical depth-dependent oxygen profiles. In MO/POPC and MO/DOPE, curvature substantially increases oxygen accessibility at interfacial and midchain positions while maintaining an oxygen-rich core, whereas order parameters and TEMPO line widths change only modestly. In MO/DOPC, mid- and deep-chain labels perturb the cubic-like mesophase and retain more vesicle-like oxygen-accessibility profiles, revealing that spin-labels can, in some compositions, perturb the MO/phospholipid mesophase and therefore must be treated as active mesogens rather than strictly “innocent” reporters. Across all systems, we relate ΔP1/2(oxy)to curvature-controlled free volume and depth-resolved oxygen landscapes, establishing a direct connection between mesoscopic morphology and local transport properties. In this way, our measurements extend spin-label oximetry concepts developed for planar lipid bilayers to highly curved MO-rich nanostructures, where depth-dependent oxygen profiles are known to influence membrane chemical reactivity. These results demonstrate that membrane morphology can outweigh headgroup identity in setting depth-resolved oxygen landscapes in MO-rich nanostructured membranes and provide physicochemical design rules for tuning oxygen transport in polymer-free cubic-like nanoparticles.