A comprehensive understanding of lipid structure and dynamics is essential to understanding biomembrane functions. Based on the flexible surface model (FSM), we highlight the significance of proteolipid interactions and their out-of-plane couplings, which influence membrane functionality through curvature elastic forces. These forces, underscored by spontaneous curvature and bending rigidity, are paramount to distinguishing the FSM from the earlier fluid-mosaic model. Investigating membrane mimics consisting of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) may contribute to addressing this question. Here we used solid-state deuterium NMR spectroscopy to investigate the structural dynamics of unsaturated phospholipid mixtures with varying PC to PE headgroups. Using POPC-d₃₁ as a probe lipid, we analyzed the structural and mechanical properties of 1,2-dioleoy-sn-glycero-3-phosphoethanolamine (DOPE), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and their binary mixtures. Measurements were performed on liquid-crystalline state multilamellar dispersions as a function of temperature. The order parameter (S_CD) profiles indicated that the quadrupolar splitting remains constant across a probe lipid concentration of 1-15 mol% for both PE and PC lipids, while the mol% of POPC-d₃₁ influences the transition from lamellar to hexagonal phase in DOPE lipids. In the liquid-crystalline phase, DOPE lipids exhibited a constrained acyl chain mobility, leading to a more pronounced molecular order than DOPC. Increasing DOPE concentration reduced the area per molecule at the lipid/water interface, affecting acyl chain configurational freedom and increasing order. Additionally, DOPE membranes showed higher bending rigidity than DOPC, highlighting it is driven primarily by increased lipid packing, correlating with the area compressibility modulus. The use of a probe lipid elucidates the intricate properties of complex lipid systems, facilitating a deeper understanding of how membrane mechanics are intertwined with critical biological functions.

Constantin Job

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