Packing and elasticity in membrane mechanical properties.

Here we address how membrane properties involving head group size, acyl chain unsaturation, and lipid packing affect their structure and dynamics. Strong out-of-plane proteolipid couplings considered by the flexible surface model (FSM) affect protein function through elastic forces described by the spontaneous curvature and the bending rigidity. We hypothesized that the mechanical properties and the molecular packing resulting from different lipid types are captured by introducing a perdeuterated POPC-d31 probe lipid. Di-monounsaturated phosphatidylcholine and phosphatidylethanolamine (DOPC, DOPE), and different ratios of DOPC/DOPE mixtures were studied using MD simulations combined with experimental solid-state 2H NMR spectroscopy. Residual quadrupolar couplings obtained from de-Paked powder-type spectra yielded order parameter (SCD) profiles for the individual acyl segments giving insights into average membrane structural properties such as area per lipid and bilayer thickness [1]. The corresponding spin-lattice relaxation rates (R1Z) for the resolved peaks followed a theoretical square-law dependence on SCD providing the bending rigidity [2]. Furthermore, MD simulations for the lipid systems were carried out and order parameters were calculated and compared against the NMR-observed SCD values. Utilization of the probe lipid POPC-d31 (1−10 mol%)affected the lamellar to hexagonal phase transition temperature of DOPE lipid systems but did not affect the quadrupolar splittings of DOPE and DOPC systems. Higher bending rigidity was observed for DOPE membranes versus DOPC supporting the idea that the membrane bending rigidity is primarily governed by increased lipid packing and is related to the area-compressibility modulus. Use of a probe lipid to capture membrane mechanical properties of complex lipid systems allow investigation of the relationship of membrane mechanics in vital biological functions.