Characterization of lipid chain order and dynamics in asymmetric membranes by solid-state NMR spectroscopy

Abstract

We studied the structure and dynamics of asymmetric POPC^out/(POPE/POPG)ⁱⁿ and POPS^out/(POPE/POPG)ⁱⁿ lipid membranes. To this end, the outer layer of multilamellar POPE/POPG (molar ratio 9 : 1) vesicles was exchanged (using methyl-β-cyclodextrin) by either chain deuterated POPC-d₃₁ or POPS-d₃₁, for which ²H NMR order parameters were measured. As controls, we prepared symmetric POPC-d₃₁/POPE/POPG and POPS-d₃₁/POPE/POPG membranes of the composition of just the outer membrane of the asymmetric multilamellar vesicles and pure POPC-d₃₁ or POPS-d₃₁ multilamellar vesicles. Compared to symmetric membranes of the same lipid composition, chain order parameters (S) of the asymmetric preparations were higher in the upper half of the chain and lower in the lower half. This reshuffling of acyl chain order is also expressed in higher ²H NMR Zeeman order relaxation rates (R_1Z) of the chain segments in asymmetric membranes indicating alterations in the elastic properties of asymmetric bilayers as inferred from plots of R_1Z vs. S². Asymmetric membranes showed increased stiffness and rigidity although the lipid acyl chain composition between the inner and outer leaflets were identical. There were no indications for chain interdigitation between the two leaflets in the NMR spectra, which led us to speculate that the interleaflet coupling could be accomplished by sensing the differences in lipid packing densities between the two leaflets. These alterations in leaflet properties should have consequences for lipid protein interaction and ultimately protein function.