Interdigitation-Induced Order and Disorder in Asymmetric Membranes

Abstract

We studied the transleaflet coupling of compositionally asymmetric liposomes in the fluid phase. The vesicles were produced by cyclodextrin-mediated lipid exchange and contained dipalmitoyl phosphatidylcholine (DPPC) in the inner leaflet and different mixed-chain phosphatidylcholines (PCs) as well as milk sphingomyelin (MSM) in the outer leaflet. In order to jointly analyze the obtained small-angle neutron and X-ray scattering data, we adapted existing models of trans-bilayer structures to measure the overlap of the hydrocarbon chain termini by exploiting the contrast of the terminal methyl ends in X-ray scattering. In all studied systems, the bilayer-asymmetry has large effects on the lipid packing density. Fully saturated mixed-chain PCs interdigitate into the DPPC-containing leaflet and evoke disorder in one or both leaflets. The long saturated acyl chains of MSM penetrate even deeper into the opposing leaflet, which in turn has an ordering effect on the whole bilayer. These results are qualitatively understood in terms of a balance of entropic repulsion of fluctuating hydrocarbon chain termini and van der Waals forces, which is modulated by the interdigitation depth. Monounsaturated PCs in the outer leaflet also induce disorder in DPPC despite vestigial or even absent interdigitation. Instead, the transleaflet coupling appears to emerge here from a matching of the inner leaflet lipids to the larger lateral lipid area of the outer leaflet lipids. Graphical abstract

Distributing Aminophospholipids Asymmetrically Across Leaflets Causes Anomalous Membrane Stiffening

“…( 3), the neutral plane, ℎ and the hydrocarbon thickness 𝐷 𝐶 . Assuming that ℎ coincides with the position of the glycerol backbone (35), we arrive at ℎ/(2𝐷 𝐶 ) = 0.53 ± 0.07 using previously reported data for DPPC bilayers (12). This result is well within the range of previously reported values for ℎ/(2𝐷 𝐶 ) (34).…”

“…SANS and SAXS data were analyzed jointly in terms of Gaussian-type volume distribution functions of quasi-molecular lipid fragments across the bilayer, using our recently reported advanced scattering density model for aLUVs (12) (for details, see the SI). Compared to this previous study, we did not use lipids with deuterated hydrocarbons.…”

“…In the next step, we determined the corresponding membrane structural parameters using the joint analysis of SANS and SAXS data. The transbilayer structure was parsed into the volume distributions of phosphate (PCN), choline-CH 3 (CholCH3), glycerol or sphingosine backbone (BB), methylene/methine (chains), and methyl (CH3) groups, including also bound water in the headgroup region (12). The applied model is in principle capable of deriving leaflet-specific structural details (12).…”

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes

“…DMPC-ceramide mixtures produced smaller vesicles than when only DMPC was present [ 171 ]. It is also possible to investigate vesicles in which the inner leaflet of the membrane does not have the same composition as the outer one [ 172 , 173 , 174 , 175 , 176 ].…”

Energetic and Structural Insights into Phospholipid Transfer from Membranes with Different Curvatures by Time-Resolved Neutron Scattering

“…32 However, unlike C 6 NBD-PC, this fluorescent lipid is hardly incorporated into bilayers when simply added to vesicles. Alternatively, the method of creating asymmetric vesicles by lipid exchange 33,34 can be applied to the asymmetric labeling with fluorescent probes. Therefore, in the present study, DPH-PC was introduced through lipid exchange between the LUV and the SUV.…”