Plasma membranes (PMs) are composed of hundreds of different lipids varying in structure and chemical properties. A major challenge in membrane biophysics is to determine how different classes of lipids contribute to the physical properties of the PM. One particularly interesting type of lipid is characterized by a different number of carbons in its two hydrocarbon chains. These chain asymmetric lipids are thought to have a higher propensity for interdigitation with lipids in the opposing leaflet and thus, to have a unique effect on interleaflet coupling. We have used molecular dynamics (MD) simulations to systematically examine fluid-phase bilayers composed of fully saturated PC lipids, each with a total of 32 acyl chain carbons, but increasing asymmetry ranging from zero to four carbons difference between the two chains. We quantified interdigitation in these bilayers using three novel methods and compared the results to standard approaches; we found general agreement in the trends, namely that increasing chain asymmetry results in increased interdigitation. We hypothesized that a greater extent of interdigitation should result in stiffer bilayers and thus, higher values of the bending modulus. To test this hypothesis, we measured various structural and mechanical properties of the simulated bilayers including thickness, lipid packing, and bending and compressibility moduli. Surprisingly, we found that the bilayers with the greatest extent of interdigitation are also the softest at a common temperature of 50°C. These results provide new insights into the structural and mechanical effects of lipid chain asymmetry and open up further questions about the nature of the coupling between lipid chain interdigitation and membrane biophysical properties.

Effects of interdigitation on the biophysical properties of lipid bilayers: A molecular dynamics study of chain-asymmetric lipids

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