Molecular Insight into the Line Tension of Bilayer Membranes Containing Hybrid Polyunsaturated Lipids

Line tension (y) is a key parameter for the structure and dynamics of membrane domains. It was proposed that hybrid lipids, with mixed saturated and unsaturated acyl chains, participate in the relaxation of y through different mechanisms. In this work, we used molecular dynamics simulations of the coarse-grained MARTINI model to measure y in liquid-ordered liquid-disordered (Lo Ld) membranes, with increasingly larger relative proportion of the hybrid polyunsaturated lipid PAPC (4:0-5:4PC) to DAPC (di5:4PC) (i.e., XH). We also calculated an elastic contribution to y by the Lo Ld thickness mismatch, tilt moduli, and bending moduli, as predicted by theory. We found that an increase in XH decreased the overall y value and the elastic contribution to line tension. The effect on the elastic line tension is driven by a reduced hydrophobic mismatch. Changes in the elastic constants of the phases due to an increase in XH produced a slightly larger elastic y term. In addition to this elastic energy, other major contributions to y are found in these model membranes. Increasing XH decreases both elastic and nonelastic contributions to y. Finally, PAPC also behaves as a linactant, relaxing y through an interfacial effect, as predicted by theoretical results. This study gives insight into the actual contribution of distinct energy terms to y in bilayers containing polyunsaturated hybrid lipids.