The Influence of Periodic Size Effects and Membrane Undulation on Phase Separation in a DPPC/DOPC/Chol Coarse Grain Martini System

Ternary mixtures containing a high melting temperature lipid (such as di-palmitoyl-phosphatidylcholine, DPPC), a low melting temperature lipid (such as di-oleoyl-phosphatidylcholine, DOPC), and cholesterol (CHOL) form bilayers consisting of up to three different lipid phases. The potential lipid phases that can form are liquid-disordered (Ld), liquid-ordered (Lo), a gel-like phase (Lb), or any combination of the three. The phase(s) present within these membranes are dependent on the specific percentage composition of the three components within the membrane. The mixture-dependent phases formed by these specific constituents have been well mapped experimentally to construct detailed phase diagrams. In our recent work, we presented a modified version of the CG Martini DPPC and DOPC lipid parameters. These parameters not only reproduced the characteristics of individual lipid species (such as area per lipid), but also reproduced the majority of the correct experimental phase separation. Our previous studies to iteratively improve and evaluate the lipid parameters was carried out using standard system sizes of 3,000 lipids that were restrained to maintain a planar lipid bilayer.Here we further expand on this work to investigate how the size of the system, as well as membrane undulations influence the phase separating behavior of the various lipid mixtures. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Release numbers: LLNL-ABS-759357