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Comparing an all-atom and a coarse-grained description of lipid bilayers in terms of enthalpies and entropies: from MD simulations to 2D lattice models.

Simulations of lipid bilayers at different levels of details is of major interest in computational biophysics. With a recently proposed algorithm, by mapping the information from a Molecular Dynamics (MD) simulation of a lipid bilayer on a 2D lattice model, it is possible to gain insight into the enthalpic and entropic contributions, governing the interaction of adjacent lipids and the individual chain entropy, respectively. The contributions are obtained as a function of the lipid chain order parameter. Here we use this approach for the case of pure DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) to compare the all-atom CHARMM36 DPPC lipid with the respective MARTINI coarse-grained DPPC lipid. We individually compare the enthalpy and entropy functions as well as the resulting free energies. This allows us to gain new insight into the comparison of both levels of description. When numerically solving the 2D lattice model via Monte Carlo (MC) simulations, the 2D model displays the gel/liquid transition at the same temperature as the respective MD simulation. The dramatic increase in efficiency of the 2D lattice model as compared to the MD simulation allows us to estimate the equilibrium order parameters of the gel phase, inaccessible by the MD simulations even after 3 µs.