Combined Coarse-Grained Molecular Dynamics and Neutron Reflectivity Characterization of Supported Lipid Membranes

Supported lipid bilayers on planar surfaces constitute an archetypical experimental system for the study of biological membranes. The popularity of these ordered molecular layers in the literature, is on one hand related to the simplicity of their preparation using the method of vesicle fusion and on the other hand to their compatibility with a multitude of surface sensitive experimental probes. Neutron reflectivity has proven as an important experimental method for the investigation of such systems with the ability to provide subnanometer structural information perpendicular to the supporting plane. Traditionally reflectivity data are compared to theoretical curves of simplified models consisting of stratified layers representing the hydrophilic (lipid heads) and hydrophobic (lipid tails) parts of the bilayer. In the present work we explore the combined use of molecular simulations and neutron reflectivity for the characterization of supported membranes. By performing coarse-grained molecular dynamics simulations based on the MARTINI force field of supported 1,2d-ihexadecanoyl-sn-glycero-3-phosphocholine (DPPC) bilayers close to a hydrophilic substrate, we compared the obtained reflectivity profiles with neutron reflectivity data for this system at a series of temperatures above and below the main phase transition. It is found that the use of an imperfectly smooth substrate in the coarse grained simulation is of vital importance for avoiding the artificial freezing of water that is trapped between the surface and the bilayer. The observed quantitative agreement between simulation and experiment using rough supporting surfaces, especially for the liquid lipid phase, exhibits that the presented methodology may serve as a basis for the detailed and assumption-free investigation of more elaborate systems.