Simulation of the spontaneous aggregation of phospholipids into bilayers.

Abstract

The self-aggregation of lipid molecules to form bilayer membranes is a process fundamental to the organization of life. Although qualitatively explained by the hydrophobic effect, 1 the molecular aggregation itself is a complex phenomenon that has not been possible to study in detail experimentally. Here, we report a series of molecular dynamics computer simulations that for the first time demonstrate the possibility to observe the entire process at atomic detail with realistic lipids. Starting from random solutions, bilayers are formed on time scales of 10-100 ns, with properties matching experimental data. Several key steps and approximate time scales of the aggregation can be identified. The final rate-limiting process is the reduction and disappearance of large hydrophilic transmembrane water pores, of biological relevance for, for example, ion permeation. Singer and Nicholson were the first to recognize the implications of the extreme flexibility of membranes for the structure of cellular walls, leading to the famous fluid-mosaic model 2 with diffusing lipids and proteins. The bilayer formation process is, however, extremely fast and involves subtle rearrangements at the molecular level, making it elusive to current experimental methods. Simplified computer models have been used to mimic aggregation of surfactant-like molecules into monolayers and micelles, 3 bilayerlike structures, 4,5 and even vesicles. 6 These models are theoretically important to extend length and time scales, but they do not include atomic detail like hydrogen bonds and represent the collective entropic effects driving aggregation 1 as pairwise interactions. Detailed molecular dynamics simulations have on the other hand, provided accurate models of up to nanometer and nanosecond scales, but previously only for preassembled bilayers. 7-11 This work demonstrates the first simulations of aggregation of lipids into bilayers with atomic detail of the structure and interactions. Compared to micelle aggregation studies, 12,13 bilayer formation is considerably more challenging due to the balance between hydrophobicity and solvation, and the aggregation involves collective mesoscopic dynamics. The phospholipid dipalmitoylphosphatidylcholine (DPPC) was initially chosen for the study, since it is present in biological