Coarse-grained model for PEGylated lipids: effect of PEGylation on the size and shape of self-assembled structures.

Self-assembly of polyethylene glycol (PEG)-grafted lipids at different sizes and concentrations was simulated using the MARTINI coarse-grained (CG) force field. The interactions between CG PEG and CG dipalmitoylglycerophosphocholine (DPPC)-lipids were parametrized by matching densities of 19-mers of PEG and polyethylene oxide (PEO) grafted to the bilayer from all-atom simulations. Mixtures of lipids and PEG-grafted (Mw = 550, 1250, and 2000) lipids in water self-assembled to liposomes, bicelles, and micelles at different ratios of lipids and PEGylated lipids. Average aggregate sizes decrease with increasing PEGylated-lipid concentration, in qualitative agreement with experiment. PEGylated lipids concentrate at the rims of bicelles, rather than at the planar surfaces; this also agrees with experiment, though the degree of segregation is less than that assumed in previous modeling of the experimental data. Charged lipids without PEG evenly distribute at the rim and planar surfaces of the bicelle. The averag...