Lipid Packing in Lipid-Wrapped Nanoparticles

Equilibrium simulations of lipid-wrapped nanoparticles (LNP) were performed using a hybrid molecular dynamics/Monte Carlo (MD/MC) approach. The radius, R, of a spherical nanoparticle (NP) core was adjusted with MC moves while a surrounding lipid bilayer was treated with MD. A wide range of LNP sizes, with the largest R ∼ 40 nm, were studied to determine the average NP radius for a given total number of lipids, N, the number of lipids in each layer, and configurational information. A three-bead lipid model was used to allow large N. A nonequilibrium Jarzynski free energy calculation of the optimal R for a given N, was also demonstrated validating the MD/MC method. An order/disorder transition was described, unique to LNP and distinct from lamellar bilayers, that is weak and continuous with small N, but sharpens to a first order transition with N > 10000 at T ≈ 1.1, shifting to higher T with increasing N. The radius and the overlap of the inner and outer layers were used as order parameters charactering the whole system, and the density vs distance from the origin served to describe the transition in individual layers. The ordering effect of the core on the inner layer, and the disordering effect of curvature, are evident. Excellent fits for the number of lipids in the inner and outer layers vs R are presented, based on the idea that the inner layer is described as usual by an area and area/lipid but that the outer layer is slaved to the inner. The most ordered states exhibit interdigitation of the inner head groups with themselves.