Computer simulation of lipid diffusion in a two-component bilayer. The effect of adsorbing macromolecules

Abstract

We have modelled the effects of macromolecular adsorption upon lipid lateral diffusion in a two-component lipid bilayer or monolayer, which is at a temperature above both of the main transition temperatures. One set of lipids (binders, b) can bind to the macromolecules with a free energy of binding, F B, while the other set does not bind (non-binders, nb). We assumed that no phase separation of the lipids occurs in the absence of adsorbed macromolecules. We represented the lipid bilayer/monolayer by a triangular lattice, each site of which is occupied by a lipid molecule. Adsorbed macromolecules were represented by hexagons covering n H sites, and we defined a probability per unit of time, p, that a hexagon attempts to adsorb onto the lattice. We considered two sizes of hexagons, n H  7(Size-1) and n H  19 (Size-2) and disallowed or permitted adsorbed hexagons to move laterally on the lattice. We calculate the lipid relative diffusion coefficients, D nb, and D b, for three characteristic time-regimes, (i) Γ c ⪡ Γ a, (ii) Γ c ≈ Γ a and (iii) Γ c ⪢ Γ a, where Γ c and Γ a are the times for proteins to adsorb/desorb or for lipids to move from site to site, respectively. We obtain analytical expressions for D nb and D b in the first case and calculate them using computer simulation in the other two cases. We found that (i) D α (iii) ≤ D α (ii) ≤ D α (i) ( α  nb, b); (ii) D α could display a shoulder as a function of F B for low values of p; (iii) compared to cases in which lateral diffusion was disallowed, the lateral diffusion of absorbed hexagons appeared to have little effect on D nb, but could cause D b to increase by 50%. (iv) Scatter in the calculated values of D via simulation appeared to be largest for Size-1 hexagons, and could be understood as a consequence of the large interfacial region between areas free of hexagons and areas ‘covered’ by hexagons. Our results suggest that it is advisable to measure D b since D nb might show little change from 1.0 for the values of F and p appropriate to the system being studied.