Anomalous freezing behavior of nanoscale liposomes

Abstract

The effect of the finite size of one-component liposomes on their phase behavior is investigated via simulations of an implicit-solvent model of self-assembled lipid bilayers. We found that the high curvature of nanoscale liposomes has a significant effect on their freezing behavior. While the location of the freezing temperature of liposomes with diameters larger than 30 nm is the same as that of a planar membrane, the transition is broadened as the liposome diameters are decreased. For very small liposomes, with diameters smaller than 30 nm, the transitions of the two leaflets become increasingly decoupled, and their respective freezing temperatures become different. In particular, the outer leaflet gels at a temperature, T^(u)_g, which is higher than that of the inner leaflet, T⁽ⁱ⁾_g. We argue that this decoupling is due to the difference in configurational entropies of the lipid chains in the two leaflets. At temperatures higher than T^(u)_g, the liposomes are spherical and the lipids in both leaflets lack both positional and chain order. At temperatures between T⁽ⁱ⁾_g and T^(u)_g, the lipids in the outer leaflet exhibit chain order but lack positional order. In contrast the lipids in the inner leaflet lack both chain order and positional order in this temperature range. In this range of temperatures, liposomes are essentially spherical. At temperatures lower than T⁽ⁱ⁾_g, lipids in both the inner and outer leaflets exhibit chain order as well as positional order, and the liposomes become faceted. The structure of the outer leaflet for T⁽ⁱ⁾_g < T < T^(u)_g is surprisingly akin to that of the liquid ordered phase, which is observed in binary mixtures of saturated phospholipids and cholesterol.