Lipids with bulky head groups generate large membrane curvatures by small compositional asymmetries.

Abstract

Glycolipids such as GM1 have bulky head groups consisting of several monosaccharides. When these lipids are added to phospholipid bilayers, they generate large membrane curvatures even for small compositional asymmetries between the two leaflets of the bilayers. On the micrometer scale, these bilayer asymmetries lead to the spontaneous tubulation of giant vesicles as recently observed by optical microscopy. Here, we study these mixed membranes on the nanometer scale using coarse-grained molecular simulations. The membrane composition is defined by the mole fractions ϕ1 and ϕ2 of the large-head lipid in the two leaflets of the bilayer. Symmetric membranes are obtained for ϕ1 = ϕ2 ≡ ϕle, and asymmetric ones for ϕ1 ≠ ϕ2. In both cases, we compute the density and stress profiles across the membranes. The stress profiles are used to identify the tensionless states of the membranes. Symmetric and tensionless bilayers are found to be stable within the whole composition range 0 ≤ ϕle ≤ 1. For these symmetric bilayers, both the area compressibility modulus and the bending rigidity are found to vary non-monotonically with the leaflet mole fraction ϕle. For asymmetric bilayers, we compute the product of bending rigidity and spontaneous curvature from the first moment of the stress profile and determine the bending rigidities of the asymmetric membranes using the ϕle-dependent rigidities of the single leaflets. When we combine these results, the compositional asymmetry ϕ1 - ϕ2 is found to generate the spontaneous curvature (ϕ1 - ϕ2)/(0.63 ℓme) with the membrane thickness ℓme ≃ 4 nm. Therefore, the spontaneous curvature increases linearly with the compositional asymmetry. Furthermore, the small compositional asymmetry ϕ1 - ϕ2 = 0.04 leads to the large spontaneous curvature 1/(63 nm) and the increased asymmetry ϕ1 - ϕ2 = 0.2 generates the huge spontaneous curvature 1/(13 nm). These large values of the spontaneous curvature will facilitate future simulation studies of various membrane processes such as bud formation and nanoparticle engulfment.