Membrane shape deformations are essential for many cellular processes. Their underlying energetics is largely determined by curvature elasticity, which is quantified by a bending modulus. However, biological membranes contain diverse lipid species with distinct interactions and properties, and so the effective bending rigidity is a generally complicated function of the composition. For binary mixtures, a first expectation is that the rigidity of the mixture is the molar average of the pure phase rigidities, but because local composition couples to curvature, the correlated fluctuations of these two fields give rise to diffusional softening , where the bending rigidity of a mixture is lower than this linear interpolation. Using coarse-grained simulations, we examine binary lipid mixtures across varying molar ratios and quantify their shape fluctuations, from which—via the usual power spectrum analysis—we extract an effective bending rigidity. Our theoretical framework predicts how the correlated fluctuations between local spontaneous curvature differences and local shape amplify the shape undulations, which results in softening, and how non-ideal mixing amplifies this effect. Simulation results qualitatively support these predictions: trends align with theory but the observed degree of softening is smaller. We attribute this discrepancy to hidden local ordering, which limits the validity of mean-field assumptions at the lipid length scale.
Winstel et al. (Sun,) studied this question.