Bending moduli of mixtures: Diffusional softening and interactions

The apparent bending moduli (K_C) of bilayers composed of binary mixtures of lipids with different spontaneous curvatures have been obtained using x-ray diffuse scattering (XDS). The mixtures that were studied are POPC/POPE, POPC/POPA, POPC/POPS, and DLPC/DiPhyPC. The data are qualitatively consistent with what is expected from the theory of diffusional softening for lipids with different spontaneous curvatures. However, the derived spontaneous curvature differences are larger than those obtained from the hexagonal_II (H_II) phase and from a recent giant unilamellar vesicle (GUV) study. We propose that the interactions between lipids, which we have added to the theory, also play an important role in the values of K_C obtained at the short length scale of XDS. Inclusion of a mean field term in the analysis brings the calculated difference in spontaneous curvatures ΔC₀ of the two lipids closer to the values from the H_II and GUV methods. The use of XDS opens a new experimental window on diffusional softening and the interactions between lipids in mixtures.