Structural study of the inverted cubic phases of di-dodecyl alkyl-β-D-glucopyranosyl-rac-glycerol

Abstract

We present a quantitative study of the bicontinuous cubic phases in the didodecyl alkyl-β-D-glucopyranosyl-rac-glycerol (rac-di-12:0 β-GlcDAG) lipid-water system. A temperature-composition phase diagram determined using X-ray diffraction shows Ia3d and Pn3/Pn3m cubic phase regions in addition to inverted hexagonal (H^(II)) and several lamellar phase regions. The diffraction data was used to determine the lattice repeat vector, d, and, using suitable models, the lipid monolayer thickness, d_L, for all temperatures and compositions in the diagram. The models chosen for the cubic phases were based on lipid bilayers straddling infinitely periodic minimal surfaces (IPMS) as described previously in the literature [1]. Using the structural data derived from the phase diagram, the lyotropic phase transition between the two cubics was modeled by a simple curvature free energy theory. A new result of this theory was an estimate for the Gaussian curvature bending modulus of the lipid monolayer. The model was found to quantitatively describe the phase transition, particularly for phase behavior at low hydration. Our model suggests some universal features should be present in any system that shows non-lamellar phases and we discuss those features with respect to the phase diagrams available in the literature.