Abstract
Self-assembled structures having a regular hollow icosahedral form (such as those observed for proteins of virus capsids) can occur as a result of biomineralization processes1, but are extremely rare in mineral crystallites2. Compact icosahedra made from a boron oxide have been reported3, but equivalent structures made of synthetic organic components such as surfactants have not hitherto been observed. It is, however, well known that lipids, as well as mixtures of anionic and cationic single chain surfactants, can readily form bilayers4,5 that can adopt a variety of distinct geometric forms: they can fold into soft vesicles or random bilayers (the so-called sponge phase) or form ordered stacks of flat or undulating membranes6. Here we show that in salt-free mixtures of anionic and cationic surfactants, such bilayers can self-assemble into hollow aggregates with a regular icosahedral shape. These aggregates are stabilized by the presence of pores located at the vertices of the icosahedra. The resulting structures have a size of about one micrometre and mass of about 1010 daltons, making them larger than any known icosahedral protein assembly7 or virus capsid8. We expect the combination of wall rigidity and holes at vertices of these icosahedral aggregates to be of practical value for controlled drug or DNA release.
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Acknowledgements
We thank G. Sukhorukov and O. Tiourina for help in confocal optical microscopy, and I. Erk for cryo-TEM. We also thank J.-L. Sikorav, P. Timmins and B. W. Ninham for critical reading of the manuscript and suggestions.
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Fig. S1 (GIF 14 KB)
Differential scanning calorimetry (DSC) of a catanionic mixture taken at a heating rate of 2°C/min at initial mole ratio r= 0.6. The chain melting transition is located at Tf= 50°C.
Fig. S2 (GIF 12 KB)
Schematic phase diagram showing the formation path of icosahedra (partial diagram for Φ 0.10). Above the chain melting temperature, stable unilamellar vesicles are formed for volume fractions in the range 0.1% to 1%. For the same composition, this sample is in a biphasic domain at room temperature. Tie lines between a lamellar phase with cristallised bilayers (Lβ) and nearly pure water (W) are shown. The "water" phase contains of the order of 1mmole of charged surfactant and the excess counter-ions, as shown by conductivity measurements. Slow cooling towards room temperature induces a liquid-liquid phase separation between a Lβ and a large volume of optically isotropic flowing solution which contains dispersed icosahedra (ICO). The surfactant volume fraction is of the order of Φ = 10-3. At rest and room temperature, icosahedra slowly flocculate after a few months following the tie-lines. These equilibrium lines connect the water corner to the Lβ phase at maximum swelling.
Fig.S3 (GIF 15 KB)
Wide angle scattering shows the in-plane distribution of deuterated chains in the Lβ phase in equilibrium with icosahedra (volume fraction 10%). (H/D) designs mixed deuterated anionic and protonated cationic surfactant at a molar ratio r = 0.65. (H/H) corresponds to a sample of same composition, but containing only hydrogenated chains. The main sharp peak corresponds to the frozen hydrocarbon chains in Lβ configuration (qf = 1.52 Å-1), whereas a broad superstructure peak appears around qf= 1.52/√3, showing the local hexagonal alternated packing. The superstructure peak is broad, showing defects in the triangular tiling, consistently with the quasi-equivalence principle.
Fig. S4 (JPG 4 KB)
Optical confocal microscopy images taken with an oil immersion objective (Leica TCS SP, excitation wavelength 525 nm), using a cationic dye (Rhodamine 6G). Left is the transmission micrograph. Two other images are confocal cuts through the same object obtained by displacing the focal plane by steps of 0.5 mm. Vertices are preferentially coloured by the positively charged dye molecule. This is an indication that excess anionic component is more pronounced around the vertices of the icosahedra. Central vertix can be seen on the top of the object, five-fold symmetry on the second cut. Distances between vertices are close to microscope resolution.
Fig. S5 (JPG 16 KB)
Electron micrographs of microstructures obtained not enough (A) or when too much (B) excess anionic surfactant is present to form icosahedra. (A) Fragments coexisting with closed objects are observed when r=0.52. Near equimolarity, there is not enough myristic acid to form the required minimal number of pores per vesicle. Some aggregates than fragment into nanodiscs upon cooling. (B) Large planar structures decorated with holes do not fold into closed objects when r=0.65. Micrographs show the coexistence of flat fragments with some facetted vesicles (bar is 1µm)
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Dubois, M., Demé, B., Gulik-Krzywicki, T. et al. Self-assembly of regular hollow icosahedra in salt-free catanionic solutions. Nature 411, 672–675 (2001). https://doi.org/10.1038/35079541
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DOI: https://doi.org/10.1038/35079541
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