Self-assembly of Lipid A-phosphates: Supramolecular Liquid Crystals - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 280

Figure 7. (Caption Continued)
subunits ( E. coli autovaccines ) arranged in icosahedral cubic symmetry. A layer packing can be
arranged which corresponds to a Penrose local isomorphism class, where a decomposition may
include addition of rhombohedra above and below layer packing, (E) Model packing of lipid
A-diphosphates for non-identical lipid A-phosphates in a random order for a cubic icosahedron
(yellow, blue, red, and green), in (F) a similar packing is depicted for a BCC cubic lattice (as in A),
but in an ordered array of the non-identical lipid A-phosphate subunits, (G) Rhombohedral packing
( a = 1.55 nm, α = 67 ° , space group or R32) of identical lipid A-phosphate molecules (Figure 1(A)).
The inset shows the shape of a cubic crystal ( a = 36.5 nm, red), in which a rhombohedral unit cell
has been fitted (blue), and (H) A 3-D packing of the lipid A-diphosphate (Figure 1(A)) is shown similar
as in Figure 6(B), derived form rhombohedral symmetry assuming that the lipid A-diphosphate anion
is completely orientationally ordered, and lowering the symmetry from cubic to rhombohedral and
finally to monoclinic (P2 1 or C2). A rope model of lipid A diphosphate (Figure 1(A)) is shown, based
on an image of lipid A-diphosphate aligned along the 2-fold axis screw axis, the b-axis of the unit cell
with 2a = 3.78 nm, 4b = 7.11 nm, c = 3.94 nm, and b = 62.5 ° . Atoms are presented as stick models:
Phosphorus violet, oxygens red, carbons black, and hydrogens white.
The polyhedral shapes of the lipid A-phosphates, E.coli autovaccines and stoichio-
metric mixtures of lipid A-diphosphate and antagonistic lipid A-diphosphates (Figure
1) in general, may be attributed to the deformable nature of the fluid clusters. The clus-
ters were composed of a small high electron density core (460 e/nm 3 ) approximately
1.95 nm in diameter. These were surrounded by a large, flexible, aliphatic fatty-acid
chain shell 2.70 nm in diameter. The polyhedra deformations conform to the different
symmetry sites in Pm3n , or icosahedrons and truncated octahedron structures. This
leads to a space-filling arrangement consisting of dodecahedra and tetrakaidodecahe-
dra networks ( Figure 5) . For clusters that exhibit flattened surfaces, the film curvature
will be distributed along the cell edges and at all of the vertices.
CoNClusioN
It was possible to construct different Wigner-Seitz polyhedra that make up the over-
all volume of the Frank-Kasper type unit cells with complexes comprised of lipid
A-diphosphate, antagonistic and non-toxic lipid A-phosphate analogs depending on
volume fraction and nature of the counterions (Faunce et al., 2005). They form by
spontaneous self-assembly and appear to obey the principles of thermodynamically
reversible self-assembly but once self assembled strongly resists disassembly. Base
on the principles outlined in this contribution, lipid A-phosphate assemblies can be
designed which form large unit cells by containing more than hundred lipid A-phos-
phates. The range of lipid A-phosphate structures may also be increased further by
employing various different (“non-identical subunits”) and identical subunits of lipid
A-phosphate in analogy with block copolymers.
The rational design of such assemblies including those of biocompatible quantum
dots for biological imaging, nucleation of polymorphic inorganic minerals, production
of suitable aerosols for immunization, and structure-function relationships will be im-
pacted by a theoretical and practical understanding of these spherical assemblies, rod-
like assemblies and the mixtures thereof. Given the theoretical and practical impor-
tance of this system, we expect that the attention given to it will substantially increase
our knowledge on LPS , innate immunity, mineral nucleation, the driving forces for the