Self-assembly of Lipid A-phosphates: Supramolecular Liquid Crystals - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 276

Before the lipid A-diphosphate specimens were dried for TEM or SEM inspection,
the initial concentration of the dispersions was C = 0.5C*. From C = 0.5 C* to C = 0.85
C* an isotropic phase appeared as rod-like clusters, some of which form individual
particles, while others were aligned side by side and/or head to tail. With increasing
particle density more clusters appear, grew, and finally formed Sm structures. Below
an ionic strength of I = 5 mM NaOH at pH 8.5 the nematic region increases at the
expense of the I -phase. With an increase in the particle-number density it is possible
to observe the Sm -phase and the formation of the N- phase, which is induced with a
dilution of the Sm phase. The N -phase forms in a lipid A-diphosphate suspension with
an ionic strength of I = 1.5 × 10 -4 M NaCl, but at pH 5.6, spherical particles of diam-
eters of 90-100 nm are noticed, and at higher concentrations (C @ 11.0 C*) colloidal
crystals are observed, which have a cubic FCC structure with a = 57.3 nm. Individual
elongated lipid A-diphosphate particles may be seen, oriented in a direction approxi-
mately normal to the arrow. When the particle-number density is increased, lateral
cluster growth takes place and the contours becoming clearer and cluster layering
becomes more apparent. These simulations predicted a critical minimal L/d ratio of
4.5-5.0 for the stable I -phase where the I-N -triple point was also located. The Sm layer
period and in-layer particle separation were found to be 2.0 and 2.8 nm, respectively
(C = 5C*). The packing-particle fraction h for this Sm phase was estimated to be 0.38,
smaller than the 0.45 predicted if electrostatic interactions are taken into account. This
can only be explained (adding NaOH) as being due to an increase in the number of
free counterion upon charging the particles for example self-screening implying like
charge attraction and many-body effects which might be attributed to macroion shield-
ing but not in enhancing aggregation.
Moreover, high-resolution electron microscopy and electron diffraction on lipid
A-diphosphate rods (length of the order of µm and diameter of several nanometers) re-
vealed that the rods were held to the truncated polyhedral with a 5-fold symmetry (see
also Figure 7 and legend). It was possible to show that most of the lipid A-diphosphate
particles were orientated in the [001] direction with respect to the substrate for one of
the five deformed tetrahedral subunits that is the 5-fold axis was parallel to the surface
of the substrate.
CuBiC mimiCry oF the selF-assemBly aNd deNse PaCKiNG oF liPid
a-PhosPhate
Both 2- and 3-D assemblies of lipid A-mono or diphosphate constructed from single
or multiple chemical entities of known chemical composition (“coded subunits”) were
quite complex. These assemblies were very unlikely to be loose or unordered combi-
nations of lipid A molecules or of their analogs. The “coded subunit” addresses the
chemical appearances of single or identical chemical structures of lipid A-phosphate.
However, they were non-identical and differed in chemical structure for example num-
ber of chiral fatty acid chains, length of the hydrocarbon chain (number of carbons),
inserted double bonds in the hydrocarbon chain, and hexose compositions (Christ et
al., 2003). The assemblies were characterized by being comprised of two phosphates
residues: one at the reducing end and the other one at the non-reducing end for the