Self-assembly of Lipid A-phosphates: Supramolecular Liquid Crystals - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 275

and Wette at al. (2010) for charged silica particles supported the idea that a counterion
condensation took place at the particle surface, over large surface charge densities
with an increase in pH to 8.5 or 9.0. Changes in the NaOH concentrations from µM
to mM raised the pH but did not necessarily raise the particle surface charge density.
However, this concentration adjustment resulted in a change form a spherical to a
rod-like assembly (Faunce and Paradies, 2008). Two current models explain the con-
centration-dependent findings at pH 8.0. At C @ C* the orientational entropy and the
electrostatic repulsion terms of the free-energy expression for a dispersion of charged
rods favor anti-parallel alignment. The anti-parallel alignment of the rods, gives rise
to a cubic lattice-like interparticle structure. However, for C >> C*, a hexagonal pack-
ing of parallel rods was anticipated to be an appropriate model, Where C* denotes
the critical concentration of lipid A-diphosphate, C* = particle/l 3 , which amounts to
1.5 × 10 12 particles per mL. Both models would fulfill the conditions at C >> C* be-
cause the separation between rod-like particles must be considerably less than the rod
length, the rods may be considered to be infinitely long. The rods were charged and
consequently the surfaces must be as far apart as possible. For both models, Q max L =
2pg (C/C*) 1/2 scales with g = 0.58 for a cubic structure, and for a hexagonal structure
with g = 0.93. The experimental values obtained of 0.57 for C @ C* and 0.90 for C
>> C* were close to the theoretical values. If the correlation of nearest-neighbor rods
of lipid A-diphosphate were considered, then parallel alignment of nearest-neighbor
rods occurred, with a local ordering parameter S = 0.071 at C = 2.7 C*. Accordingly,
monodisperse rod-like particles with a standard deviation of < 5.5% and an L/d range
from 20-25 were seen on the TEM and SEM images (Figure 6).
Figure 6. The TEM images of various lipid A-diphosphate assemblies at pH 8.0 in the presence of 100
µ M NaOH. (A) Rod-like lipid A-diphosphate assemblies comprising of lipid A-diphosphate ( Figure
1(A)) and nontoxic but antagonistic lipid A-diphosphate (Figure 1(C)). Scale bar is 10 nm, (B) A 3-D
assembly of an antagonistic lipid A-diphosphate analog (Figure 1(B)) modeled from TEM images and
arranged in a hexagonal packing (R3), (C) TEM image of a 3-D assembly with two antagonistic lipid
A-phosphates (Figure. 1(B) and 1(C)) and endotoxic lipid A-diphosphate. Scale bar is 5 nm, and (D)
Single lipid A-diphosphate (Figure 1(A)) particles forming a highly ordered chain-like structure at pH
8.5 in the N -phase. The scale bar is 100 nm. (D)