Biomedical Engineering Reference
In-Depth Information
Colloidal clusters from dispersions with low polydispersity in shape, size and
charge of lipid A-diphosphate and antagonists of lipid A-diphosphate were investi-
gated using small-angle X-ray scattering, light scattering (LS), and electron micros-
copy. These aqueous dispersion and colloidal crystalline phases were tailored with
different chain lengths (C
12-16
) with even or odd numbers of covalently attached chiral
(R-[-])-fatty-acid chains, or the number of chains (
Figure 1(A)
).
The clusters were
successfully prepared by self-assembly at very low ionic strength (I @ 10
-4
M, at 25°C)
and physically analyzed and studied for their clinical efficacy. The colloidal crystalline
phases of lipid A-diphosphate
were identified by the presence of resolution-limited
Bragg peaks that were indexed according to their structures which were as follows:
a BCC (
Im3m
) lattice (
a
= 37.5 ± 2.0 nm) at f = 3.5 × 10
-4
, an FCC (
Fd3m
)
lattice (
a
= 57.5 ± 2.5 nm) at f = 5.0 × 10
-4
with I = 5.0 mM NaCl and a second FCC (
Fm3m
)
The peaks observed, for lipid A-diphosphate dispersion
(f = 3.5 × 10
-4
), which
were assigned a BCC lattice (
h
2
+
k
2
+
l
2
: 2, 4, 6, 8 and 10, and
h
+
k
+
l
= 2n) with a =
37.6 nm. A possible space group was
Im3m
(
Figure 2(A))
.
The molecular weight cal-
culations for this phase of lipid A-diphosphate were made using the (110) diffraction
planes, d = d
110
=
a
/(2)
1/2
. For this BCC lattice type, the estimated molecular weight
was approximately 10.1 × 10
6
g/mol (formula monomer molecular weight @ 1,950 g/
mol). Under these experimental conditions the observed cubic phases were all non-
lamellar and none belonged to the space group
Pn3m
, with a spacing ratio of 1, 1:√2,
1:3; 1:√4, 1:6, and so on, or to the
Pm3n
structure with diffraction lines of the fol-
lowing ratios: √2, √4, √5, √6, √8 and √10, and so on. Both the
Im3m
and
Pm3n
struc-
tures were possibilities and the overall structure consisted of disconnected clusters of
lipid A-diphosphate separated by a continuous film of water. The body-centered cubic
phase was generally present at higher water contents and low ionic strengths. The
structure could be modeled by the packing of clusters of identical rigid spheres which
were embedded in water. The closest packing of the spheres took place in the array and
the radius of the closely packed rigid sphere was R
sp
= (√3/4×
a
) = 16.4 nm. In contrast
to the
Im3m
structure
,
the phase represented by the space group
Pm3n
was thought to
contain two types of clusters.
Interestingly, when employing colloidal dispersions prepared from lipid A-diphos-
phate and the antagonistic molecules from either 1(B) or 1(C), using the same protocol
and at low ionic strength (I @ 10
-4
M), like results were obtained. For low volume
fractions of lipid A-diphosphate (Figure 3(A)), very similar SAXS-diffraction spectra,
These assemblies were consistent with a colloidal assembly for a BCC lattice (
Im3m
)
with
a
= 35.5 nm. However, a mixture of equimolar concentrations of the two antago-
nistic molecules 1(B) and 1(C), revealed a SAXS-powder diffraction pattern and a LS
profile that could be indexed for a much larger face-centered (
Fd3m)
unit cell, with
a
= 58.0 nm. For dilute solutions the LS profile conformed to a hydrodynamic sphere of
radius of R = 34.7 nm, similar values were found for lipid A-diphosphate
dispersions
at high volume fraction f = 5.4 × 10
-4
. An investigation of another aqueous preparation
which contained lipid A-diphosphate and each of the antagonistic molecules B and C
in Figure 1 was carried out at volume fractions 2.5 £ f ≤ 3.4 × 10
-4
. Here, a primitive
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