Chemistry Reference
In-Depth Information
10
9
3mM
10
9
3mM
30mM
10mM
10
8
50mM
20mM
10
8
100mM
50mM
100mM
10
7
200mM
10
7
10
6
10
6
10
5
10
5
10
4
10
4
10
3
10
3
0.001
0.01
0.1
1
0.001
0.01
0.1
1
q(nm
-1
)
q(nm
-1
)
(a) (b)
Figure 5 The q-dependence of the intensities from light scattering (low q) and X-ray
scattering (high q) of highly diluted very large aggregates formed by (a) OA
and (b)
b
-LG close to the gel point at pH
¼
7 after prolonged heating at
different NaCl concentrations.
21
The solid lines represent fits to the structure
factor of semi-flexible chains
formed in two steps at pH
¼
7: in the first step relatively small pre-aggregates
are formed, which in a second step assemble into large fractal clusters.
30,31
The increase in the density of the aggregates with increasing ionic strength
has been confirmed by analysing the dependence of M
w
on R
gz
.
32
The results
obtained for b-LG are plotted in Figure 6. The slopes are equal to the fractal
dimension [see Equation (1)], while the pre-factor depends on the density of the
elementary unit of the fractal structure. We have drawn solid lines through the
data with slopes that are consistent with the structure factor of the large
aggregates, i.e., d
f
¼
2 for NaCl concentrations 430 mM and d
f
¼
1.7 in the
absence of NaCl and at 30 mM. The lines are compatible with the data for large
aggregates. However, a clear deviation can be found for the data obtained at
400 mM NaCl, for which M
w
has a stronger dependence on R
gz
. This can be
interpreted by assuming either that the fractal dimension is larger at this ionic
strength or that the elementary unit is larger so that the fractal structure is only
revealed for large aggregates. In any case it is obvious that the density of the
elementary unit increases with increasing ionic strength, especially for NaCl
concentrations above 100 mM. Hagiwara et al.
26
observed that BSA aggregates
formed close to pI were denser than at pH
¼
7, but the fractal dimension was
close to 2 in both cases. Recently we have studied the structure of clusters
formed at different values of pH. We have also found
33
that the fractal
dimension remains close to 2, but the density increases when the pH approaches
the isoelectric point.
The structure factors obtained for b-LG at pH
¼
2 indicate the formation of
long rod-like structures at low ionic strength.
14
At 100 and 200 mM NaCl, the
results are closer to those obtained for OA at pH
¼
7 than for b-LG at pH
¼
7.
The aggregates globally have a self-similar structure and are locally rod-like
with a diameter close to that of monomeric b-LG. The persistence length of the
rod-like structure decreases with increasing ionic strength, but the diameter
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