Chemistry Reference
In-Depth Information
For a given protein concentration both R
a
and M
a
increase with increasing
salt concentration. The range of salt concentrations that can be investigated
with cross-correlation DLS is rather limited, however, because the gels become
rapidly too turbid. The range can be extended using turbidimetry. The turbidity
t is directly related to the structure factor S(q)by
t
¼
K
0
CM
a
Z
Z
2
p
p
S
ð
q
Þð
1
þ
cos
2
y
Þ
sin
y
d
y
d
j
;
ð
5
Þ
0
0
where K
0
is an optical constant.
28
If S(q) is known, M
a
and R
a
can be calculated
from the wavelength dependence of
t
. It turns out that the structure factor of
globular protein gels in the q-range relevant for the turbidity can be well
described by Equation (4) (replacing R
gz
with R
a
), so that an analytical
expression for
t
can be derived.
28
We have recently carried out a systematic study of the turbidity for b-LG gels
as a function of pH and ionic strength.
33
The values of M
a
and R
a
were derived
from the wavelength dependence using Equations (4) and (5). Figure 11(a) shows
M
a
of b-LG gels formed at pH
¼
7withC
¼
100gL
1
as a function of the
concentration of added NaCl. Small values of M
a
characteristic of transparent
samples were obtained using light-scattering measurements. The value of M
a
was
found to increase exponentially with the ionic strength between 0 and 200 mM.
At higher salt concentrations, the transmission became less than a few percent
even if we used path lengths of 1 mm, and M
a
could no longer be determined
accurately. (The upper limit of the turbidity that could be determined accurately
was
20 cm
1
.) The dependence of M
a
on pH plotted in Figure 11(b) shows a
strong increase of M
a
with decreasing pH starting at pH
¼
6. We find that
M
a
increases slightly more strongly than R
a
, which is to be expected because,
B
10
11
10
11
10
10
10
10
10
9
10
9
10
8
10
8
10
7
10
7
10
6
10
6
10
5
10
5
10
4
10
4
0.00
0.05
0.10
0.15
0.20
5.0
5.5
6.0
6.5
7.0
7.5
8.0
pH
(a)
C
NaCl
(mol/L)
(b)
Figure 11 Dependence of apparent molar mass M
a
on (a) NaCl concentration at pH
¼
7
and (b) pH in absence of added salt, for gels formed by extensive heating of
b
-LG solutions at C
¼
100 g L
1
. (Taken from ref. 33.)
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