Chemistry Reference
In-Depth Information
protein overcharging than GdnHCl. This might be explained by a higher
accessibility and diffusion rate of Na
1
, but also by additional charges on
arginine HCl due to the amino acid group.
12.3.2 Physicochemical Properties of Protein Aggregates
in
Relation to Interfacial and Foaming Properties
Figure 6A shows the evolution of the hydrodynamic diameter D
h
of the soluble
aggregates formed during heat treatment of b-LG+arginine HCl or NaCl. It
was found that b-LG alone heated at 801C for 10 min gives a higher D
h
value
(65
1.8 nm) than at low cosolute concentrations (5-30 mM for arginine HCl
or GdnHCl, and up to 80 mM for NaCl). The effect of the low concentration of
cosolute could be related to a salting-in effect, as described by Arakawa and
Timasheff.
38
However, it is more likely that a few larger aggregates dominate
the scattered signal in a mainly monomeric and dimeric protein solution (b-LG
alone and mixtures with low cosolute concentration). With increasing particle
size and denaturation degree this effect gets less important as higher quantities
of larger particles dominate the signal. With increasing cosolute concentration,
D
h
increases up to a maximum of
130 nm for A70 and N150 corresponding
to the critical cosolute concentration where insolubility of the proteins
occurred. At concentrations higher than C
cs
, the soluble aggregates still present
in the sample decrease in size until D
h
reaches values of 8-14 nm. Figure 6B
shows the molecular weight M
w
of the soluble aggregates formed in presence of
arginine HCl and NaCl as a function of the hydrodynamic radius of the
aggregates. Interestingly, a master curve is obtained, even though the cosolute
concentration to obtain aggregates with similar D
h
is different for the two
cosolutes (arginine HCl or NaCl). In other words, it is possible to produce
B
140
12
A
B
120
10
100
8
80
6
60
4
40
2
20
0
0
0
20
40
60
80
100 120 140
05
0
100
150
200
Concentration in cosolute[mM]
Aggregate size [nm]
Figure 6 Dynamic light scattering of heated
b
-LG solutions (pH 7.0, protein concentra-
tion 10 g L
1
, 0-200 mM cosolute, heated at 801C for 10 min):
K
, arginine
HCl;
J
, NaCl. (A) Aggregate size D
h
plotted as a function of the cosolute
concentration. (B) Molecular weight M
w
plotted as a function of the aggregate
size. Vertical error bars represent the standard deviation
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