Chemistry Reference
In-Depth Information
ratio of 8 and higher. In the case of HMP, an increase in scattered light intensity
is observed as well. Based on the lower charge density of HMP as compared with
LMP, aggregation could have been expected at a lower mixing ratio, but no
aggregation was actually observed with this pectin sample.
To study the impact of the presence of polysaccharide on the adsorption
kinetics of the protein, the surface pressure was measured as a function of time
for b-Lg solutions and b-Lg + pectin solutions, both at the air
water interface
[Figure 3(a)] and at the oil-water interface [Figure 3(b)]. The protein concen-
tration in all the samples was 0.1 g L
1
, and the protein/polysaccharide mixing
ratio was 0.5. Under these conditions, the majority of the protein is present in
soluble complexes with the polysaccharide because the charge on the poly-
saccharide is in excess. Both LMP and HMP were used, in order to compare the
effect of their different protein-binding affinities due to their different charge
densities. Compared to proteins, the polysaccharides on their own do not give a
significant increase in surface pressure at the concentrations used (
o
2mNm
1
in 2
10
4
s for a concentration of 0.05 g L
1
, data not shown). The pure b-Lg
solution demonstrated a fast increase in surface pressure at both types of
interfaces. The presence of LMP led to a 'lag time' before the surface pressure
increased at the air-water interface. The lag time is defined as the time from the
beginning of the experiment - with a clean interface - until the surface pressure
starts increasing, or, more precisely, the time of the cross-over point between
the initial nearly horizontal slope and the steepest slope in the surface pressure
versus time curve. The presence of HMP causes a lag time at the air-water
interface as well, but it is much shorter compared to the one with LMP. This
can be explained by the lower charge density and thus the lower binding affinity
for b-Lg. At the oil-water interface this lag time is absent, but the increase in
surface pressure is clearly delayed with LMP, and to a lesser extent with HMP.
One could imagine that the difference in shape of the curves for the air-water
and oil-water interfaces is caused by a difference in the effect of pectin at both
interfaces. However, Figure 4 shows that the surface pressure versus time curve
at the oil-water interface for a pure b-Lg solution of lower concentration also
25
15
β
-Lg
β
-Lg
20
β
-Lg + HMP
10
β
-Lg + HMP
15
10
5
β
-Lg + LMP
5
β
-Lg + LMP
OIL/ WATER
AIR / WATER
0
0
0
500
1000
1500
0
500
1000
1500
(a)
Figure 3 Surface pressure as a function of time for
b
-Lg and mixed
b
-Lg + pectin
solutions (pH
¼
4.5, ionic strength
¼
2 mM) with mixing ratio
¼
0.5: (a) air-
water interface and (b) oil-water interface
(b)
Time (s)
Time (s)
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