Chemistry Reference
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(ii) On lowering the pH close to the isoelectric pH of the protein (or below),
soluble protein-polysaccharide complexes are formed at low ionic
strength (below
100 mM, depending on the binding affinity).
(iii) A further decrease in pH leads to aggregation of the soluble complexes
and subsequently to complex coacervation.
(iv) At pH values below pH
B
2.5, the complexation can be suppressed by
protonation of the acidic groups on the polysaccharide.
5
E
The adsorption kinetics of proteins has been extensively studied in relation to
foam and emulsion formation. Several steps in protein adsorption have been
identified:
6-8
transport of the molecules to the interface by diffusion/convection;
adsorption at the interface; and possibly conformational changes once adsorbed
at the interface. Large variations have been found between different proteins -
for example, b-lactoglobulin (b-Lg) is known to increase surface pressure rap-
idly,
9
whereas for lysozyme the process is much slower.
10
According to Wierenga
and co-workers,
11,12
a kinetic barrier for protein adsorption can explain these
differences: when a protein molecule approaches the air-water interface, a
balance between its hydrophobic exposure and its net charge determines the
probability of adsorption. Obviously all this holds for pure protein solutions; but
the presence of anionic polysaccharide molecules that interact with the protein
molecules also affects the adsorption kinetics to air
water interfaces.
13-15
Because protein adsorption kinetics at air-water and oil-water interfaces can
be different,
16
the impact of protein-polysaccharide interactions on adsorption
kinetics could also differ for the two types of interfaces.
Based on previous observations at the air
water interface, a mechanistic
model for mixed protein + polysaccharide adsorption has been proposed
(Figure 1).
17
The protein-polysaccharide-binding affinity determines how much
protein is uncomplexed in solution and how much is bound to the polysaccharide
(Figure 1, equilibrium A). Protein molecules can diffuse freely to the air-water
interface when not bound to a polysaccharide (route D). Once protein
approaches the interface, it might encounter a kinetic barrier for adsorption
(depicted by E). Protein-polysaccharide complexes can also diffuse to the
interface (route B), but their diffusion coefficient is much lower than that for
the free protein due to the larger hydrodynamic radius.
One may wonder how a protein
polysaccharide complex adsorbs at the air-
water interface. Presumably it is the protein molecule(s) in the complex - and
not so much the polysaccharide one(s) - that are responsible for the increase in
surface pressure. Complexes that are highly negatively charged may have a low
density. If a polysaccharide can trap protein molecules in such a diffuse
complex, it may prevent efficient packing of protein at the interface. In this
case the mobility of the protein molecules within the complex determines how
fast a compact protein layer can be formed. This is represented by factor C in
Figure 1. This factor most likely depends on the protein-polysaccharide-
binding affinity. Understanding the mechanism of mixed adsorption should
allow better control of adsorption kinetics to the air-water interface
(as monitored, for example, by drop tensiometry) and therefore the processes
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