Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 43

10 7

0.003M
0.01M
0.02M
0.05M
0.1M
0.2M
0.4M

10 6

10 5

10 4

10 3

10 100 1000

R gz (nm)

Figure 6 Dependence of M w on R gz for b -LG aggregates formed at pH ¼ 7 and different
NaCl concentrations. (Taken from ref. 32.)

remains the same. The difference between OA at pH ¼ 7 and b-LG at pH ¼ 2,
on the one hand, and b-LG at pH ¼ 7, on the other, is that in the former case
the proteins form chains of monomers or dimers, while in latter they first form
small pre-aggregates, which themselves assemble into more-or-less branched
self-similar structures.
Before discussing the structure of interacting aggregates and gels, we mention
a related light-scattering study on large b-LG clusters formed at room temper-
ature. 34 In this work, small b-LG aggregates formed at pH ¼ 7 in the absence of
added salt were self-assembled by reducing the pH closer to the isoelectric point
or by adding CaCl 2 . Here large self-similar aggregates were also observed with
fractal dimensions close to 2.

3.3 Structure of Interacting Aggregates and Gels
The scattering intensity of protein solutions increases during heating because
both the numbers and the sizes of the aggregates grow. The rate of the initial
increase can be used to estimate the aggregation rate. 35-37 However, the
interaction between the aggregates influences the structure factor to an extent
that depends on the protein concentration and the strength of the interactions.
This is already evident for native proteins that show a peak in S(q) at low ionic
strength caused by correlations between the positions of the proteins. 38-42 The
peak position q p indicates the preferred distance between the proteins, and it
increases approximately with the cube root of the concentration. The height of
the peak increases with increasing protein concentration because the degree of
order increases. The repulsion between the proteins decreases when salt is