Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 40

Four domains can be distinguished in the plot of I/KC versus q:

(i) At the smallest q-values, I is independent of q, and we have I(0)/KC ¼
M w .
(ii) For q R gz 1 , the q-dependence crosses over to a power-law behav-

iour, I/KC ¼ bq df characterizing the self-similar structure. The pre-
factor b is larger if the local structure of the aggregates is denser, and it
is directly related to the pre-factor a of Equation (1). 25 The value of R gz
can be determined from the initial q-dependence of I:

E

I ¼ I(0)/[1 + (qR gz ) 2/3 ]. (4)

L p 1 , the q-dependence crosses over to I/KC ¼ cq 1 , because a
characteristic structure of the rods is probed on length-scales below the
persistence length (L p ). The parameter L p characterizes either the aver-
age length-scale over which the aggregates do not bend or the average
distance between branch points. The pre-factor c is determined by the
molar mass per unit length of the rods (M l ), i.e., c ¼ p M l .
(iv) At q 4 R c 1 , the q-dependence becomes steeper again because the
internal structure of the rods (and branch points) is probed on length-
scales smaller than their radius (R c ).
In order to cover all the relevant length-scales, light scattering has to be
combined with neutron or X-ray scattering. As mentioned earlier, it is dicult
to obtain information about the degree of branching using scattering tech-
niques. One cannot distinguish between increased flexibility of linear aggregates
and an increase in the branching density. Both lead to a decrease of L p . Neither
can one easily distinguish between a change in the structure of the rods and a
change in the number of branch points; both will influence b and R c .
Full characterization of aggregate structure by combined light scattering and
X-ray scattering has so far been done only for ovalbumin (OA) and b-LG at pH
¼ 7, 21,23,24 and for b-LG at pH ¼ 2. 14 In each case the amount of added NaCl
was varied. Light-scattering measurements have also been carried out for BSA
aggregates formed by heating at pH ¼ 7. 26 Extensive heating at different
protein concentrations leads to the formation of clusters with values of M w and
R gz that increase with increasing C. A gel is formed above a critical protein
concentration C g . The rate of aggregation increases strongly with increasing
temperature, but the structure of the aggregates is independent of the heating
temperature. 27 The temperature dependence of the growth rate, and thus the gel
time t g ,

(iii) For q

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300 kJ mol 1 ,
suggesting that it is determined by the protein denaturation step.
Figure 4 shows the q-dependence of I in the range covered by light scattering
for b-LG aggregates formed at different concentrations in the presence of 100
mMNaCl at pH ¼ 7. 28 The largest aggregates show a power-law behaviour over
the whole q-range, and from the slope we can calculate d f ¼ 2.0. If the structure
of the aggregates is self-similar, then the structure factor S(q) ¼ I(q)/I(0) is a

is characterized by a large activation energy of B