Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 46

with the notable exception of lysozyme. The transition between transparent gels
and very opaque gels occurs over a relatively narrow range of salt concentra-
tions or pH. For instance, b-LG gels formed at pH ¼ 7 are transparent if o 50
mM NaCl is added, and are very opaque if 200 mM NaCl is added. In the
absence of salt, the gels are transparent at pH ¼ 6.2 and opaque at pH ¼ 5.8.
The transition depends mainly on the combination of ionic strength and pH,
and it occurs approximately in the same range for gels formed by other globular
proteins. 47
The strong increase in turbidity is caused by the loss of the order due to the
weakening of the repulsive interactions between the aggregates. The structure
factor of more heterogeneous gels cannot be determined by standard light-
scattering equipment because the contribution of multiple scattering, which
gives the gels the turbid aspect, can no longer be neglected. Utilizing cross-
correlation, dynamic light-scattering allows one to correct for the effect of
multiple scattering and thus to obtain the true structure factor. 48,49 This
technique has recently been used to obtain the structure factors of solutions
and gels formed by heating b-LG, 28,50 OA, 19,29 or BSA 43 at pH ¼ 7. Figure 9
shows examples of the q-dependence of the intensity for heated b-LG solutions
at different protein concentrations. For heterogeneous gels we observe a power-
law q-dependence indicating that the gels have a self-similar structure over a
range of length-scales. The fractal dimension was found to be close to 2, i.e., the
same as that for diluted aggregates formed under the same conditions.
The scattering data for the gels were analysed using the same method as
described above for the dilute aggregate solutions. Instead of M w and R gz one
obtains an apparent molar mass M a and apparent radius of gyration R a . The

26g/ L
33g/ L 10 8

12g/ L

50g/ L

68g/ L

90g/ L

8.1g/ L

10 7

6.3g/ L

2.1g/ L

0.001 0.01

q(nm -1 )

Figure 9 The q-dependence of the light-scattering intensity of extensively heated b -LG
solutions (pH ¼ 7, 100 mM NaCl) at different protein concentrations. 28 Open
points represent aggregated solutions; filled points represent gels (C 4 C g )