Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 41

2.0 g/ L
4.3 g/ L
6.9 g/ L
9.9 g/ L

1

10 9

12.1 g/ L

10 8

14.2 g/ L

0.1

10 7

0.01

0.01 0.1 1 10

q.R gz

q (nm -1 )

(a) (b)
Figure 4 The q-dependence of the light-scattering intensity of highly diluted b -LG
aggregates formed after prolonged heating in aqueous solution (pH ¼ 7, 0.1
M NaCl): 28 (a) I/KC versus q at different concentrations; and (b) same data
after normalization of I/KC by M w and q by R gz

universal function of qR gz (Figure 4). This was found to be the case for
aggregates formed (at pH ¼ 7 for different ionic strengths) for OA, 29 b-LG, 28
or BSA. 26 The structure factor for all these systems is very similar at large
length-scales and the inferred fractal dimension is close to 2, except at low ionic
strength where it is slightly smaller (d f E 1.7).

The structure on smaller length-scales has been investigated using small-
angle X-ray scattering (SAXS). Figure 5 shows the q-dependence of I for large
aggregates of OA and b-LG formed at different ionic strengths over a wide
q-range combining light scattering and X-ray scattering. 21 The structure factor
of aggregates formed at low ionic strength shows a cross-over from the fractal
structure at low q to a rod-like structure for q 4 L p 1 , so that both the
persistence length and the molar mass per unit length can be calculated. The
value of L p decreases with increasing concentration of NaCl for both these
proteins, indicating increased flexibility and/or branching density. Conse-
quently, the elementary unit of the fractal structure at large length-scales is
denser at higher ionic strength, and so the pre-factor b is larger. The
q-dependence at large q can be used to calculate the radius of gyration of the
cross-section of the rod-like structure (R c ). At higher ionic strength it is no
longer possible to detect a rod-like local structure for b-LG clusters.
Although the values of the fractal dimensions of OA and b-LG aggregates
are close, and the general features of the S(q) data are similar, the local
structure is quite different. For OA, the behaviour at large q-values can be
well described by a homogeneous rod-like structure for all the NaCl concen-
trations tested. The diameter (7 nm) is only slightly larger than that of
monomeric OA (6 nm). For b-LG, the local structure is more complex. The
diameter of the rod-like structure at low ionic strength (10 nm) is significantly
larger than that of monomeric b-LG (4 nm). Furthermore, already at 100 mM
NaCl a rod-like structure can no longer be detected and the local structure
becomes denser with increasing ionic strength. It appears that b-LG clusters are