Structure of Self-Assembled Globular Proteins - Food Colloids: Self-Assembly and Material Science - page 50

Figure 13 SEM image of a b -LG gel formed at C ¼ 40 g L 1 ,pH ¼ 7, and 0.6 MNaCl. 55
The total width of the image is 15 m m
However, TEM has also shown 56 that the micrometre-size particles are clusters
of smaller units. If the gels are truly made up of homogeneous domains with
well-defined interfaces, one should observe a very steep decrease of the scat-
tering intensity, i.e., I p q 4 . 23 This q-dependence has so far been observed in
only one case for BSA gels formed at the isoelectric point using neutron
scattering. 57 However, BSA aggregates formed under the same conditions were
found to show a self-similar structure with d f ¼ 1.8-1.9 at the larger length-
scales probed by light scattering. 26
The formation of homogenous micrometre-size protein particles is not
compatible with the scattering results discussed above. It is clear that, on the
basis of the results shown in Figure 11, one cannot make an unambiguous
distinction between fine-stranded gels and particulate gels in the pH and salt
concentration range covered. Perhaps homogeneous micrometre-size particles
are formed under conditions of higher salt concentrations, or closer to pI, but it
is also possible that the structure is partly modified by the treatment (fixation,
dehydration, embedding, and staining) that is necessary for TEM or SEM. A
more systematic comparison of electron microscopy and scattering results for
the same globular protein gels is needed to resolve this issue.
The principal origin of the strong, but gradual, increase of the turbidity with
increasing ionic strength or pH is the gradually increased heterogeneity with
relatively little change of the local structure. A possible origin for the increased
heterogeneity is that the repulsive interaction between the aggregates decreases.
This would lead to increased concentration fluctuations that are subsequently
frozen in by gelation. One might even imagine that this process leads to micro-
phase separation into well-defined spherical domains. 9,16 This latter mechanism
has been clearly established for the gelation of b-LG at pH ¼ 7 in the presence
of anionic polysaccharides.
When protein aggregates are mixed at room temperature with polysaccha-
ride, they phase separate into well-defined micrometre-sized domains that have
a tendency to agglomerate into larger clusters 58,59 (Figure 14). Micro-phase
separation is not observed for native proteins, and it occurs at lower poly-
saccharide concentrations when the aggregates are larger. Light scattering