Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 270

TEM image of hen lysozyme amyloid fibrils (scale bar = 200 nm). Reproduced from Corrigan et al.
( 2006 ) © American Chemical Society.

Figure 9.7

and dialysis-related amyloidoses, which affect patients on haemodialysis and cause
severe discomfort. At the same time, protein amyloid fibrils have many features that

are potentially of great value for the construction of functional materials. Natural
examples provide a model for novel

fibrous biomaterials for application in materials
science and nanobiotechnology (MacPhee and Woolfson, 2004 ).
Figure 9.7 shows a TEM image of the very dilute amyloid

fibrils forming randomly

oriented long fibrils with a constant cross-section.

In view of the main focus of this volume, we concentrate on the

case of
globular proteins heated as with the particulate or colloidal systems, but at these lower
pHs. Early studies of temperature-induced denaturation, especially those with lysozyme
(hen egg white), insulin and serum albumin (both human and bovine), revealed that

' simple '

bril
(and gel) formation could be induced by thermal denaturation, and that these form highly

fibrillar networks.
Aggregates formed at pH 2 have been studied by a combination of light and neutron
scattering (Aymard et al., 1996a ; Aymard et al., 1996b ). Rod-like structures formed at
pH 2, rather than the globules and aggregates of globules formed at pH 7. The rods were
thought to occur when disulphide exchange (implied in globule formation) was inhibited.
Increasing the salt concentration increased the level of branching (or rod flexibility),

since high salt screens out repulsive electrostatic interactions. The -Lg dimer was

suggested to be the fundamental repeat unit in these linear structures, although an
unfolded monomer could not be excluded.
A number of studies have been carried out using TEM (Langton and Hermansson,
1992 ; Stading et al., 1992 ), wide-angle X-ray diffraction and, most particularly, Fourier
transform IR (Kavanagh et al., 2000a ) to try to understand what factors in

β

uence the

relative propensity for fibrillar growth in the resultant gel. Microscopy showed long

linear aggregates forming in solutions at pH 2 (and sometimes 2.5) after prolonged
heating. Wide-angle X-ray diffraction (WAXD) showed a diffuse β -sheet halo in patterns