Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 257

fibrillar systems have in common is that they
are formed from proteins in aqueous (or electrolyte) solutions that have become partly
denatured, generally irreversibly, by some physical or chemical treatment. This distin-
guishes the particulate gels from those prepared by treating a protein with strong
denaturant such as 8M urea (van Kleef et al., 1978 ). This treatment reduces the protein
to its

What most of the simple particulate and

flexible polypeptide backbone, and such materials, not discussed here, have much
more in common with chemical gels.
It is now appreciated that in, for example, simple heat-induced denaturation, the
protein size and shape is only mildly perturbed. Instead some of the hydrophobic groups,
which at ambient temperatures remain buried in the protein core, become exposed above
some minimum unfolding or denaturation temperature. This ' hydrophobic effect ' leads

to aggregation, forming either the fine-stranded networks or the amorphous particulate

structures of the physical gel (Clark and Ross-Murphy, 1987 , 2009 ; Clark, 1998 ; van der
Linden and Foegeding, 2009 ). In succeeding sections we discuss the various mechanisms
and their structural and macroscopic consequences.

9.2 Colloidal gels formed from partially denatured proteins

9.2.1 Structural aspects

The production of gels from solutions of globular proteins usually requires a degree of
unfolding of the protein as a

first step. The details of secondary and tertiary structure
of globular proteins, and the mechanisms of folding into a speci

c globular structure, are
outside the scope of this volume, but it is now clearly established that the various globular
structures are stabilized by a combination of -sheet and other secondary

structure components, further held together by hydrogen bonding between non-adjacent
peptide residues.
As mentioned above, it was once assumed that protein gels were formed only if this
entire secondary and tertiary structure was disrupted, as in the urea example above.
However, structural work carried out by a number of groups some 30 years ago, using
X-ray diffraction, Fourier transform infrared (FTIR) and electron microscopy, estab-
lished quite clearly that this was not the case. The picture instead is that, under appro-
priate mildly denaturing conditions, a protein will partly denature via various
intermediates (Chiti and Dobson, 2006 ) and then partially renaturate in a slightly differ-
ent manner to create non-covalent interactions between various proteins, and this is what
subsequently leads to gelation.
Most of the published work involves either serum albumen, particularly from bovine
sources (BSA), or the milk protein

α -helix, β

-Lg) (Donald, 2008 ). This is not
because these are interesting or particularly special in their own right, but more pragmati-
cally because they can be obtained in reasonably pure form at comparatively low cost.
For the class of experiments of interest to a gel researcher, grams

β -lactoglobulin ( β

-
of material are required. In much of this chapter, then, because of the amount of published
work we will naturally tend to concentrate on gels formed from these two proteins.

- even tens of grams