Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 256

9 Colloidal gels from proteins

and peptides

9.1 Introduction

One of the most interesting and scienti cally richest class of physical gels are those

formed from proteins and polypeptides (Doi, 1993 ; Clark, 1998 ; Foegeding, 2006 ). Here
we distinguish gels from loose colloidal aggregates formed, for example, by

coagulation and collapse of otherwise stable colloids, by the extra requirement that the
'

' true '

cannot be redispersed simply by agitation; in other words, that the interaction
between particles is of the order of 10kT or more. The simplest common example of this
class of materials is the easily recognized gel formed by boiling an egg. Classically, all of
these networks were once thought of as being formed from the polypeptide backbone
itself (i.e. through complete unfolding), much in the way

particles '

fibrous networks of denatured
collagen are formed. However, it is now appreciated that they are usually constructed
in a multi-step process from

' partially folded still globular entities ' (known as the

' string-of-beads interpretation).

In the distinction

'

first made explicit by Hermansson and co-workers in the 1980s, these
may be divided into coarse-stranded (sometimes simply referred to as or

colloidal) gels, i.e. those having a heterogeneous and phase separated appearance; and

' particulate '

fine strands (Stading
and Hermansson, 1991 ; Langton and Hermansson, 1992 ; Stading et al., 1992 ). The latter
are now often referred to as

fibrillar gels, i.e. those formed of more uniform assemblies of

fine-stranded materials have been
studied very extensively over the last few years, revealing many apparent similarities
with the amyloid structures found in disease conditions such as Alzheimer

' amyloid ' , because such

s.
We will discuss the latter systems later, as this is an active area of research. However,
since the

'

at one
level. Moreover, the emphasis seen in some of the literature might suggest that all relevant
work has been performed in the last decade, but this is certainly not the case. Indeed highly

fibrils are formed from particles, they can still be regarded as ' particulate '

fibrillar insulin gels were known and studied in the 1950s (Koltun et al., 1954 ), and again
by Clark and co-workers in the 1980s (Clark et al., 1981a , 1981b ). Later in this chapter, we
include a brief description of the very speci

cordered fibrous assemblies produced by

actin, tubulin and c and we cannot

really consider them to be full members of this class. As with the other physical gels in this
volume, the principal aims are to investigate phenomena over a range of length scales and if
possible to establish descriptive models. Finally we introduce, albeit brie

fibrinogen, although the biology of these is rather speci

y, those gels

formed from specially synthesized peptides - again a very topical subject.