Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 182

7 Helical structures from neutral

biopolymers

7.1 Introduction

This chapter is devoted to gelatin and agarose, two systems which form gels via the transition
from a disordered and relatively flexible coil conformation in solution at high temperatures

(60 80°C) to an ordered and partly helical structure on cooling to ~30°C, provided the

concentration is large enough. At these lower temperatures the network strands consist of
both helical and coil-like portions. On heating, the gels

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and the individual molecular
chains revert to the coil state. At one level, the gel network consists of long and thin

' melt '

bres

which are interconnected. However, they differ fundamentally from the fibrillar networks to

be discussed in Chapter 9 , which are formed from globular protein precursors such as actin or
the so-called acid and irreversibly heat-set

' '

gels, since in these cases the strands are
assembled not from molecular chains but from globular macromolecules derived from
proteins.
In the present chapter, we analyse the mechanisms of network formation in gelatin, coil
to triple helix, and agarose, coil to double helix to multiple strand

amyloid

bres. Among the other
thermoreversible multiple helix-stranded networks, carrageenan and gellan gels were
discussed in Chapter 5 . There, gelation occurs in the presence of various types or
amounts of salts, and the role of speci c ions is a major factor, which requires separate

analysis. By contrast, gelatin and agarose aqueous solutions undergo a sol

gel transition
mainly driven by cooling from the high-temperature form, and the gelation is not strongly
in

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uenced by ionic interactions.
Both systems are derived from natural polymers; gelatin is proteinaceous, extracted
from collagen by acid or alkaline treatment, whereas agarose is a polysaccharide
extracted from marine algae. Gelatin is widely used in food preparations (desserts,
confectionery), technical formulations (paper coating, inks and

- until digital cameras

became ubiquitous film) and well-known pharmaceutical applications

(hard and soft capsules). Agarose, or the cruder product known as agar, is used in similar
food applications (and sometimes as a vegetarian replacement for gelatin), as a common
laboratory culture medium for microbes, in the laboratory technique of gel electropho-
resis and as the base of some dental impression materials.

- photographic

7.2 Gelatin

Collagen is the major structural component of mammalian tendon, bone, skin and cornea,
and gelatin is a name applied to degraded and denatured collagen (Ward and Courts,