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2.2. THE SHAPE OF BIOPOLYMERS
IN SOLUTION
2.2.1.
Introduction and examples
Figure 33. Haug's triangle illustrates the basic shapes of polymers in solution.
We will now be concerned with the properties of biopolymers when they are
dissolved in a solvent, usually water.
The diversity in terms of size and shape of biopolymers varies enormously,
but they can roughly be categorized into three basic shapes as shown in
Figure 1: solid spheres, rigid rods, and random coils.
With size we usually mean the chain length, or equivalently, the molecular
weight or the DP. The size has always a pronounced effect of the properties
of a polymer. Generally speaking, a long chain behaves differently from a
short chain having the same chemistry. However, the effect of size alone
depends strongly on the basic shape, which itself plays an important role. For
example, a small, compact (globular) protein behaves totally different than its
denatured form, which is usually more open and randomly coiled
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Most of this chapter will focus on flexible, expanded chains, which include
most polysaccharides in aqueous solution. Proteins deserve a brief comment,
because many of them adopt compact shapes with little flexibility. Protein
scientists take advantage of the fact that many proteins can form crystals
without changing their native shape. The shape can then be determined
accurately using X-ray diffraction, obtaining data with atomic resolution.
Thousands of examples may be found in standard protein databases.
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See example at the end of this document.