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Most proteins, except for structural proteins (collagen, silk etc), are small and
compactly folded, and behave in many ways as essentially solid spheres. One
example is haemoglobin:
Figure 34. Haemoglobin (tetramer): A compact protein 5.5 nm across.
Examples of polysaccharides are alginate and xanthan (Figure 3). We note
that, in contrast to haemoglobin, these biopolymers have irregular and
expanded structures, and chains have different lengths (polydispersity).
Alginate appears more compact than xanthan, a fact that has large
consequences.
Figure 35. Alginate (top) and xanthan (bottom) visualized by electron microscopy after being
absorbed to a surface. Bar: 200nm.
In order to determine whether a polymer behaves like a solid sphere, rigid rod,
or wormlike chain in solution we need to determine both the molecular weight
and a parameter or property reflecting the actual shape of the polymer.
Haemoglobin
5.5 nm
Proteins:
Small
Compact, well-defined
physical structures
Monodisperse