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2.2.5. The RG-­‐M relationships for randomly coiled chains.
Biopolymers behaving like flexible chains in solution are common:
• Most denatured proteins (-S-S- bridges cleaved, high T 31, 6 M urea..)
(See also Section 2.2.13)
• Denatured (single-stranded) DNA, RNA (high T)
• Many linear (unbranched) polysaccharides:
• Pullulan (very flexible)
• Alginates (Na+-form, no Ca+)
• Chitosans (pH < ca. 6)
• Hyaluronan
• Amylose (in DMSO or alkali)
• Cellulose (in special cellulose solvents)
The flexibility is caused by the properties of the linkages between monomers.
In proteins, the peptide linkage has a certain freedom of rotation (Figure 9).
This applies in particular to the Cα-C and N-Cα linkages, whereas the partial
C=N double bond is much more rigid due to resonance:
Figure 41
In polysaccharides, the sugar rings are usually very rigid, but some rotation is
allowed for the glycosidic linkages (figure 10):
Figure 42
In total, even a small amount of rotational freedom in each linkage gives rise
to flexible chains.
It is important to realize that flexible chains do not adopt a single, fixed shape
or conformation. Due to thermal forces, the chains will constantly change
31 There are exceptions (cold denaturation)
OH
O
-OOC
OH
HO
O
O
-OOC
HO
O