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In addition to the perfectly linear structure, synthetic amyloses can be tailored
to give any desired molecular weight with extremely low polydispersity. Such
amyloses are very useful tools to study the properties of starches.
In Japan, synthetic amylose is being produced commercially as a food
additive.
1.5.5. Amylopectin
The amylopectins are very large, hyperbranched structures. A single
macromolecule may contain up to 3.5 million glucose residues, although
values closer to those of amylose seem to be common.
Amylopectins are extensively branched (
α
-1,6), and the branching pattern is
far from random. The branches are clearly clustered. By cleaving the
α
-1,6
linkages (by isoamylase or pullulanase), the
unit chains
are released. It turns
out the unit chains can be grouped into different classes, with average DP
values in the range 17-26. The unit chain length distribution is again not
random, but centered around populations of short chains (DP range 6-30),
and long chains up to DP 100.
Amylopectins also contain a small amount of phosphate ester (potato starch:
1 P per 317 Glc), giving the molecules a weak polyelectrolyte character.
In the granules, the unit chains of amylopectin molecules form intramolecular
double helices as described above (A- or B-type). The molecules are further
organized to give starch grains their characteristic ultrastructure of alternating
semi-crystalline and amorphous ‘growth rings’.
1.5.6. Cyclic
α
-‐1,4 glucans
Cyclodextrins are cyclic amyloses with 6, 7 or 8 sugars per ring (
α
,
β
,
γ
-CD).
They are formed from starches in an enzymatic process. They are used
commercially both as food additives and as separation agents in HPLC
columns. The circular architecture provides a hydrophobic environment inside
the rings, which has affinity for hydrophobic molecules in aqueous solutions.
Larger cyclic amyloses have recently been produced and studied.