Basic HTML Version
63
Figure: Amylose chain showing the
ax-eq
linkage C1-O-C4.
Naturally occurring amylose tends to have a small number of branches, which
are linked to the oxygen in position 6.
Because of the
ax-eq
linkage the amylose chains tend to form helices rather
than simple, extended chains such as found in cellulose.
In DMSO and in freshly prepared aqueous solutions, amyloses behave as
randomly coiled polymers. However, in water the structure is not stable, and
crystallization and precipitation occurs.
Amylose helices have been intensively studied, mainly by X-ray scattering of
the crystalline state. Most amyloses crystallize into forms (polymorphs) called
A-type or B-type. A-amylose occurs mainly in cereal starches whereas B-
amylose is mainly found in tuber starches. They both consist of the parallel
packing of left-handed, double helices associated with a number of water
molecules.
Crystals may be formed in the presence of many small molecules (ethanol,
butanol, fatty acids etc.), sometimes referred to as inclusion complexes. The
structures of these helices, collectively termed V-amyloses, are still debated
(2010), but a recurring motif is a left-handed single helix with (in some cases)
6 residues per turn.
1.5.4. Synthetic amylose: Perfect model
substances?
Recently, synthetic amylose has become available. These amyloses, which
have no branches, are synthezised
in vitro
, starting from a primer (typically
maltopentaose), and successively adding glucose residues by the reaction:
...(Glc)
n
+ Glc-1-P = ..(Glc)
n+1
+ P
The preferred enzyme for this reaction is glucan phosphorylase. Other
enzymes catalyze a similar reaction based on different substrates, for
example UDP- or ADP-glucose.
O
O
HO
O
OH
OH
O
HO
O
OH
OH
O
HO
O
OH
OH