Page 101 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: LLL/LLL P2: FJU Final Pages
Encyclopedia of Physical Science and Technology EN002C-80 May 25, 2001 20:18

Carbohydrates 409

ilar to that of iodine in nonoxygenated solvents, such as
hydrocarbons. This observation has been attributed to the
lack of interaction between the equatorial oxygen atoms
of the six α-D-glucopyranose rings forming a turn of the
helix and an atom at its center. The latter is in a lipophilic
surrounding (of the axial C-H groups), very similar to that
of iodine in a hydrocarbon solution. The lipophilic cen-
ter of the helix would also make it difficult to remove a
FIGURE 16 Partial structures of the two forms of starch; the linear
lipid molecule located in its center, which explains why
amylose and the branched amylopectin.
13
the C-NMR spectrum of pure amylose invariably shows
lipid absorptions.
The industrial preparation of starch starts by loosening the Amylopectin: Amylopectin is a highly branched
starch granules away from their matrices with cold water α-D-glucose polymer that possesses a much larger
and letting them settle in tanks before filtration and drying. molecular weight than amylose. It contains a number
Purification of starch is achieved by repeated cycles of of linear chains of disubstituted (1 → 4)-linked α-D-
dispersion in cold water, sedimentation, and filtration. If glucopyranosylresiduessimilartothosefoundinamylose,
desired, such treatment may be followed by fractionation in addition it contains some (1 → 6) linked branches orig-
to separate the linear polymer of starch, amylose from the inating from 1,4,6-tri-O-substituted α-D-glucopyranosyl
branched polymer, amylopectin (see Fig. 16). residues. Light scattering data gives amylopectin a DP
5
Amylose: Amylose is the linear form of starch; it is pre- value in the order of 10 or higher if the starch from
cipitated from a cold aqueous dispersion of starch granules which it was obtained had matured for a long period in
with alcohols such as thymol or 1-butanol and filtered. The the plant. The accepted shape of the amylopectin macro-
accompanying amylopectin is left in the supernatent solu- molecules is that of a three-dimensional tree or bush
tion and if needed can be precipitated. Usually the ratio of (shown two dimensionally in Fig. 17). This conformation
amylose to total starch ranges between 15 and 35%, how- is based on data obtained from methylation experiments
ever, certain plant hybrids have been developed which are and enzymatic methodologies. For example, methylation
capable of producing a much larger excess of one poly- of amylopectin yields a smaller amount amount of 2,3,6-
mer over the other. Thus, amylomaize contains more than tri-O-methyl-D-glucose than amylose. Because 2,3,6-tri-
80% of amylose; whereas waxy maize starch contains O-methyl-D-glucose is produced from linear portions of
only 2% of this linear polymer. Amylose is composed (1 → 4) linked α-D-glucopyranose polymers and because
of (1 → 4)-linked α-D-glucopyranosyl residues attached it is formed in a smaller ratio from amylopectin than from
by acetal bonds. Due to lengthy manipulation during end amylose, one can conclude that the chains of amylopectin
group assay, the DP of amylose measured chemically is in- are considerably shorter than those of amylose. The high
variably lower than the DP measured by physical methods, degree of branching of amylopectin can also be deduced
such as light scattering or ultracentrifugation. For exam- from the relatively large amount (4%) of 2,3,4,6-tetra-O-
ple, the DP calculated from the amount of glucose present methyl-glucose, produced from the nonreducing ends of
at the nonreducing end of the chain and estimated from the amylopectin molecule, as compared to the amount pro-
theamountof2,3,4,6-tetra-O-methyl-D-glucoseproduced duced from amylose (<0.4%). In addition, amylopectin
upon methylation and hydrolysis of amylose ranges be- yields a unique product not usually found when amylose is
tween 200 and 300. This is much lower than the DP methylated and hydrolyzed, namely of 2,3-di-O-methyl-
obtained by light scattering or ultracentrifugation (about D-glucose, produced from the points of branching.
6000). The difference has been attributed to degradation
during the chemical treatment. In neutral solution amy-
lose exists as a random coil, but other conformers can be
produced by retrogradation, i.e., separating the different
insoluble fractions of amylose that deposit on standing.
In the presence of complexing agents amylose assumes
a more organized conformation; namely that of a right-
handed helix made up of 150 to 1000 turns and containing
six α-D-glucopyranose rings per turn. The dimensions of
FIGURE 17 A two-dimensional depiction of the bush model of
the amylose helix are such that it can accommodate an
amylopectin. The reducing end of the molecule is represented by
iodine atom, or a lipid molecule of appropriate size. The a solid black circle; the other circles represent the nonreducing
light absorption of iodine-amylose complexes is very sim- ends.