4.4 Polysaccharides 321Fig. 4.32.Gelatinization temperature of potato starch
as a function of water activity aw(top) and of the nat-
ural logarithm of the quotient of activity awto volume
fraction vlof water (bottom);•glycerol,◦maltose,
saccharose,glucose,♦ribose,⊗NaCl, CaCl 2 (ac-
cording toGalliard, 1987)
4.4.4.14.3 StructureandPropertiesofAmylose
Amylose is a chain polymer of α-D-gluco-
pyranosyl residues linked 1→4:
(4.149)Enzymatic hydrolysis of the chain is achieved by
α-amylase,β-amylase and glucoamylase. Often,
β-amylase does not degrade the molecule com-
pletely into maltose, since a very low branching
is found along the chain withα( 1 → 6 )linkages.
The molecular size of amylose is variable.
The polymerization degree in wheat starch lies
between 500 and 6000, while in potatoes it can
rise up to 4500. This corresponds to a molecular
weight of 150–750 kdal. X-ray diffraction ex-
periments conducted on oriented amylose fibers
make possible the assignment of the types of
starch mentioned above to definite molecular
structural elements. Oriented fibers of the A-type
were obtained by cutting and stretching thin
films of acetylamylose at 150◦C, deacetylation
in alcoholic alkali, and conditioning at 80%
relative air humidity and 85◦C. Type B fibers
were obtained in a corresponding manner by
conditioning the deacetylated material at room
temperature for three days at 80% and for
another three days at 100% relative air humidity,
followed by aftertreatment in water at 90◦C
for 1 h. The diffraction patterns obtained with
these oriented fibers corresponded to those of
typesAandBgivenbynativestarchpow-
ders, allowing the development of structural
models.
The structural elements of type B are left-hand
double helices (Fig. 4.34a), which are packed
in a parallel arrangement (Fig. 4.33). One
turn of the double helix is 2.1 nm long, which
corresponds to 6 glucose residues, i. e., three
residues from each glucan chain. Hydrogen
bridges between the amylose molecules stabilize
the double helix. The central channel surrounded
by six double helices is filled with water (36
H 2 O/unit cell). The A-type is very similar to
the B-type, except that the central channel is
occupied by another double helix, making the
packing more close. In this type, only eight
molecules of water per unit cell are inserted
between the double helices. The transition from
type B to type A achieved by wet heating has
been described already (4.4.4.14.2, Fig. 4.28).
It is difficult to bring the postulated antiparallel
arrangement of the double helices into line with
the requirements of biosynthesis, where a parallel
arrangement can be expected. It is possible that
the present experimental data do not exclude
such an arrangement.