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1.1.9. The
shape of hexoses
Cyclic sugars without double bonds
2
cannot form flat structures as indicated
by the Haworth formulae. The situation is similar to that of simple cyclic
compounds such as cyclohexane. This is because of the
sp
3
hybridization of
the carbons of the molecule, which forces the sugars to adapt specific
shapes. The most common
shapes of hexoses are:
a) The
4
C
1
chair form (very
abundant)
b) The
1
C
4
chair form (less
abundant, but important)
c) Boat forms (uncommon except when chains are mechanically stretched)
The naming refers to the position (above or below) of C1 and C4 relative to
the plane formed by C2-C3-C5-O (shaded area in the figure):
In the
4
C
1
conformation carbon 4 is above the plane, and carbon 1 is below
the plane.
1.1.10. How
to determine whether a
sugar
is
1
C
4
or
4
C
1
Most of the hexoses that we encounter in this course as part of oligo-and
polysaccharides exist only in the
4
C
1
chair form. A typical example is
cellulose:
Cellulose consists only of
β
-1,4-linked D-glucose, which in the
4
C
1
chair form
has all its heavy substituents (-OH and –CH
2
OH) in a stable, equatorial
position.
Axial substituents destabilise the hexose rings. If a sufficient number of
hydroxyls or –CH
2
OH groups (or other heavy/bulky groups) are axial, the
2
Unsaturated sugars do exist, for example the structure formed by alginate lyases of by
alkaline
β
-elimination of alginates or pectins
O
1
2 3
4
5
O
CH
2
OH
OH
HO
O
O
O
CH
2
OH
OH
HO
O
O
OH
OH
O
CH
2
OH
O
OH
OH
O
CH
2
OH
O
OH
OH
O
CH
2
OH
O
OH
OH
O
CH
2
OH
O
O
1
2 3
4
5
1
2 3
4
5
4
C
1
1
C
4