472
CARBOHYDRATES
conformers, in that the left-hand conformer of glucose
is termed
4 C 1 , and the right-hand one
1 C 4 . The ‘C’
indicates chair conformation, the superscript numeral
is the carbon atom that is above the plane of the
ring, and the subscript numeral is which carbon atom
is below the plane of the ring. For this description,
we consider the pyranose ring as originally planar,
distorted to a chair by pushing carbons 1 and 4 out
of the plane.
O
HO
HO
OH
HO
OH
β-D-glucose
O
OH
OH
OH
OH
OH
1
4
4
1
4 C 1 conformer
1 C 4 conformer
At first glance, the preferred conformation for
L-hexoses, e.g. α-L-rhamnose, appears different from
that of the D-hexoses. This is readily rationalized by considering the preferred conformation of
α-L-glucose – the α-anomer is chosen simply because
we can easily follow the anomeric substituent. Since
α-L-glucose is the enantiomer of α-D-glucose, we can
draw the mirror image representation, then rotate this
so that the heterocyclic oxygen comes to the required
position.
O
HO
HO
OH
HO
O
OH
OH
HO
HO
O
HO
HO
HO
OH
α-D-glucose
preferred conformation
α-L-glucose
mirror image
≡
rotate 180º
α-L-glucose
OH
HO
OH
Note that you may also encounter another version of the cyclic form referred to as the Haworth
representation (see Box 3.16). This shows the ring
as a planar system, and is commonly used in
biochemistry books. However, we know that fivemembered and six-membered rings are certainly not
planar. The Haworth representation nicely reflects
the up–down relationships of the various substituent
groups, but is quite uninformative about the shape of
the molecule, and whether the substituents are equatorial or axial. In really bad cases, authors omit the
hydrogen atoms, giving an ambiguous structure – do
lines mean methyl or hydrogen? Haworth representations may be easier to draw, but you are strongly
encouraged to use the more informative conformational structures.
O
HO
HO
OH
HO
OH
O
HO
OH
HO
OH
H
O
HO
OH
H
H
OH
H
OH
CH 2 OH
H
HOH 2 C
H
OH
H
H
OH
OH
H
O
O
HO
OH
OH
OH
CH 2 OH
often seen, but careless
versions that omit
hydrogens; don't think
the lines mean methyls!
β-D-glucose
β-D-ribose
Haworth
representation
conformational
representation
Haworth
representation
conformational
representation
HOH 2 C
OH
OH
OH
O
CARBOHYDRATES
conformers, in that the left-hand conformer of glucose
is termed
4 C 1 , and the right-hand one
1 C 4 . The ‘C’
indicates chair conformation, the superscript numeral
is the carbon atom that is above the plane of the
ring, and the subscript numeral is which carbon atom
is below the plane of the ring. For this description,
we consider the pyranose ring as originally planar,
distorted to a chair by pushing carbons 1 and 4 out
of the plane.
O
HO
HO
OH
HO
OH
β-D-glucose
O
OH
OH
OH
OH
OH
1
4
4
1
4 C 1 conformer
1 C 4 conformer
At first glance, the preferred conformation for
L-hexoses, e.g. α-L-rhamnose, appears different from
that of the D-hexoses. This is readily rationalized by considering the preferred conformation of
α-L-glucose – the α-anomer is chosen simply because
we can easily follow the anomeric substituent. Since
α-L-glucose is the enantiomer of α-D-glucose, we can
draw the mirror image representation, then rotate this
so that the heterocyclic oxygen comes to the required
position.
O
HO
HO
OH
HO
O
OH
OH
HO
HO
O
HO
HO
HO
OH
α-D-glucose
preferred conformation
α-L-glucose
mirror image
≡
rotate 180º
α-L-glucose
OH
HO
OH
Note that you may also encounter another version of the cyclic form referred to as the Haworth
representation (see Box 3.16). This shows the ring
as a planar system, and is commonly used in
biochemistry books. However, we know that fivemembered and six-membered rings are certainly not
planar. The Haworth representation nicely reflects
the up–down relationships of the various substituent
groups, but is quite uninformative about the shape of
the molecule, and whether the substituents are equatorial or axial. In really bad cases, authors omit the
hydrogen atoms, giving an ambiguous structure – do
lines mean methyl or hydrogen? Haworth representations may be easier to draw, but you are strongly
encouraged to use the more informative conformational structures.
O
HO
HO
OH
HO
OH
O
HO
OH
HO
OH
H
O
HO
OH
H
H
OH
H
OH
CH 2 OH
H
HOH 2 C
H
OH
H
H
OH
OH
H
O
O
HO
OH
OH
OH
CH 2 OH
often seen, but careless
versions that omit
hydrogens; don't think
the lines mean methyls!
β-D-glucose
β-D-ribose
Haworth
representation
conformational
representation
Haworth
representation
conformational
representation
HOH 2 C
OH
OH
OH
O