Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Substituted cyclohexanes with equatorial substituents, and also corresponding
the six-membered rings with heteroatoms, are usually more stable. Conformational
analysis of the pyranose sugars accepts identical geometry for the pyranose and
cyclohexane rings. Thus, the substituted pyranose ring may take one of two chair
conformations denoted as 4 C1 (2.2) or 1 C4 (2.3). The superscript corresponds to the
number of the carbon atom in the upper position of the chair and the subscript to
that in the lower position. In an aqueous solution these conformations reside in
equilibrium, but usually one of conformers preponderates. The preponderance of
either one or the other chair conformation can be estimated by conformational
analysis.
4
H
HO
HO
H
CH 2OH
H
4 C1
H
O
OH
H
OCH3 1
CH 2OH
OCH 3
In monosaccharides, being pyranoses, substituents usually reside in the equatorial
position. These preferences can be rationalized in terms of the energy of interactions
between the axial and equatorial substituents (Table 2.1). Apart from the
nonbonding interactions, the so-called anomeric effect has to be taken into account.
Thus, a polar group at the anomeric carbon atom favors the axial position. The
magnitude of the anomeric effect depends on the configuration of the hydroxyl group
on C-2. In an aqueous solution, the energy associated with the anomeric effect of
the hydroxyl group is estimated at 2.30 and 4.18 kJ mole -1 for the equatorial and
axial group, respectively. In organic solvents of lower dielectric constant, an increase
in the energy of the anomeric effect has been observed. Calculated energies fit
experimental values.
The data in Table 2.1 show that a conformation of the aldohexopyranoses in
aqueous solution depends to a great extent on the orientation of the hydroxymethyl
group. Stability of the conformation is reduced by the bulky axial CH 2OH group,
and most of the β-D-anomers take predominantly the 4 C 1 form, because the 1 C 4 form
involves a strong interaction (10.45 kJ mole -1 ) between the axial hydroxymethyl
group and the axial hydroxyl group.
Changes in free energy associated with the transformation of equatorial into the
axial groups point to the priority for the equatorial alkyl groups at the anomeric
carbon atom, but for polar groups (e.g., the hydroxyl group), the axial position is
preferable. The same reason rationalizes a higher stability of methyl-α-D-glucoside
over its β-isomer. Estimated free energy differences between equatorially and axially
substituted pyranose rings help determine the proportions of stereoisomers. Mutarotation
of D-glucose (Chapter 1) is an excellent example of the application of
H
H
4
OH
H
OH O
1 C4
2.2 2.3
OH
1
H
H