2009_Book_FoodChemistry

254 4 Carbohydrates
The reaction rate for the conversion of the
α-and β-forms has a wide minimum in an aqueous
medium in a pH range of 2–7, as illustrated
in section 10.1.2.2 with lactose, and the rate
increases rapidly beyond this pH range.
4.2.1.3 Conformation
A series of physicochemical properties of
monosaccharides can be explained only by the
conformation formulas (Reeves formulas).
The preferred conformation for a pyranose is
the so-called chair conformation and not the
twisted-boat conformation, since the former has
the highest thermodynamic stability. The two
chair C-conformations are 4 C 1 (the superscript
corresponds to the number of the C-atom in the
upper position of the chair and the subscript to
that in the lower position; often designated as
C1 or “O-outside”) and 1 C 4 (often designated
as 1C, the mirror image of C1, and C-1 in
upper and C-4 in lower positions, or simply the
“O-inside” conformer). The 4 C 1 -conformation
is preferred in the series of D-pyranoses, with
most of the bulky groups, e. g., HO and, especially,
CH 2 OH, occupying the roomy equatorial
positions. The interaction of the bulky groups is
low in such a conformation, hence the conformational
stability is high. This differs from the
C 4 -conformation, in which most of the bulky
groups are crowded into axial positions, thus
imparting a thermodynamic instability to the
molecule (Table 4.4).
β-D-Glucopyranose in the 4 C 1 -conformation is an
exception. All substituents are arranged equatorially,
while in the 1 C 4 all are axial (Formula 4.15).
α-D-Glucopyranose in the 4 C 1 -conformation has
one axial group at C-1 and is also lower in energy
by far (cf. Table 4.5).
(4.15)
Table 4.4. Free energies of unfavorable interactions between
substituents on the tetrahydropyran ring
Interaction
Energy
kJ/mole a
H ax − O ax 1.88
H ax − C ax 3.76
O ax − O ax 6.27
O ax − C ax 10.45
O eq − O eq /O ax − O eq 1.46
O eq − C eq /O ax − C eq 1.88
Anomeric effect b
for O c2
eq 2.30
for O c2
ax 4.18
a Aqueous solution, room temperature.
b To be considered only for an equatorial
position of the anomeric HO-group.
The arrangement of substituents differs e. g., in
α-D-idopyranose. Here, all the substituents are
in axial positions in the 4 C 1 -conformation (axial
HO-groups at 1, 2, 3, 4), except for the CH 2 OHgroup,
which is equatorial. However, the 1 C 4 -
conformation is thermodynamically more favorable
despite the fact that the CH 2 OH-group is axial
(cf. Table 4.5):
(4.16)
A second exception (or rather an extreme case) is
α-D-altropyranose. Both conformations (O-outside
and O-inside) have practically the same stability
in this sugar (cf. Table 4.5).
The free energy of the conformers in the
pyranose series can be calculated from partial
interaction energies (derived from empirical
data). Only the 1,3-diaxial interactions (with
exception of the interactions between H-atoms),
1,2-gauche or staggered (60 ◦ ) interactions of
two HO-groups and that between HO-groups
and the CH 2 OH-group will be considered.
The partial interaction energies are compiled
in Table 4.4, the relative free enthalpies G ◦
calculated from these data for various conformers
are presented in Table 4.5. In addition to the
interaction energies an effect is considered