Food Chemistry

254 4 Carbohydrates

The reaction rate for the conversion of the
α-andβ-forms has a wide minimum in an aque-
ous 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 (Reevesformulas).
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 ofD-pyranoses, with
most of the bulky groups, e. g., HO and, espe-
cially, CH 2 OH, occupying the roomy equatorial
positions. The interaction of the bulky groups is
low in such a conformation, hence the confor-
mational 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 equatori-
ally, 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 be-

tween substituents on the tetrahydropyran ring

Energy

Interaction kJ/molea

Hax−Oax 1. 88

Hax−Cax 3. 76

Oax−Oax 6. 27

Oax−Cax 10. 45

Oeq−Oeq/Oax−Oeq 1. 46

Oeq−Ceq/Oax−Ceq 1. 88

Anomeric effectb

for Oc2eq 2. 30

for Oc2ax 4. 18

aAqueous solution, room temperature.

bTo 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 OH-

group, which is equatorial. However, the^1 C 4 -

conformation is thermodynamically more favor-

able despite the fact that the CH 2 OH-group is ax-

ial (cf. Table 4.5):

(4.16)

A second exception (or rather an extreme case) is

α-D-altropyranose. Both conformations (O-out-

side and O-inside) have practically the same sta-

bility 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 enthalpiesG◦

calculated from these data for various conformers

are presented in Table 4.5. In addition to the

interaction energies an effect is considered