2009_Book_FoodChemistry

256 4 Carbohydrates
tion a mixture exists of conformers similar in energy
(cf. Formula 4.18).
(4.18)
An anomeric effect preferentially forces the
anomeric HO-group into the axial position.
This is especially the case when the HO-group
attached to C-2 is also axial. When pyranose
ring formation is prevented or blocked, as in 5-
O-methyl-D-glucose, the twisted 3 T 2 -conformer
becomes the dominant form:
(4.19)
A pyranose is generally more stable than a furanose,
hence, the former and not the latter conformation
is predominant in most monosaccharides
(Table 4.6).
The composition of isomers in aqueous solution,
after equilibrium is reached, is compiled for
a number of monosaccharides in Table 4.6.
Evidence for such compositions is obtained by
polarimetry, by oxidation with bromine, which
occurs at a much higher reaction rate with β-than
α-pyranose and, above all, by nuclear magnetic
resonance spectroscopy ( 1 H-NMR).
In proton magnetic resonance spectroscopy of
sugars, the protons bound to oxygen, which
complicate the spectrum, are replaced by derivatization
(O-acyl derivatives) or are exchanged for
deuterium when the sugar is in D 2 O solution.
The chemical shift of the retained protons
bound covalently to carbon varies. Due to less
shielding by the two oxygens in α position, the
proton on the anomeric carbon atom appears at
a lower magnetic field than other protons, the
chemical shift increasing in the order pyranoses
< furanoses in the range of δ = 4.5−5.5 (free
monosaccharides). As a result of the coupling
with the H-atom at C-2, the anomeric proton
appears as a doublet. Furthermore, an axial
proton (β-form of D-series) appears at higher
field than an equatorial proton (α-form of
D-series). The sugar conformation is elucidated
from the coupling constant of neighboring
protons: equatorial–equatorial, equatorial–axial
(small coupling constants) or axial-axial (larger
coupling constants).
The proton resonance spectrum of D-glucose
( 1 C 4 -conformation) in D 2 O is shown in Fig. 4.3.
The figure first shows the signals of the protons
at C-2 to C-6 in the range of 3.2–3.9 ppm.
The large coupling constant of the doublet at
δ 4.62 (7.96 Hz) shows a diaxial position of the
H-atoms at C-1/C-2 and, thus, the equatorial
position of the hydroxy group at C-1. This
indicates the β-D-glucopyranose conformation.
The equatorial proton in α-D-glucopyranose
(5.2 ppm) appears at lower field (higher ppm).
The smaller coupling constant of the doublet at
δ 5.2 (J= 3.53 Hz) confirms the axial/equatorial
arrangement of the H-atoms at C-1/C-2 of
α-D-glucopyranose.
The content of both anomers in aqueous solution
can be calculated from the signal areas. α- and
β-Glucofuranoses are not present in aqueous solution
(Table 4.6).
Elucidation of sugar conformation can also be
achieved by 13 C-nuclear magnetic resonance
spectroscopy. Like 1 H-NMR, the chemical shifts
differ for different C-atoms and are affected by
the spatial arrangement of ring substituents.
4.2.2 Physical Properties
4.2.2.1 Hygroscopicity and Solubility
The moisture uptake of sugars in crystallized
form is variable and depends, for example, on the
sugar structure, isomers present and sugar purity.
Solubility decreases as the sugars cake together,
as often happens in sugar powders or granulates.
On the other hand, the retention of food moisture
by concentrated sugar solutions, e. g., glucose
syrup, is utilized in the baking industry.
The solubility of mono- and oligosaccharides in
water is good. However, anomers may differ substantially
in their solubility, as exemplified by
α-andβ- lactose (cf. 10.1.2.2). Monosaccharides