2009_Book_FoodChemistry

4.2 Monosaccharides 257
Fig. 4.3. Proton resonance spectrum of D-glucose (in D 2 O)
are soluble to a small extent in ethanol and are
insoluble in organic solvents such as ether, chloroform
or benzene.
molecular weights:
(4.21)
4.2.2.2 Optical Rotation, Mutarotation
Specific rotation constants, designated as [α] for
sodium D-line light at 20–25 ◦ C, are listed in Table
4.8 for some important mono- and oligosaccharides.
The specific rotation constant [α] t λ
at
a selected wavelength and temperature is calculated
from the angle of rotation, α, by the equation:
(4.20)
where l is the polarimeter tube length in decimeters
and c the number of grams of the optically
active sugar in 100 ml of solution. The molecular
rotation, [M], is suitable for comparison of
the rotational values of compounds with differing
where M represents the compound’s molecular
weight. Since the rotational value differs for
anomers and also for pyranose and furanose
conformations, the angle of rotation for a freshly
prepared solution of an isomer changes until an
equilibrium is established. This phenomenon is
known as mutarotation. When an equilibrium
exists only between two isomers, as with glucose
(α- and β-pyranose forms), the reaction rate
follows first order kinetics:
(4.22)
A simple mutarotation exists in this example,
unlike complex mutarotations of other sugars,
e. g., idose which, in addition to pyranose, is also
largely in the furanose form. Hence, the order of
its mutarotation kinetics is more complex.