Food Chemistry

4.2 Monosaccharides 257

Fig. 4.3.Proton resonance spectrum ofD-glucose (in D 2 O)

are soluble to a small extent in ethanol and are
insoluble in organic solvents such as ether, chlo-
roform or benzene.

4.2.2.2 OpticalRotation,Mutarotation

Specific rotation constants, designated as [α]for
sodiumD-line light at 20–25◦C, are listed in Ta-
ble 4.8 for some important mono- and oligosac-
charides. The specific rotation constant[α]tλ at
a selected wavelength and temperature is calcu-
lated from the angle of rotation,α, by the equa-
tion:

(4.20)

where l is the polarimeter tube length in decime-
ters and c the number of grams of the optically
active sugar in 100 ml of solution. The molecu-
lar rotation, [M], is suitable for comparison of
the rotational values of compounds with differing

molecular weights:

(4.21)

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-andβ-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.