232 M. Luz Sanz and Isabel Mart ́ınez-Castro
7.2 Chemical Structure
Monosaccharides are the basic pieces which constitute all higher carbohydrates.
They are polyhydroxyaldehydes and polyhydroxyketones, with 3–8 carbon atoms.
Sugars that contain four or more carbons can exist in a chain (open) form or a hete-
rocyclic ring form. The carbonyl groups (aldehyde or ketone) react reversibly with
an intramolecular hydroxyl to form hemiacetals, giving cyclic forms of five mem-
bers (furanoses) or six members (pyranoses). When a reducing sugar is in solution,
the different forms ( and , pyranose and furanose) spontaneously interconvert
through the open form until they reach equilibrium. This process can take several
hours at room temperature. The most common monosaccharides are glucose and
fructose, in aqueous solution the most abundant isomers being the pyranose forms
of glucose and the anomers (both pyranose and furanose) of fructose (Fig. 7.1).
Other functional groups are also found in natural sugars such as carboxylic
acids (aldonic, uronic, aldaric) able to form lactones (reversibly), amines or amides
(aminosugars) and both acid and amine groups (sialic acids).
7.3 Properties
7.3.1 Isomerism
Carbohydrates display a really high degree of isomerism. It is possible to find:
- Anomers, stereoisomers which differ in the position of thesubstituentof the
hemiacetal or acetal linkage (anomeric center). They are named when exo-
cyclic O at the anomeric center is on the opposite face to the−CH 2 OH group,
and when the exocyclic O at the anomeric center is on the same face as the
−CH 2 OH group (See Fig. 7.1).- Enantiomers, stereoisomers that are mirror images of each other likeL-andD-
arabinose (Fig. 7.2).
Most natural sugars areD-isomers (glucose, fructose, sucrose, cellulose), but
there is a number of sugars appearing in natural products which correspond toL-
series (arabinose, sorbose, rhamnose):
- Diastereomers, stereoisomers that are not enantiomers (e.g.D-ribose andD-
arabinose; Fig. 7.3)
- Epimers, diastereomers that differ only in a single chiral carbon that is not the
reference carbon, such asD-glucose andD-galactose which differ only in -OH
orientation at C-4. (e.g.D-glucose andD-galactose are epimers in C-4; Fig. 7.4)- Regioisomers,which differ in the position of a substituent on the ring, as maltose
and kojibiose (Fig. 7.5)