Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
other. The aqueous solution of one enantiomer twists the plane of passing polarized
light to the left and that of the other twists it to the right. Because of the environment
of the H–2C–OH moiety (a rotaphore), the angle of the twist in both directions is
identical. The angle of the twist in other chiral carbohydrates, including those with
higher number of rotaphores, results from the additivity of the chiral effects specific
for every rotaphore present in a given molecule.
In his fundamental studies, Emil Fischer assigned the direction of optical rotation
in (1.1) and (1.2) to the structure of its rotaphore. Thus, (1.1) appeared to be a righttwisting
compound [(D) from dextra ] and (1.2) a left-twisting compound [(L) from
levo]. The following principle linked the steric configuration of (1.1) and (1.2) to
experimental chirality. The one that takes position at C-2 of a given saccharide
looking straight toward the H-atom of that rotaphore has to turn right in order to
see the most oxidized group of the molecule (the aldehyde or keto group) and belongs
to the D-family of saccharides. Because this condition is met in (1.1), it is the mother
compound of the D-family, and for the same reason (1.2), the L-molecule, opens
the L-family of saccharides.
Subsequent members of the families of oses are formally formed by inserting
one, two, or more H–C–OH or HO–C–H moieties between the aldehyde group and
the before last H–C*–OH group in (1.1) and the before last HO–C*–H group in
(1.2) and subsequently formed family members. Structures of two tetroses, four
pentoses, eight hexoses, and so on are designed in such a manner in each group.
Structures (1.1) and (1.2) are mirror images, because of which they are called
enantiomers. It is easy to check that designed structures of higher oses within a
given D- or L-family cannot be enantiomers with respect to one another. Because
they are isomeric but not enantiomeric, they are named diastereoisomers. It is also
easy to check that enantiomers of particular members of the D-family can be found
among members of the L-family. Such cases also exist within the families of D- and
L-uloses. The operations start from inserting the H–C–OH or HO–C–H fragments,
or both, between the C=O and before last H–C*–OH and HO–C*–H groups of Dand
L-tetruloses, respectively.
The Fischer link of absolute configuration to direction of the experimental
chirality is valid only for trioses and tetruloses. Thus, for (1.1) and (1.2) one may
write D(+)-glyceric aldehyde and L(−)-glyceric aldehyde, where the notations (+)
and (−) are related to experimental chirality of right and left, respectively. Saccharides
being D(−) and L(+) are common.
1.3 FURTHER REMARKS ON THE STRUCTURE
One should note that chain structures designed in the manner presented above reflect
only to a limited extent the real structure of these saccharides. Open-chain structures
are the only structures of trioses and tetroses, logically, tetruloses. Pentoses, hexoses,
pentuloses, and hexuloses take cyclic structures. Such cyclization is common in
chemistry because it is energetically beneficial. The energy of five- and six-membered
cyclic saccharides is lower than that of corresponding open-chain isomers.
Cyclization in open-chain structures involves the electron gap at the carbonyl carbon
atom and a lone electron pair orbital of the oxygen atom of this hydroxyl group,