Page 61 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-80 May 25, 2001 20:18

Carbohydrates

Hassan S. El Khadem
The American University

I. Introduction
II. Monosaccharides
III. Oligosaccharides
IV. Polysaccharides
V. Oligonucleotides and Polynucleotides

GLOSSARY Furanoses Aldoses having four carbon atoms or more
and uloses having five carbon atoms or more can form
Aldoses Chiral polyhydroxyalkanals having three or five-membered rings. Such cyclic monosaccharides are
more carbon atoms (polyhydroxyalkanals having fewer called furanoses because they are related to tetrahy-
than three carbons are achiral). Aldoses having enough drofuran. Aldopentoses having such five-membered
carbon atoms to form five- or six-membered oxy- rings are designated aldopentofuranoses, and hexu-
genated rings readily undergo cyclization by in- loses, hexofuranuloses. In solution, furanoses exist
tramolecular nucleophilic attack of suitably located hy- mainly in envelope and twist conformations, alternat-
droxyl groups onto the aldehydic groups. Saccharides ing in a wavelike motion between them.
possessing five-membered rings are called furanoses, Monosaccharides Monomeric saccharides, which are
and those possessing six-membered ones, pyranoses. grouped according to the number of carbon atoms
Anomeric configuration The cyclization of an aldose or in their skeleton into pentoses, hexoses, and so on.
a ulose to form a furanose or a pyranose ring occurs Members of these groups may exist in the form of
by intramolecular nucleophilic attack of a suitably lo- polyhydroxyaldehydes,calledaldoses,orpolyhydroxy
cated hydroxyl group on a carbonyl function (present ketones, called uloses. Accordingly, certain monosac-
in the same molecule). This cyclization results in the charides are designated aldopentoses, other hexuloses,
formation of a chiral hemiacetal group in place of an and so on.
achiral carbonyl group, which increases the number Mutarotation Designates changes in optical rotations
of chiral centers present in the molecule by one. The observed over a period of time. The phenomenon oc-
configuration of the newly created center is designated curs when chiral substances equilibrate in solution
by the greek letter α or β, followed by the D or L no- with one or more isomeric forms. Saccharides usu-
tation of the chiral center farthest from the carbonyl. ally crystallize in structurally pure form; for example,
Thus, one speaks of α-D-aldopento-furanoses or β-L- D-glucose usually separates in the α-D-glucopyranose
hexopyranuloses. form. When this is dissolved in water, it undergoes

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