Page 66 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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374 Carbohydrates

TABLE IV Polysaccharides
Homopolysaccharides Heteropolysaccharides
Linear Branched Linear Branched
Simple polysaccharides
Amylose (α-D-glucan) Amylopectin Mannans Gums
Cellulose (β-D-glucan) Glycogen Xylans Mucilages
Chitin (D-glucosaminan) Pectins
Algin
Agar
Bacterial polysaccharides
Conjugate polysaccharides Peptidoglycans
Glycoproteins
Lectins

afford on depolymerization only mono- and oligosac- carbohydrate-containing antibiotics may be monosaccha-
charides or their derivatives (esters or ethers) and ride or oligosaccharide in nature.
(2) conjugate polymers made up of a polysaccharide
linked to another polymer, such as a peptide or a protein
(to form a glycopeptide or a glycoprotein). True polysac- II. MONOSACCHARIDES
charides are, in turn, grouped into two major classes: (1)
homopolysaccharides, which are polymers having as a re- Monosaccharides (monomeric sugars) exist as chiral poly-
peating unit (monomer) one type of monosaccharide; and hydroxyalkanals, called aldoses, or chiral polyhydrox-
(2) heteropolysaccharides, which are made up of more yalkanones, called ketoses, which are further classified
than one type of monosaccharide. Because the shape of (see Tables I and II) according to the number of carbon
polymers significantly influences their physical proper- atoms in their chains into trioses, tetroses, pentoses, hex-
ties, each of these types of polymer is further divided into oses, and so on, and according to the type of ring they form
linear and branched polysaccharides (Table IV). into furanoses and pyranoses (five- and six-membered
rings).

4. DNA, RNA, Nucleotides, and Nucleosides
A. Structure, Configuration, and Conformation
Unlike oligo- and polysaccharides, which are polyacetals of Monosaccharides
linked by oxygen bridges, DNA and RNA are polyesters 1. Structure
linked by phosphate bridges. DNA is the largest known
polymer; its DP exceeds 10 12 in human genes and de- The presence of an aldehyde group in aldoses such as
creases as the evolution ladder is descended. This giant D-glucose was established by the fact that (1) aldoses react
molecule plays a key role in replication and in transcrip- with carbonyl-group reagents, giving oximes and hydra-
tion. It achieves the latter by doubling one of its strands zones; and (2) aldoses are oxidized to acids that possess
with a smaller polymer, mRNA, which, in turn, binds the same number of carbon atoms. Furthermore, by es-
with a string of oligomers, tRNA, to form the peptide timating the number of acetyl groups in fully acetylated
chain. The monomers of both DNA and RNA are made aldoses, it is possible to determine the number of hydroxyl
up of phosphorylated 2-deoxy-D-erythro-pentofuranosyl- groups present in the starting aldoses (e.g., five OH groups
and D-ribofuranosyl-purine and -pyrimidine bases, des- in aldohexoses) and to determine their structural formulas.
ignated nucleotides. The latter can undergo hydrolysis of
their phosphoric ester groups to afford simpler monomers,
2. Configuration
the nucleosides. Thus it is apparent that this group can also
be divided according to DP into monomers (nucleosides The system commonly used to represent three-
and nucleotides), oligomers (tRNA), and polymers (DNA dimensional linear molecules such as monosaccharides
and mRNA). two dimensionally is the Fischer projection formula. This
Some carbohydrate derivatives may belong to more affords an unambiguous way of depicting monosaccha-
than one of the above-mentioned groups. For example, rides, as follows. (1) The carbon chain is drawn vertically,