Page 105 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Carbohydrates 413

pure mannan, and was, until the advent of plastics, used 15 and 30% pectin, respectively. The pectin extracted from
for making buttons. Many palm kernels contain less pure theseagriculturalwastesisusedinthemanufactureofjams
polymers of mannose, often containing galactose and re- and jellies. It is also used as a stabilizer of fruit drinks and
ferred to as galactomannans. pharmaceutical suspensions because of its great stability
at low pH.
6. Chitin and Chitosans
Chitin and Chitosans are polymers of β-D-glucosamine; V. OLIGONUCLEOTIDES AND
the main difference between them is in DP, the first poly- POLYNUCLEOTIDES
5
mer has a DP around 10 whereas the second has only
4
10 . As a result chitosan is much more soluble than chitin Oligonucleotides are DNA or RNA segments of low
and is therefore easier to handle and to shape; it is used in molecular weight; they are composed of nucleotide
photographic films, for immobilization of enzymes, and monomers (see Fig. 20) linked by phosphoric ester bridges

its phosphate esters as fire retardants. spanning C-5 of one unit and C-3 of the other. Synthetic
oligonucleotides have been widely used in the study of
DNA,inproteinbiosynthesis,andininducedmutagenesis.
7. Hemicelluloses
They were instrumental in deciphering the genetic code,
Hemicellulose is the most abundant naturally occurring and they have since been used to introduce mismatches in
organic compound after cellulose. It differs from cellu- specific sites of DNA strands to induce specific changes
lose in that it dissolves in alkali to form a dark brown in enzymes.
solution known in the paper industry as “black liquor.” More recently, antisense oligonucleotides have been
Some hemicelluloses are linear while others are highly used in the fight against AIDS and cancer. Anitisense
branched. For example, xylans, the most common compo- oligonucleotides were synthesized that complement por-
nent of hemicellulose, are mostly linear polymers made tions of mRNA which translate to the protein cover of the
up of (1 → 4) linked β-D-xylopyranosyl residues, which HIV virus or to a cancer specific oncogene. In the cell,
may occasionally branch at the 3-position and arabino-D- the oligonucleotides complex with the targeted RNA and
galactans which are highly branched polymers. arrest the synthesis vital proteins (see Fig. 21).

8. Pectin A. Synthesis of Oligonucleotides

Pectin is produced in large quantities as by-products of the Both oligodeoxyribonucleotides (DNA-type oligomers)
apple and orange juice productions. The pulp of apples and and oligoribonucleotides (RNA-type oligomers) have
the peels of orange left over after pressing contain about been synthesized, but the first have attracted most of the

FIGURE 20 The four 2-deoxyribonucleotides found in DNA and the four ribonucleotides found in RNA.