Page 338 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Plants 301

the name given to a number of sulfated polysaccharides found in many red seaweeds where they
play a structural role. The approximate repeat units for two industrially important carrageenans
are given below. Both are able to form double helices containing two parallel staggered chains
creating a gel. There are three disaccharide units per helix turn. The sulfate units are located on
the outside of the helix with the helical structure stabilized by internal hydrogen bonds. In nature,
red seaweeds contain an enzyme that converts the galactose-6-sulfate of the k-carrageenan to 3,6-
anhydrogalactose that causes a stiffening of the helix. It has been found that red seaweeds found
where there is strong wave action contain a high proportion of anhydrogalactose. Thus, it appears
that the seaweed is able to control its structure in response to external stimuli to minimize shred-
ding by the increased wave action.
Because of its gelling ability, carrageenan is widely used as food thickeners and emulsion stabi-
lizers in the food industry and is present in many dairy products, including less-expensive ice cream
and other desert products, providing a smooth, creamy texture. It is used as a stabilizer in foods such
as chocolate milk.

O
O
O O S O
O S O
OH O O S O OH O
O O O O H
H O O O O H
H H H H H
H H H R H OH H R
R H OH H O O O R H H OH H H H O O
S
O O S O
O
O
(9.30a) (9.30b)

O
O O S O
O S O
OH O O S O OH O
O O O O H
H O O O O H
H H H H H
H H H R H OH H R
R H OH H OH O H H H O
R H OH H OH
(9.31a) (9.31b)

The name agar refers to a family of polysaccharides that contain alternating β-d-galactopyranose
and 3,6-anhydro-α-l-galactopyranose units and is thus similar to a carrangeenin where the anhydro-
l-galactose is substituted for the anhydro-d-galactose. It is employed as the basis of many microbio-
logical media and in canned food because it can be sterilized. The latter is an advantage over gelatin
that is not able to withstand sterilization.
Agarose is the agar polysaccharide with the greatest gelling tendency. It contains no glucuronic
acid units. It can form a compact double helix with the two chains being parallel and staggered, as
in the case of carrageenan, forming a gel. Agarose gels are employed in gel-permeation chromatog-
raphy (GPC) and gel electrophoresis.

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