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1.8. CARRAGEENANS AND AGAROSE
•
Produced by red algae
•
Industrially important polysaccharides. Gelling and stabilising agents in
the food industry.
•
Repeating structure [A-B]
n
•
κ
- and
ι
-carrageenan (E-407): Sulphate esters. Strong acid, negatively
charged at all pH above 1.
•
κ
-carrageenan: Gels with K
+
(thermoreversible)
Agar (E406): Mixture of agarose (Non-sulphated) and agaropectin (often
removed)
Agarose: Gels by
cooling hot solutions
(bacteriology++)
Note: Agarose has no charged groups. This is an advantage because it does
not interact with charged molecules. For example, using agarose for
electrophoresis of DNA fragments would be difficult if the agarose contained
charges. Also, agarose does not interact with (bind) ions (Ca
++
etc). When
used in microbiology agar gels do not absorb e.g. peptides or factors essential
for bacterial growth.
A: 4-linked 3,6-anhydro-
α
-D-
Gal
B: 3-linked
β
-D-Gal-4-sulfate
A: 4-linked 3,6-anhydro-
α
-D-
Gal-2-sulfate
B: 3-linked
β
-D-Gal-4-sulfate
A: 4-linked 3,6-anhydro-
α
-L-
Gal
B: 3-linked
β
-D-Gal
(no sulphate)
ι
-carrageenan
CARRAG2.CW2
O
OH
CH
2
O
O
CH
2
OH
OH
-
O
3
SO
O
O
κ
-
carrageenan
O
CH
2
O
O
CH
2
OH
OH
HO
O
O
OH
Agarose
O
O
-
O
3
SO
OH
CH
2
OH
O
O
CH
2
OSO
3
-
O