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The diaxial GG linkage produces a cavity in the chain. Note that the
diequatorial MM results in an extended chain similar to cellulose (which
shares the same geometry). Also note that two adjacent M residues are
rotated about 180° relative to one another (also like in cellulose).
1.2.14. Gelation with calcium ions: Cross-­‐linking of G-­‐blocks
The GG cavity is almost certainly the basis for the selective binding of Ca++ to
alginates. The Ca++ cation binds (coordinates) with several –OH and –COO-
groups.
The gelling of alginates with calcium ions happens only when the alginate
contains longer G-blocks, typically NG>1 = 5-6 or
larger. Several studies have indicated that G-blocks
form the junction sones where two chains
associate, mediated by calcium ions. The regions
containing M-blocks or irregular sequences will not
associate. Instead they are ‘soluble’, but complete
dissolution is prevented by the junction zones. Ca-
alginate is an example of a hydrogel, containing
water-soluble polymers cross-linked (covalently,
ionically or by other attractive forces) at a few point
along the chain.
Upon addition of more Ca++ the junction zones in
Ca-alginate tend to associate slowly, producing
thicker junctions.
O
OH
OH
O
O-
O
O
OH
OH
O - O
O
OH
HO
O-
O
O O
OH
HO
O-
O
O
G G
M M
Figure 19
Figure 20. Gelation of
alginates with calcium
salts