Alginic acid

From Freepedia

Alginic acid (algine, alginate) is a viscous gum that is abundant in the cell walls of brown algae.

Structure

Chemically, it is a linear copolymer with homopolymeric blocks of (1-4)-linked ß-D-mannuronate (M) and its C-5 epimer α-L-guluronate (G) residues, respectively, covalently linked together in different sequences or blocks.

The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks), alternating M and G-residues (MG-blocks) or randomly organized blocks. The relative amount of each block type varies both with the origin of the alginate. Alternating blocks form the most flexible chains and are more soluble at lower pH than the other blocks. G-blocks form stiff chain elements, and two G-blocks of more than 6 residues each form stable cross-linked junctions with divalent cations (e.g. Ca²⁺, Ba²⁺, Sr²⁺ among others) leading to a three-dimensional gel network. At low pH, protonized alginates will form acidic gels. In these gels, it is mostly the homopolymeric blocks that form the junctions, where the stability of the gel is determined by the relative content of G-blocks.

Forms

Commercial varieties of alginate are extracted from seaweed. Certain bacteria, however, also produce alginate.

Alginate ranges from white to yellowish brown, and takes filamentous, grainy, granular, and powdered forms. It is insoluble in water and organic solvents, and dissolves slowly in basic solutions of sodium carbonate, sodium hydroxide and trisodium phosphate.

Uses

Purified forms of alginate are used in antacid preparations such as Gaviscon®, Bisodol®, Asilone®, and Boots Own® tablets. Alginate is used extensivly as a mold-making material in dentistry and prosthetics, and in textiles. It is also used in the food industry, for thickening soups and jellies.