Agarans: sulfated precursors and derivatives from agarose, and related sulfated galactans

MARINA CIANCIA; MARÍA C. MATULEWICZ

Sulfated Polysaccharides

Nova Science Publishers, Inc.

Año: 2015; p. 199 - 216

Agarans are sulfated galactans biosynthesized by red seaweeds; they are constituted by alternating 3-linked -D-galactose units and 4-linked -L-galactose units with different degree of cyclization to give 3,6-anhydro--L-galactose. The neutral, completely cyclized agaran is known as agarose and is the major component of the products industrially obtained mainly from seaweeds of the orders Gelidiales, Anhfeltiales, and Gracilariales. Its gelling properties have a wide range of industrial uses. The biological precursor structure of agarose is usually known as porphyran, being found in important amounts in seaweeds of the genus Porphyra (Bangiales). The ideal structure of this galactan is composed of alternating 3-linked -D-galactose and 4-linked -L-galactose 6-sulfate units. The latter residues are cyclized by specific enzymes in the cell wall regulating the physical characteristics of the amorphous matrix. The same reaction, but in the presence of alkali, is used to improve the rheological properties of extracts from different sources with commercial purposes. Besides, water extracts from agarophytes usually comprise not only major amounts of agarose, which is only obtained with hot water or hot water solutions, but also minor quantities of galactans with related structures but with a certain degree of substitution with sulfate ester groups, methyl ethers, pyruvate ketals, D-xylose and/or 4-O-methyl-L-galactose side chains, and different percentages of 3,6-anhydrogalactose, depending on the source and sample work up. These kind of polymers constitute the major sulfated galactans from Ceramiales, while those obtained from calcareous seaweeds of the order Corallinales are characterized by important amounts of single stubs of D-xylose, a complex methylation pattern, and by the absence of 3,6-anhydrogalactose units. The presence of substituents, as well as non-cyclized units in the backbone, precludes helix formation and gelling in aqueous systems.