Alginate carboxylic groups

Setting solution Percent of alginate carboxyl groups esterified ... 

Natural polysaccharides available on an industrial scale include polymers with anionic functions, like alginates (carboxylic groups) or carrageenans (sulfonic groups) derived from seaweed, or with cationic functions, like as chitosan (amino groups), obtained by deacetylation of chitin from seafood shells (Fig. 1). 

Pectins are generally classed according to their ester content as high methoxyl pectins (>50% of the carboxyl groups esterified) or low methoxyl pectins (<50% of carboxyl groups esterified) (pectic acid, methyl ester [9049-34-1]). Low methoxyl pectins, like algins, require calcium for gelation. 

There are several reports on the coating of bone-like hydroxyapatite onto natural polymer substrates. Kawashita et at. [57] reported that carboxymethylated chitin and gellan gum gels, which have carboxyl groups, can form hydroxyapatite on their surfaces in SBF if they are treated with a saturated Ca(OH)2 solution in advance, while curdlan gel, which has no carboxyl group, does not form hydroxyapatite in SBF, even if it is treated with Ca(OH)2 solution. These results support the hypothesis that carboxyl groups induce hydroxyapatite nucleation. Kokubo et at. [58,59] reported that non-woven fabrics of carboxymethylated chitin and alginate fibers also form hydroxyapatite on their surfaces in SBF if they are treated with Ca(OH)2 solution. 

Figure 7.2 Schematic representation of alginate cross-linking with divalent cations and the chemical structure of the constituent repeat units, guluronate (G) and mannuronate (M). Carboxylate groups present along the backbone (largely from the G residues) interact with multivalent cations to yield metal ion cross-linked gels.

Solubility is a very important criterium for the different uses of polysaccharides in pharmacy. Table 4 lists the solubility criteria of some polysaccharides in water. Solutions containing sugars and alcohol generally depress the solubility of polysaccharides. Polysaccharides containing carboxyl groups, i.e., pectins, alginates, and carboxy-methylcellulose, are insoluble at low pH values. They will be precipitated when the pH is lowered below 3. 

To explain the appreciable solubility of the product, Rogovin et al. suggest that the carboxylic groups of alginic acid are probably internally esterified with hydroxyl groups. Thus (as with pectic acid nitrates, see above) no cross-links can be formed of the type assumed by the same authors to exist in celluronic acids and in celluronic acid nitrates (see below). 

The Cdi ions are ordered along rows bridging intra or inter molecular carboxylic groups. This arrangement is consistent with the egg and box model proposed for alginates and pectates [96] (Fig. 16). 

Pectin consists predominantly of sequences of galacturonic acid residues (which are quite similar to the G units in alginate), with occasional interruptions by rhamnose residues. At least some of the carboxyl groups are methyl esterified, the precise distribution depending upon the plant source and age, and an important aspect still not fully understood. Reasonably in view of their structural similarity, pectins of low degree of esterification behave like alginates, and gel with divalent ions. The more esterified materials gel under conditions of low pH and decreased water activity, Le. where intermolecular electrostatic repulsions are reduced in this case the junction zones are thermoreversible at, say, 40 C. 

Alginates are formed from poly-D-mannopyranosyluronic acid by the action of a C-5 epimerase that inverts the C-5 carboxyl group to give approximately 33% (w/w) of the... 

Amination can be achieved by reductive amination of carboxyl groups using sodium cyano-borohydride [124]. By this method, 2-amino-2-deoxy functionalized amylose and cellulose have been produced. Reductive amination can also be applied to the synthesis of other amino derivatives of various oxidized polysaccharides such us xanthan [125] or alginic acid and galactomannans [126]. 

The PhEur 2005 describes alginic acid as a mixture of polyuronic acids [(CgHgOeU composed of residues of D-mannuronic and L-glucuronic acid, and is obtained mainly from algae belonging to the Phaeophyceae. A small proportion of the carboxyl groups may be neutralized. 

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