Alginic acid, molecular weight

Alginates are polysaccharides that are linear copolymers of (3-D-mannuronic acid and a-L-guluronic acid residues with (1 4) linkages, as shown in Figure 14.13 [24]. The molecular chain is formed by homopolymeric blocks or heteropolymeric blocks of the two monomers, (3-D-mannuronic acid or a-L-guluronic acid. Alginates exhibit molecular weights between 20 and 600 kDa [3]. 

Microspheres (10 to 200 pm in diameter) of poly(fumaric acid—cose-bacic acid) anhydride (20 80) [P(FA SA)] were shown to exhibit very strong and pronounced mucoadhesive properties both in vitro and in vivo.143 145 The microspheres were tested for their effect on GI transit of low-molecular-weight drugs salicylic acid and dicumarol. As compared with the control (drug-loaded alginate microspheres of similar size), the P(FA SA) microspheres significantly delayed the GI transit of these drugs in rats.145... 

Investigations by X-ray methods have indicated that alginic acid possesses a fiber structure analogous to that of cellulose.86 An estimate of the molecular weight was obtained by Heen87 from a study of the viscosity of solutions of the acid in 2 JV sodium hydroxide. The values found, (14,100-15,400), corresponded to a degree of polymerization of about 80, but these were considered to be approximations only, as it was uncertain whether or not Staudinger s formula could be applied. 

The molecular weight of the sodium salt of alginic acid has been determined by measurements of osmotic pressure in sodium chloride solution.88 The n/C versus C curve was found to be independent of salt concentration for values between 0.2 N and 0.5 JV, and the slope of the curve was the same for a series of seven fractions with values of molecular weight between 48,000 and 186,000 (220-860 D-mannuronic acid units). Intrinsic viscosity measurements showed the coefficient a in the modified Staudinger s equation88 to be 1.0 and the approximate value of Km to be 15(10)-4. 

A)H Dextran and alginic acid Hydroxyethyl cellulose and hydroxypropyl cellulose Polyvinyl alcohol, polyhydroxyethyl methacrylate, and low-molecular-weight polyethylene oxide... 

In contrast, low-methoxy (LM) pectins gel in the presence of divalent cations, especially calcium, by the egg box mechanism proposed for alginates. Moreover, calcium pectinate gels prepared at neutral pH are heat stable, whereas acidic pH gels are thermo-reversible. Gel strength depends on the extent of esterification (levels from 30% to 50% are optimal), the distribution of ester groups on the chain, and the average molecular weight. LM pectins have been used traditionally in antidiarrheal formulations with kaolin. HM pectins were evaluated in controlled release matrix formulations. Pectin microspheres were reported to improve ophthalmic bioavailability of piroxicam in rabbits compared with commercial piroxicam eye drops. ... 

In the present study, the polysaccharide alginic acid (AlA) with molecular weight (Mw) of 250 000 was chosen as a polyanion. The strong basic protein protamine sulfate (PtS) with Mw about 5000 was used as a polycation. The calcium carbonate matrices were successively employed as a core material because they dissolve in mild conditions and are non toxic. Our motivation to study this combination was to reach the more biocompatible shell composition. 

Alginic acid is a linear glycuronan polymer consisting of a mixture of P-(l—>4)-D-mannosyluronic acid and a-(1 4)-L-gulosyluronic acid residues, of general formula (CeHgOlK. The molecular weight is typically 20 000-240 000. 

Viscosity (dynamic) various grades of alginic acid are commercially available that vary in their molecular weight and hence viscosity. Viscosity increases considerably with increasing concentration typically a 0.5% w/w aqueous dispersion will have a viscosity of approximately 20 mPa s, while a 2.0% w/w aqueous dispersion will have a viscosity of approximately 2000 mPa s. The viscosity of dispersions decreases with increasing temperature. As a general rule, a 1°C increase in temperature results in a 2.5% reduction in viscosity. At low concentrations, the viscosity of an alginic acid dispersion may be increased by the addition of a calcium salt, such as calcium citrate. See also Sections 11 and 18. 

Alginic acid hydrolyzes slowly at warm temperatures producing a material with a lower molecular weight and lower dispersion viscosity. 

Crosslinking addition of a calcium salt, such as calcium citrate or calcium chloride, causes crosslinking of the alginic acid polymer resulting in an apparent increase in molecular weight. Films crosslinked with triphosphate (tripolyphosphate) and calcium chloride were found to be insoluble but permeable to water vapor. Drug permeability varies with pH and the extent of crosslinking. ... 

Draget, K.I. Skjak-Braek, G. Smidsrod, O. Alginic acid gels the effect of alginate chemical composition and molecular weight. Carbohydr. Polym. 1994, 25, 31. 

Alginates are composed of (1—>4)-linked (3-D-mannuronic acid (M units) and a-L-guluronic acid (G units) monomers (Figure 16.9), with different sequential distribution along the polymer chain [142]. The composition, sequence, and molecular weight generally determine the physical properties of alginate [143]. 

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