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Galactose moieties have been introduced on the uronic groups of alginates from different sources via an N-glycosidic bond, thus affecting the net charge on the polymer chain. The modified polymers have been analyzed by means of viscosity and of high-performance size-exclusion chromatography combined with refractive index multiple angle laser light scattering (HPSEC-RI-MALLS) measurements. The latter technique enabled us to determine the molecular weight of the modified polymers, proving that the synthetic procedure did not affect the chemical integrity of the chain. The intrinsic viscosity and the radius of gyration data showed that the hydrodynamic properties of the polymer chain varied with the degree and the pattern of substitution. In the presence of a relatively low galactose content (up to 19%), a decrease of the hydrodynamic dimensions of the coil was experienced, while on increasing the degree of substitution (especially on GG diads) a re-extension of the chain was discovered. Measurements of intrinsic viscosity at different values of the degree of dissociation have demonstrated that this effect cannot be solely explained by the reduction of the charge density of the polymer. Rather, it implies the occurrence of conformational changes of the chain that are specific to the chemical nature of the site of substitution. These data have been supported by the values of the persistence length of the natural and modified polymers obtained with the Doty-Benoit equation. The chiro-optical properties of the modified polymers studied by means of circular dichroism (CD) spectroscopy confirmed that conformational variations occurred to the polymeric chain upon introduction of galactose residues.

Coupling of alginate with 1-amino-1-deoxygalactose in the presence of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide results in a substituted polymer containing galactose side linked via an amide bond. To clarify the degree and pattern of substitution, a (1)H NMR study on the anomeric region of modified alginate, polymannuronate, alginate enriched in guluronic acid (G-enriched alginate), and polyalternating MG, was carried out (G, alpha-l-guluronic acid; M, beta-d-mannuronic acid). From the resonance of the proton at position 1 of galactosylamine, it was possible to determine the amount of galactose linked to mannuronic and to guluronic residues, respectively. Furthermore, (1)H NMR spectroscopy revealed a higher reactivity of guluronic residues for low degrees of conversion. Modified alginates with 7% and 19% of substitution are both able to form stable beads in the presence of calcium ions. The effect of galactose substitution on the dimensions, swelling, and stability of the beads has been studied and the cytotoxicity of the modified polymer evaluated in preliminary biological tests.