Mechanism Of The Binding Of Alkaline Earthmetal Ions To Alginate

As a natural polysaccharide, alginate was applied widely in both traditional industries and emerging fields such as pharmaceutical, biotechnology and environmental industries. The complexation between alginate and multivalent cations is the base of its applications. Therefore, probing the mechanism between alginate and various multivalent cations would be meaningful for the theory and application of alginate. This work has investigated the mechanism of the binding of alkaline earth metal ions to sodium alginate. Firstly, the molecular parameters of the alginates used in this study were characterized. Then the binding of alginate with alkaline earth metal ions was studied through relative viscosity and circular dichroism spectra. Furthermore, critical exponents and self-similarity for sol-gel transition was investigated via rheological investigations. The formation of mixed “egg-box” between high G-block and alginate or pectin in the presence of calcium was analyzed by fluorescence labeling, oscillation rheology and ion chromatography. The main conclusions and results were as follows:1. The binding of calcium ion and strontiumion to alginate occurs in multiple steps. The “egg-box” model applied to the complexation of alginate and calcium but not to that of alginate and strontium. There was negligible binding between magnesium and alginate. The binding of barium with alginate occurs in two steps. The molecular size increased firstly and then decreased, indicating that barium ion played a role of cross-linker in the reaction.2. By in situ acidification of the mixtures of alginate and alkaline earth metal ions via GDL, the viscoelastic properties of the mixtures at different concentration of ions were measured, and the values of the critical gel point for Ca, Sr and Ba were 0.15, 0.05 and 0.0125, respectively. The critical exponent of calcium alginate and barium alginate obtained from Winter-Chambon method, relaxation modulus and scaling law were close to the prediction, suggesting a random electrical network and self-similarity structure. But the growth of network structure for strontium alginate was not completely random and did not follow the scaling law.3. No mixed “egg-box” structure was found between high G-block and alginate under low ratio of calcium to guluronic acid. At high ratio of calcium to guluronic acid, high G-block could promote the gel formation of alginate. The complexation was found between high G-block and LMP when adding calcium ion.