Alginate oligoguluronates as a tool for tailoring properties of Ca-alginate gels

摘要

Alginates represent a family of structural polysaccharides derived from either seaweeds or bacteria. They readily form ionotropically-induced physcial gels in aqueous solution in the presence of multivalent ions. In this study, the homogeneous release of Ca~(2+) from Ca-ethylene glycol tetraacetic acid (Ca-EGTA) within an alginate solution is utilized to prepare homogeneous gelation conditions. The pH-dependence of the multiple equilibria involved, i.e. between Ca~(2+), the various states of EGTA, the alginate and its protonation and ion binding, is modeled and used as a basis for designing the glucono-δ-lactone concentration to reduce the pH. This method is used for the perturbation of the Cα-induced gelation of alginates in the presence of oligoguluronate oligomers. These oligoguluronate oligomers are able to be involved in the binding of Ca~(2+), but sufficiently short not to mediate connectivity through their chains. The results indicate that the oligoguluronate oligomers perturb the gelation of alginate differently in the Ca~(2+)-limited and non-Ca~(2+)-limited regimes. In the calcium limited regime, the oligoguluronate oligomers appear to sequester calcium either by binding to oligoguluronate sequences of the network, or between the free oligoguluronates, yielding an overall net effect of reduced gel strength. In the non-Ca~(2+) limited regime, the experimental data shows increased gel strength in the presence of oligoguluronate blocks. These results show that oligoguluronates can be used as modulators of gelation kinetics as well as local network structure formation and equilibrium properties in alginate gels.