Similarities and differences between alginic acid gels and ionically crosslinked alginate gels

A Na-alginate solution can, under given conditions, undergo a sol–gel transition at pH below pKa for the uronides. These gels are presumably stabilised by intermolecular hydrogen bonds. This paper summarises the effect of alginate variables such as chemical composition and sequence and molecular weight on mechanical properties and network fine structure. Alginic acid gels resemble ionically crosslinked alginate gels in the sense that guluronic acid blocks are the building elements of most importance with respect to gel formation. The main difference is that mannuronic acid blocks, although with less efficiency compared polyguluronate, also support alginic acid gel formation. With the exception strictly alternating alginates made by treating mannuronan with the AlgE4 epimerase, the so-called alternating (random) sequence of the alginate molecule seems to destabilise alginic acid gel formation due to its lack of repeating structure. Small Angle X-ray Scattering (SAXS) reveal a scattering intensity for alginic acid gels being typically and order of magnitude higher than Ca-alginate gels, suggesting junction zones composed of random aggregates with a high degree of multiplicity. A molecular understanding of alginic acid gels is of importance in the general understanding of polyuronides at low pH as well as for the utilisation of alginates in drug delivery systems.