Ionic and acid gel formation of epimerised alginates; the effect of AlgE4

Abstract

AlgE4 is a mannuronan C5 epimerase converting homopolymeric sequences of mannuronate residues in alginates into mannuronate/guluronate alternating sequences. Treating alginates of different biological origin with AlgE4 resulted in different amounts of alternating sequences. Both ionically cross-linked alginate gels as well as alginic acid gels were prepared from the epimerised alginates. Gelling kinetics and gel equilibrium properties were recorded and compared to results obtained with the original non-epimerised alginates. An observed reduced elasticity of the alginic acid gels following epimerisation by AlgE4 seems to be explained by the generally increased acid solubility of the alternating sequences. Ionically (Ca 2+) cross-linked gels made from epimerised alginates expressed a higher degree of syneresis compared to the native samples. An increase in the modulus of elasticity was observed in calcium saturated (diffusion set) gels whereas calcium limited, internally set alginate gels showed no change in elasticity. An increase in the sol–gel transitional rate of gels made from epimerised alginates was also observed. These results suggest an increased possibility of creating new junction zones in the epimerised alginate gel due to the increased mobility in the alginate chain segments caused by the less extended alternating sequences.