Effects of molecular weight and elastic segment flexibility on syneresis in Ca-alginate gels

Syneresis in Ca-alginate gels was studied as a function of the alginate mol ecular weight and the degree of flexibility of the elastic segments. Small angle X-ray scattering of alginate gels reveals an increased lateral associ ation of junction zones when entering a Ca2+ regime giving syneresis. This suggests growth of junction zones to be the primary driving force for syner esis. Ca-alginate gels prepared from alginates with different molecular wei ghts show a reduced syneresis with decreasing M-w A reduced syneresis is al so observed when fractions of a high M-w alginate is replaced by short algi nate molecules enriched in guluronate residues. The effect of altering the monomer sequence of the elastic segments spanning the junction zones was st udied by converting poly- mannuronate regions to alternating guluronate/man nuronate sequences by the mannuronan C5-epimerase AlgE4. This epimerisation reaction gives a more flexible elastic segment. The epimerased alginates y ielded gels with larger syneresis compared to the non-epimerased, native al ginate samples. Thus, both molecular weight and elastic segment flexibility are needed in a molecular model for describing syncretic behaviour in algi nate gels. These parameters will to a large extent determine to which degre e the non-equilibrium nature of the alginate gel is macroscopically express ed (syneresis). (C) 2001 Elsevier Science Ltd. All rights reserved.