Molecular basis for some physical properties of alginates in the gel state

Alginate is a binary linear heteropolymer containing 1,4-linked β-D-mannuronic and α-L-guluronic acid residues in varying proportions. The monomers are arranged in a blockwise fashion along the chain; it contains the two homopolymeric blocks (MM and GG) together with blocks of the alternating sequence (MG) in the same molecule. Data from light scattering and viscometric experiments show that their relative unperturbed dimensions increase in the order: MG-blocks < MM-blocks < GG-blocks. Ion exchange data show that GG-blocks are characterized by a selective binding of calcium ions in solution, a strong autocooperative binding of calcium between the chains in the gel state, and hystrisis due to a slow dissociation of the cooperatively-formed functions between the chains. The two other blocks have much lower selectivity for calcium ions, no autocooperative binding mechanism and no detectable hysteresis. The mechanical strength of calcium alginate gels is mainly due to junctions formed by the GG-blocks. the modulus of rigidity of gels formed by different cations is directly dependent on their ability to bind to the polyuronides by a cooperative inter-chain binding mechanism.