DISTRIBUTION OF URONATE RESIDUES IN ALGINATE CHAINS IN RELATION TO ALGINATE GELLING PROPERTIES

Alginate is a family of polysaccharides isolated from seaweeds or produced by certain bacteria composed of (1 --> 4)-linked residues of alpha-L-guluronic acid (G) and beta-D-mannuronic acid (M) varying both in abundance and sequence along the chain. Chain sequences from five different sources were reconstructed by using experimentally determined fractions of the eight possible triad arrangements as parameters in second-order Markov chains. The Monte Carlo generated chain sequences were analyzed with respect to gelling ability, assuming a cooperative model for junction formation. The cooperative model was approximated by a step function at LG(min) representing the minimum level of G blocks being able to form a junction. By varying LG(min) from 4 to 12, it was found that there was a fraction, the loose-end fraction, of chains within each sample that had less than two possible junction zones. The loose-end fraction was calculated to decrease with increasing chain length and with decreasing LG(min). The gel strength was found to correlate to the calculated number of elastically active chains. The sol fraction constitutes chains with no G blocks of length exceeding LG(min). The sol fraction was found to depend on the source of the alginate, to decrease with increasing chain length and decreasing LG(min), and to constitute approximately 35-50% of the loose-end fraction. The sol fraction was further predicted to be enriched in beta-D-mannuronic acid residues, the enrichment being larger for smaller LG(min). The model unites properties at the polymer level to those of practical interest, such as gel strength and porosity and enrichment of beta-D-mannuronic acid in leaking material, properties that are of prime importance when applying alginates as immobilization material for implantation purposes.