DISTRIBUTION OF URONATE RESIDUES IN ALGINATE CHAINS IN RELATION TO
ALGINATE GELLING PROPERTIES
Macromolecules, 24 (16):
4637 -- 4645 (August 1991)Abstract
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.