1.2. Preferential exclusion of cosolvents from polysaccharide chains
This hypothesis also originates from biomolecular hydration
thermodynamics; the enhancement of biomolecular association in
the presence of cosolvents can be rationalised by the preferential
exclusion of cosolvents (or equivalently, preferential hydration)
from the biomolecular surface which has been supported by experiments.
The notion of preferential cosolvent exclusion has sometimes
been invoked to rationalise the cosolvent-induced enhancement of
biopolymer properties. Shimizu and Matubayasi have recently
extended the rigorous statistical thermodynamic theory of the
cosolvent effect to gelation equilibria, which afforded a rationalisation for the sugar- and
polyol-induced enhancement of gelatin and agarose gelation in
terms of cosolvent preferential exclusion. What has emerged from
this rigorous statistical thermodynamic theory is the clarification
that preferential exclusion is actually caused not by the increased
hydration but by the exclusion of cosolvents from biomolecular
surfaces. Hence this hypothesis is actually about the exclusion of cosolvents. Whether this is the case also for k-carrageenan remains to be examined.
1.3. Binding of cosolvents on polysaccharide gels
Sugars and polyols bind directly to k-carrageenan, increasing
the number of junction zones, with shorter average length, per unit
volume. This is attributed to the stabilization of the characteristic intermolecular
hydrogen bonding between individual k-carrageenan strands in a
typical junction zone by the formation of intermolecular, crosslinking
hydrogen bonds between the OH-groups of the sugar/polyol
cosolvent and k-carrageenan. Such a change in cross-linking came originally from rheological
evidence, as well as the cosolvent-induced elevation of the gel denaturation temperature
for agarose gels and kcarrageenan gels. Such evidence, more specifically, includes:
i) the shorter average length, and therefore increased number per
unit volume, of junction zones for k-carrageenan gels in the presence
of sugars/polyol cosolvents; ii) the decreased free energy of junction zone
formation; due to stabilisation of intermolecular cross-linking of
individual k-carrageenan molecules in the junction zones by
intermolecular hydrogen-bonding between cosolvent OH-groups
and individual k-carrageenan strands; iii) the increased setting and melting temperature of k-carrageenan gels in the presence of sugars; iv) the increased rigidity
of k-carrageenan gels with increasing sugar/polyol concentration ; v) the change in rheological properties of k-carrageenan gels above a critical sugar concentration (Loret et al.,
2009); and vi) the positive correlation between the number of
equatorial-OH groups on a sugar and the increased thermal stability
of k-carrageenan gels as equatorial groups are suspect to form
cross-linking hydrogen bonds between biopolymers.
Similar observation and explanations have been reported for