1.1. Cosolvent-induced change of water structure and hydration
change
This hypothesis stems originally from the classical view of biomolecular
folding and stability, in which the hydrophobic effect has
long been believed to play a dominant role. According to this
view, the hydrophobic effect is caused by the entropy penalty due
to the enhancement of hydrogen bonds between water molecules
adjacent to the hydrophobic solute. Hence cosolvent molecules that
enhance the hydrogen bond network of the surrounding water
molecules, known formally as kosmotropes, are considered to
concurrently enhance the hydrogen bond network around the hydrophobic
solutes, thereby strengthening the hydrophobic effect
and, consequently, facilitating biomoleculeebiomolecule association.
The enhancement of the water structure by the cosolvent is thus
the cause of the hydration change. Attempts have therefore been
made to measure, or obtain evidence for, the cosolvent-induced
changes in hydration accompanying gelation. A bulk of
evidence has been reported to support this view: (i) the introduction
of cosolvents changes the gelation process from an enthapically
driven to an entropically driven process, indicating a
cosolvent-induced release of water; (iii) the
observed linear correlation between the “water structure parameter”
(derived from the partial molar volume of the cosolvent) and
the cosolvent-induced increase in thermal stability of the gel
; this is underscored further by a linear correlation
between the “water structure parameter” and the number of
equatorial-OH groups on the cosolvent which correlates also with the
sugar-enhanced thermal stability of k-carrageenan gels.
In contrast to all above, our recent statistical thermodynamic
study on the gelation of agarose and gelatin has shown that the
contribution of cosolvent-induced hydration change is negligible. However, the question still remains
as to whether this is also the case for k-carrageenan gelation.
Also the previous evidence for this hypothesis, as summarised
above, should be examined critically in the light of statistical
thermodynamics.
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.