Handbook of Solvents - George Wypych - ChemTech - Ventech!

726 Ranieri Urbani and Attilio Cesàro
able pseudo-helical pattern. With respect to cellulose, the amylose chain shows a more
coiled and apparently disordered backbone topology, but from a statistical point of view
possesses a lower configurational entropy, 67
i.e., a more limited set of allowed
conformational states. The C∞ values are higher in water (5.6) than in DMSO (4.9) and both
are higher than for the unperturbed chain which is in good agreement with the earlier work
of Jordan and Brant 67 which observed a decrease of about 20% in chain dimensions of
amylose DMSO/water mixture with respect to the water alone. More recently, Nakanishi
and co-workers 68 have demonstrated from light scattering, sedimentation equilibrium and
viscosity measurements on narrow distribution samples, that the amylose chain conformation
in DMSO is a random coil, which results expanded (C∞ = 5) by excluded-volume effect
at high molecular weight. Norisuye 69 elaborated a set of published viscosity data reporting a
C∞ = 4.2-4.5 for unperturbed amylose and C∞ = 5.3 for aqueous KCl solutions, while Ring
and co-workers 70 estimated a C∞ ≅ 4.5 for amylose/water solutions by means of the
Orofino-Flory theory of the second virial coefficient.
Order-disorder conformational transitions very often occur on changing physical
and/or chemical conditions of polysaccharide solutions. DMSO, for example, is the solvent,
which is commonly used as co-solvent for stabilizing or destabilizing ordered solution conformations.
Schizophyllan, a triple helical polysaccharide with a [β-D-(1-3)-glc] n backbone
exhibits a highly cooperative order-disorder transition in aqueous solution. 71 When small
quantities of DMSO are added to aqueous solutions the ordered state is remarkably stabilized,
as has been observed in the heat capacity curves by means of the DSC technique. 71
Several efforts have been made with MD simulations in order to explicitly take into
account the solvent molecule effect on the saccharide conformation, although only
oligomeric segments have been considered, given the complexity in terms of computational
time required for such a multi-atoms system. One interesting example is that of Brady and
co-workers 56 on the stability and the behavior of double-helix carrageenan oligomer in
aqueous solution compared with the results of the in vacuo calculations. They observed a
higher relative stability of the double helix in vacuo, a fact, which is consistent with experimental
results under anhydrous conditions, as in the fiber diffraction studies. However, in
aqueous solution, the interchain hydrogen bonds that stabilize the double-helix structure appear
much less stable, as the glycosidic hydroxyl groups make more favorable interactions
with water molecules. They concluded that in the solvation step the double-helix would
seem to be unstable and an unwinding process is theoretically predicted, at least for the oligomers.
12.2.6 SOLVENT EFFECT ON CHARGED POLYSACCHARIDES AND THE
POLYELECTROLYTE MODEL
12.2.6.1 Experimental behavior of polysaccharides polyelectrolytes
Based on the experimental evidence of polyelectrolyte solutions, whenever the degree of
polymerization is sufficiently high, all ionic macromolecules are characterized by a peculiar
behavior, which sets them apart from all other ionic low molecular weight molecules as well
as from non-ionic macromolecules. A general consequence of the presence of charged
groups in a chain is a favorable contribution to the solubility of polymer in water. A strongly
attractive potential is generated between the charge density on the polymer and the opposite
charges in solution. For example, the value of the activity coefficient of the counterions is
strongly reduced with respect to that of the same ions in the presence of the univalent opposite
charged species. If the charge density of the polyelectrolyte is sufficiently high, such a