Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.2 Chain conformations of polysaccharides 709
However, thermodynamic arguments suggest that a partially disordered state is an essential
prerequisite for the stability of polymeric systems in solution. Under these circumstances,
realistic chain pictures of polysaccharides will not necessarily be generated from
the repetition of a single conformational state, which is usually identified by the minimum
energy state found in the internal conformational energy calculations. Statistical approaches
to these conformational energy surfaces 7,8 suggest a more disordered solution conformation
(Figure 12.2.2, snapshots marked with a) than the chain structures proposed so as to fit the
helical regularity deduced from x-ray fiber diffraction studies (Figure 12.2.2, structures
marked with b). Thermal fluctuations are in general sufficient to generate delocalized disorder,
unless diffuse interactions, cooperative in nature, ensure long-range order.
From the experimental point of view, several polysaccharides with different chain
linkage and anomeric configuration have been studied to determine to what extent the polymeric
linkage structure and the nature of the monomeric unit are responsible for the preferred
solvation and for the chain topology and dimensions. 9 Conversely, since it is
generally understood that the structure and topology of many macromolecules are affected
by solvation, theoretical models must include these solvent effects in addition to the internal
flexibility, in order to estimate changes in the accessible conformations as a result of the
presence of the solvent molecules.
The concept of chain conformational disordering and dynamics in solution is associated
with the existence of a multiplicity of different conformations with accessible energy
and, moreover, exhibiting their topological differentiation. The above conformational variability
of polymeric chains is implicitly recognizable by the great difficulty in crystallization
and by the typical phenomenon of polymorphism. This discussion is relevant in
particular to ionic polysaccharides (see below) because, from the polyelectrolytic point of
view, the transition from a more compact conformation (also an “ordered chain”) to an extended
coil conformation, is usually associated with a net variation in ionic charge density
along the chain.
Polysaccharides generally dissolve only in strongly solvating media. Water displays a
complicated behavior: it is a good solvent for monomers and oligosaccharides inasmuch as
it is able to compete with the specific inter- and intra-molecular hydrogen bond network
(Figure 12.2.3). In many cases it is the thermodynamic stability of the solid-state form
which protects the solute molecules from being solubilized. 10 Nevertheless, some other
strong solvents, such as, dimethylsulfoxide, DMSO, and 1,4-dioxane are known to be good
solvents for carbohydrate polymers. Many commercial applications of polysaccharides require
compatibility with different solvents and solutes (organic solvents, salts, emulsifiers,
plasticizers, enzymes, etc.), for example, in pharmaceutical matrices, paints and foods. In
this field, solvent compatibility of some glycans has been improved and controlled by
functionalization and derivatization in order to obtain a proper degree of substitution, which
determines a wide range of compatibility properties.
At the molecular level, various specific and non-specific solvent-solute interactions
may occur in polysaccharide solutions that may result in a change in the conformational
shape, solubility, viscosity and other hydrodynamic and thermodynamic properties. Hydrophilic
interactions such as hydrogen bonding and electrostatic interactions are believed to
be factors that influence the conformation of polysaccharides in solution, although the question
is being raised (more and more) as to the implication of patches of hydrophobic
intermolecular interactions, especially for chain aggregations. One important feature is the