Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.2 Chain conformations of polysaccharides 711
an increasingly important tool for understanding the structure and solution behavior of saccharides.
11-15 Although computer facilities and calculation speeds have grown exponentially
over the years some of these techniques, like quantum mechanic methods (ab initio methods),
have been shown not to be useful in dealing with the complexity of systems which involve
many atoms such as in macromolecular systems. On the other hand, these techniques
have been successfully applied to small molecules, e.g., mono- and disaccharides, in predicting
charge distribution on the atoms, conformations and transitions among accessible
conformations, 16 thus providing a background knowledge for the more complicated systems.
12.2.3 EXPERIMENTAL EVIDENCE OF SOLVENT EFFECT ON
OLIGOSACCHARIDE CONFORMATIONAL EQUILIBRIA
The problem of sugar conformation and dynamics in solution is related to the question of to
what extent are oligo- and polysaccharides intrinsically flexible under the different experimental
conditions. The answer to this question, which requires a complete knowledge of the
time-space dependence of the chain topology, 17 is often “rounded-off” by the use of empirical
terms like “flexibility”. A further problem is to what extent does the solvent contribute to
stabilizing some conformational states rather than others. Solution properties are functions
of the distribution of conformations of the molecules in the solvated states, in the sense that
the experimental data are statistical thermodynamic averages of the properties over all the
accessible conformational states of the molecule, taking each state with a proper statistical
weight. This aspect can be better illustrated by taking into consideration the accessible
conformational states of a simple sugar unit and the conformational perturbation arising
from the changes in the interactions with the surrounding solvent medium.
For an α-pyranose ring, for example, the rotation about carbon-carbon bonds and the
fluctuations of all ring torsional angles give rise to a great number of possible conformers
with different energies (or probabilities). Some of these are identified as the preferred rotational
isomeric states in various environments. The boat and boat-skew conformers of a
pyranose ring, which are higher in energy than the preferred chair form, correspond to a major
departure from the lowest energy chair conformation as illustrated by the globular
conformational surface of Figure 12.2.4. Additional conformational mobility in a
monosaccharide is due to the rotations of exocyclic groups, namely, OH and especially
CH2OH. Let us point out that in a polymer these ring deformations do not normally occur,
but, when they do, they may determine to a great extent the equilibrium mean properties and
the overall mean chain dimensions. 18,19 However, one very recent theory is that the elastic
properties of amorphous polysaccharides are related to the glycosidic ring deformation. 20
D-ribose is probably the best example of sugar that reaches a complex conformational
equilibrium, giving rise to the mixture composition shown in Figure 12.2.5. The percentage
of each form is taken from Angyal’s data 21,22 with integration of the sub-splitting between
the 4 C1 and the 1 C4 forms. The prime purpose of this analysis is to point out that the stability
of each conformer is not determined solely by the intrinsic internal energy, which can be
evaluated by means of e.g., ab-initio quantum mechanics calculations, but is strongly influenced
by all the solvation contributions. Therefore, the conformer population may be
shifted by changing temperature, solvent composition, or any other external variable such
as, for example, adding divalent cations (see Table 12.2.1). The evaluation of the actual
concentration of the several conformers involved in the equilibrium immediately leads to