Chern., Vol. 69, No. 9,
Printed in Great Britain.
Shapes and motions
David A. Brant
Department of Chemistry, University of California
Irvine, California 92697-2025 USA
Computer-based molecular modeling of single stranded polysaccharide chains is capable
of a quantitative description of the polymer chain dimensions in dilute aqueous solution. The
influence of glycosidic linkage position and stereochemistry on the local chain trajectory is now
well understood. Some illustrative results of modeling of this sort are reported here. More recent
efforts to understand the possible influence of glycosidic linkage type on the dynamics of
polysaccharide conformational change are also described.
Important differences between polysaccharides and other biopolymers arise from the multiple hydroxyl
functionality of the monomeric sugar units. Thus, aldohexopyranose sugars may be linked
(1,4) in a polysaccharide chain. The glycosidic linkage oxygen may have equatorial or axial disposition with
respect to each sugar ring, depending on the anomeric form and chemical identity of the sugars involved.
Together, the regio- and stereochemistry of the linkage govern the extent of steric conflict between
successive sugar residues, which in turn determines the inherent conformational freedom of a given linkage.
These same factors govern the general characteristics of the polysaccharide chain trajectory( l), the
equilibrium spatial distribution of the chain(2), the mean values of the characteristic metrics of that spatial
configuration(3), and the rates of transformation beween conformations characteristic of that distribution(4).
The possibility for extensive chain branching, which vastly expands the information carrying capacity of
carbohydrate polymers and oligomers, is another important consequence of multiple hydroxyl hnctionality
that will not be pursued here.
When aldohexopyranose sugars are linked (1,6), interposition of an additional chemical bond in the linkage
region between adjacent sugar residues promotes a greatly increased inherent conformational freedom as the
two sugars are separated by a greater mean distance(5). Additional sources of chain tortuosity may also
arise if there are several conformers of the pyranoid ring possessing similar energies, as in the L-iduronic
residues of heparin(6).
Sugar ring flexibility may play a much larger role in polysaccharides involving
furanoid sugars owing to the well known propensity of furanoid sugars for pseudorotation(7). All of these
factors conspire to make the polysaccharides more conformationally diverse than the other polymers of
natural origin. Less well understood are the factors that determine the rates at which individual
polysaccharide chains sample the conformational diversity to which they are entitled.