Polysaccharide conformation. Part VI. Computer model-building for linear and branched pyranoglycans. Correlations with biological function. Preliminary assessment of inter-residue forces in aqueous solution. Further interpretation of optical rotation in terms of chain conformation

Abstract

Homopolymers of glucopyranose, galactopyranose, mannopyranose, xylopyranose, and arabinopyranose, with various positions and configurations of linkage, have been compared by model-building in the computer in an attempt to formulate simple rules for conformational analysis. Regular conformations are very restricted by steric forces alone, and each polymer has one of four characteristic shapes: Type A—extended and ribbon-like, Type B—flexible and helical, Type C—rigid and crumpled, or Type D—very flexible indeed but, on the average, rather extended. Chain branching leads to increased steric restriction for many examples which involve two secondary positions.

Correlations exist, but cannot always be explained, between the chain Type and the biological function: skeletal polysaccharides are usually of Type A, reserve and network polysaccharides are often of Type B, whereas loosely jointed polysaccharides have linkages of Type D, and chains of Type C are unnatural and rare.

More evidence is given to support the method set forth in Part V for the interpretation of optical rotations of carbohydrate polymers in terms of chain conformations. When used with model-building calculations, this method shows that the flexibility of the glycosidic system is determined by the equatorial substituents on both residues which are next to the glycosidic oxygen; libratory freedom increases with the number of hydrogen atoms in these positions. For an equatorial–equatorial linkage, A → B, the conformation about the glycosidic bond, C(1)–O, is controlled by the steric bulk of neighbouring substituents on ring A, and the O–C conformation is similarly controlled by neighbouring substituents on ring B. Polar and hydrogen bonding influences seem to be small for most equatorial–equatorial linkages in aqueous solution. For α-linked disaccharides, however, the exoanomeric effect seems to dominate the conformation about C(1)–O, and to override steric considerations which are suggested by model-building calculations. In α-1,6- and β-1,6-linkages, the conformations about C(1)–O and C(5′)–C(6′) resemble those in corresponding methyl glycosides, and the conformation about O–C(6′) is, on the average, anti.