Disaccharides torsional angles

While most CA s of disaccharides have depended only on intrinsic characteristics of the molecule, experimental results depend strongly on the environment. By experiment, Kamide and Saito ( ) have shown that the degree of flexibility of cellulose and its derivatives is strongly dependent on the dielectric constant of the solvent as well as the exact type and degree of substitution. Since a substantial portion of the polymer flexibility depends on the extent of variability of the torsion angles at the intermonomer linkage, the dependence of polymer flexibility on type of solvent and substitution means that the disaccharide flexibility also should depend on those factors. Non-polar solvents allowed the molecules to have greater flexibility than did polar solvents (35). 

Our procedure depends on a new computer program, RAMM (RAndom Molecular Mechanics), which is applicable to any kind of biomolecule. It is described in detail elsewhere (KoS r, T./ Petrak, F. Galova, Z. TvaroSka, I. Carbohvdr. Res.. in Press). Only the basic characteristics of RAMM and its application to conformational analysis of disaccharides are discussed here, concentrating on the effect of the orientations of pendant groups on the energy values at the various < ) and f torsion angles. 

The orientation of the pendant groups in a disaccharide coitqposed of two hexapyranose residues can be described by 10 torsion angles. 

Figure 1. A (1 -> 4) disaccharide showing and < ), based on the torsion angles Hl-Cl-04 -C4 and Cl-04 -C4 -H4, respectively.

DFT calculations have been used to investigate the structures and conformations of four j8-[l—>4]-linked disaccharide mimics,84 the method being used to compute both optimized geometries, and values of3/Hcoc (and 3./cocc, see below) as a function of the interglycosidic torsion angles p and i//. The 3/hcoc values computed by DFT were fitted by least squares to Eq. (3) giving ... 

Scheme 2 Schematic representation of a hexopyranoses (a) and a 1 —>6 linked disaccharide (b) showing the uj torsion angle. Schematic diagram of the gt, tg, and gg staggered conformers around the C5-C6 bond.

Assuming rigid monosaccharide rings, the source of flexibility in carbohydrates is the glycosidic linkage characterized by the /T torsion angles. In this approximation the complete information about the conformation of a disaccharide fragment is embodied in the conformational distribution function It has been... 

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