Ionic polysaccharides. V. Conformational studies of hyaluronic acid, cellulose, and laminaran

Abstract

AbstractHindered rotation was studied for the disaccharides composed of basic β‐glucopyranose units A (linked at O1 and O4) and B (linked at O1 and O2) in Cl conformation. The van der Waals' interactions were calculated for the Lennard‐.Jones, Buckingham, and Kitay‐gorodsky interatomic potential functions, including a study of the effect of minimization of the first two functions for each atomic pair k, m (a) at the sum rkm* of the usual crystal atomic radii rk* + rm* and (b) at rkm* + 0.2 Å. Values of the ratio of unperturbed to free‐rotation root‐mean‐square end‐to‐end distance σ, were calculated for chains composed of the unsolvated disaccharide repeating units: AA (cellulose), AB and BA (hyaluronic acid), and BB (laminaran). For cellulose and hyaluronic acid, σ increases markedly with both assumed atomic radii rk* and θ, the supplement of theglycosidebond angle. Energy contributions from dipole–dipole and side‐chain interactions cause relatively small dimensional changes for hyaluronic acid: for cellulosic polymers bulky side groups attached near the glycoside carbon atom may increase or decrease unperturbed dimensions, depending on the potential function chosen. Inclusion of bond torsional potentials increases σ for cellulose and hyaluronic acid and decreases σ for laminaran. Hydrogen‐bonding ellects normally increase calculated σ values for unsolvaled polymers. The predicted influence of temperature on σ is small. The large decrease in σ with increasing temperature which appear to occur experimentally for cellulosic polymers are prc.‐umably due to conformational effects of solvation.