Polysaccharide conformation. Part VII. Model building computations for α-1,4 galacturonan and the kinking function ofL-rhamnose residues in pectic substances

Abstract

D-Galacturonic acid residues in α-1,4 linked chains are shown, by ¹H n.m.r. spectroscopy of oligomers in solution and by mathematical model-building to have the Reeves C1 conformation. The probable conformation of the polymer in the solid state is also derived. Calculations show that multiple-helix formation is impossible for α-1,4-galacturonan; therefore, despite physical similarities, the gel properties of pectins and pectates cannot be explained in the same terms as for agar and carrageenans. There is, however, a strong analogy with the other biological-network polysaccharides in that anomalous units are interspersed to form ‘kinks’ which interrupt any tendency to an ordered chain conformation; these units are L-rhamnose in pectic substances and correspond to galactose 6-sulphate in carrageenans and to L-iduronic or D-glacuronic acid in mucopolysaccharides.