Structure of gum arabic and its configuration in solution

Abstract

AbstractGum arabic was found to have an osmotic molecular weight of 250,000, in agreement with earlier determinations. A molecular weight of 365,000 was found by light scattering, somewhat higher than obtained earlier by sedimentation equilibrium analysis but lower than light‐scattering values reported by other investigators. The M̄w/Mn ratio, 1.46, is quite low in gum arabic. The angular dependence of light scattering exhibited the upward curvature to be expected of a spherical molecule and a radius of gyration of about 100 A. or less, as estimated from a Zimm plot. Fractionation of the original gum arabic was done by precipitation of a 0.5% solution in aqueous 0.5% NaCl with acetone. Comparison of the curves of viscosity versus molecular weight and the estimated radius of gyration shows that the hydrodynamic volume is less than that of branched dextran of similar molecular weight. The electroviscous effects for gum arabic in aqueous solution were shown by reduced viscosity curves at various acidities and in salt. The degree of dissociation was calculated for each pH level. The minimum intrinsic viscosity was found in 0.04N HCl where the degree of dissociation at pH 1.5 was found to be 0.049. When the acidity was increased, further reduction in viscosity was found to be negligible. Routine determination of the viscosity and molecular weight of the fractions was done in 0.35M NaCl at pH 10 to which 0.25% of the sodium salt of ethylenediaminetetraacetic acid was added as a sequestrant. The intrinsic viscosity in this solvent was nearly as low as in 0.04N HCl. Light‐scattering dissymmetries in water and in 0.35M NaCl plus EDTA at pH 10 were similar, 1.13 and 1.09, respectively, which showed that actual expansion of the macroion is not the cause of the large increase in viscosity of gum arabic when the ionic strength of the solvent is reduced. Periodate oxidation of the polymer confirmed the existence of a 1–3‐linked backbone of galactose. Subsequent treatment of the oxidized polymer with alkali reduced the osmotic molecular weight to 45,000 but failed to remove oxidized side branches. The oxidized polymer was fractionated by gel permeation chromatography and the intrinsie viscosity–molecular weight relation compared with relations for fractions of the unoxidized polymer and for other branched and crosslinked polymers.