Gum 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.

The Porod‐Kratky persistence lengths, monomer and polymer radii, and Flory coefficients of a number of β0–1,4 linked hexosans have been compared to each other and to the β‐1,4 linked xylan pentosans of wood by applying the Eizner‐Ptitsyn viscosity equations to intrinsic viscosity and molecular weight analyses from a number of sources. The analyses were made on fractions of cellulose acetate, cellulose nitrate, cellulose caproate, the diethylacetamide derivative of cellulose xanthate, cellulose in cadoxene and in the alkaline ferric tartrate solvent, FeTNa, the galactomannan triacetate of guar, and the glucomannan triacetate of orchid salep tuber. Xylans from spruce and birch, polystyrene, and a branched dextran were also investigated. The persistence lengths of the hexosan derivatives, except the cellulose trinitrate, were remarkably similar and were found to vary from 48 A. for the cellulose caproate determined under theta conditions to 58 A. for the galactomannan triacetate. The persistence lengths of cellulose in the two complexing solvents (72 A.) were nearly identical, confirming that these solvents do not increase the solution dimensions of cellulose greatly. The configuration of the polymers were ranked on a common graph by plotting the log of the polymer radii to contour length ratios vs. the log of the contour length. The investigation shows that the 0–1,4 linked hexosans have a common configuration which is independent of the length of the derivative side group or of single α‐1,6 linked side groups, as in the galactomannan. The configuration of the hexosans is also independent of whether the main chain is composed of glucose or mannose, or a mixture of these sugars. The greater rigidity of the hexosans, in comparison to the pentosans, is explained in the former by hindrance between the C2 group of the one sugar with the C6 group of the next, which is absent in the pentosan that has no C6. Analogy is made to the hindered racemization in substituted biphenyls to explain the great rigidity of cellulose trinitrate, whose persistence length is 132 A.

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Gelation studies have sought to establish the background for a theory of pectin gel formation. Factors investigated that control the properties of pectin gels were: pH, degree of esterification, effect of acylation, temperature, and molecular weight. Elastic moduli of pectin gels decrease as the degree of esterification or of acylation decreases; they remain practically constant from 0° to 50° C. and decrease rapidly with further increase in temperature; and they are controlled by number average molecular weight. Breaking strength is controlled by weight average molecular weight. Although a complete picture of pectin gel formation cannot be drawn until stress relaxation data are available, association of pectin may begin with the ester groups but hydrogen bonding may account for the strength of the gels. Instead of point-to-point contacts between molecules, there may be crystalline regions involving several galacturonide units. This hypothesis accounts for the effect of acylation, setting temperature, and the low-temperature coefficient of the shear modulus of pectin gels.Pectin, a long-chain polymeric galacturonide partly esterified with methanol, has been used in the preparation of jellies and similar food products for over a hundred years. The mechanism by which it forms gels is not clearly understood and more information on the factors influencing pectin gel formation is required to develop a more exact theory of gelation.The first coherent hypothesis of pectin gel formation and elucidation of the importance of each component were made about 20 years ago by Olsen (15). Sugar functions as the dehydrating agent. 2. Acid functions by reducing the negative charge on the pectin, thus permitting coalescence.3. Dehydration of pectin requires time to come to equilibrium (to account for slow setting or low temperature of setting of certain pectin gels). The rate of dehydration and precipitation increases directly as hydrogen ion concentration increases.5. The maximum jelly strength is reached when the system reaches equilibrium.6. Any component added to a pectin jelly system, including salts which cause a change in the ultimate jelly strength of that system, may function by changing the rate of gelation, by affecting the position of the ultimate equilibrium of the system, or by a combination of these effects.The question whether sugar acts solely as a dehydrating agent or enters the framework of the gel structure has been raised (29), but the fact that glycerol at the same weight concentration as sucrose forms equally strong gels with pectin (SI) may indicate that the number of hydrophyllic groups is the more important factor. The marked increase in viscosity of sugar solutions as the concentration of sugar is increased above 45% probably reinforces the rheological properties of pectin gels, but there is no direct evidence that sugar molecules actually enter the gel structure. Unpublished work at this laboratory has revealed that more than 99% of the sugar in a gel can be extracted with ethanol without a change in dim...