Dextran produced by Leuconostoc mesenteroides NRRL B‐512 was acid‐hydrolyzed and fractionated, yielding a series of fractions from which 24 were selected that ranged in molecular weight from 17,700 to 9.5 million. Light‐scattering and viscosity measurements were made on all fractions, and selected fractions were characterized by endgroup determinations and velocity ultracentrifuge measurements. Branching in these dextran fractions was evidenced by the progressively decreasing slope of the curve of logarithm of intrinsic viscosity plotted against logarithm of molecular weight, attaining a value of 0.11 at M = 107; by the upward curvature of the log‐log plot of sedimentation constant against molecular weight; and by the relatively large values of k′ in the Huggins specific‐viscosity equation for the high‐molecular fractions. Assuming Lansing‐Kraemer molecular‐weight distributions for the dextran fractions and using sedimentation data to furnish β, the measure of breadth of the distributions, physical characterizations were corrected for polydispersity. For molecular weights below 100,000, [η] = 1.0 × 10−3 Mv0.50. In the range 18,000 < M < 400,000 S200 = 0.0251 M00.44. The constant Φ′, of the Flory‐Fox theory, corrected for polydispersity of the fractions, was found to be 55.2 × 1021, which is larger than the value reported for linear polymers. It is concluded that the ratio of root‐mean‐square radius and hydrodynamic radius effective in viscosity differs for branched and linear molecules. The quotient (Φ′)1/2/P′ also appears to be significantly smaller for dextran than the value reported for linear molecules. Increase in Φ′, for dextran above the value previously assumed necessitates recalculation of data of Wales, Marshall, and Weissberg for g, the ratio of mean‐square radii of branched and unbranched dextran molecules.

The DMSO molecule behaves as a zwitterion having a n electrostatic charge density on its oxygen atom comparable to that of the fluoride ion. Therefore the DMSO anion interacts with water to form an A-region having two water molecules, i.e., a DMSO.2H2O complex a t 66% DMSO. Studies on amylose and amylopectin show that DMSO.2H2O as well as anhydrous DMSO help to stabilize the helix structure of amylose and amylopectin. Because this helix structure is absent in dextran and hydroxyethyl cellulose, their properties in DMSO/water solutions are different from those of starch. The proposed model for the interaction of DMSO or hydrated DMSO with the starch helix shows how these molecules stabilize the helix by interacting with only one hydroxyl group of the C-2 and C'-3 hydrogen bonded hydroxyl groups. The a-helix of amylose and amylopectin is more stable at acid p H values of 90 and 100 yo DMSO solutions than alkaline values because of the formation of the (CH,),S+-OH cation. The hydroxyl group of this cation salts-out the hydrogen bonds just as the case of the hydrated A-regions of Li+ and Na+ cations. Optical rotation studies show that the addition of hydrated Na+ ions also stabilizes the a-helix just as a decrease in apparent pH. The greater stability of the helix in these DMSO solutions predicts that its average segment length will also increase. The decrease in the corrected specific rotation of am)lose in going from water to DMSO solutions is most likely due to an increase in the thermal motion of the amylose rather than any decrease in the helix stability. The DMSO disperses starch because, unlike the hydrogen bonds in the helix, both of the hydrox>l groups involved in intermolecular hydrogen bonds can become complexed to DMSO anions. The lower pK of such intermolecular hydrogen bonds also allows alkali in DMSO to increase the dispersion power of DMSO. Salt (NaCI) decreases the dispersion power of DMSO because of the formation of interchain linkages through the unhydrated C1-ion. The (CH,),S+-OH cation increases the dispersion power of DMSO by favoring the formation of those hydrogen bonds involved in the helix. ZUSAMMENFASSUNG:Das DMSO-Molekul verhalt si-h als Zwitterion und tragt am Sauerstoffatom eine elektrostatische Ladungsdichte, die mit der des Fluoridions vergleichbar ist. Daher reagiert das DMSO-Anion mit Wasser und bildet eine A-Region mit zwei Wassermolekiilen, d. h., einen DMSO ' Stability of the Helix of Amylose and Amylopectin in DMSO and H,O Solutions Helixstruktur von Amylose und Amylopektin stabilisieren. Da diese Helixstruktur in Dextran und in Hydroxyathylcellulose fehlt, sind die Eigenschaften dieser Verbindungen in DMSO/Wasser-Losungen von denen der Sttirke verschieden. Das vorgeschlagene Modell fii die Wechselwirkung von DMSO oder hydratisiertem DMSO mit der Starke-Helix zeigt, in welcher Art diese Molekiile die Helix stabilisieren; es tritt dabei nur eine der durchWasserstofiriicken verbundenen Hydroxylpppen am C-2 und C'-3 in Wechselwirkung.Die a-Helix von Amylose und Amylopektin ist bei s...

synopsisThe specific rotation of starch components, corrected for refractive index variation, exhibits a discontinuity in the region of the water-dimethyl sulfoxide (HzO- DMSO) system that corresponds to the composition of the complex 2HzO-DMS0. This discontinuity is a property dependent upon the presence of a number of consecutively linked a-1,4 glucose units and, therefore, must reflect a change in symmetry of a segment of polymer chain. The optical rotation of amylose between 26.5 and 92.5'C. does not change in DMSO and is only slightly lowered in water at the higher temperature. The behavior of amylose in both DMSO and HzO is like that of a random coil, as indicated by viscosity and sedimentation measurements. These results may be interpreted either ae being compatible with models of amylose in solution in which the polymer backbone has helical twist, or as indicating removal of strong interactions between polymer chain segments by a good solvent.

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