The stability of the helix of amylose and amylopectin in DMSO and water solutions

The DMSO molecule behaves as a zwitterion having an 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 at 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 α‐helix of amylose and amylopectin is more stable at acid pH values of 90 and 100% DMSO solutions than alkaline values because of the formation of the (CH3)2S+‐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 α‐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 amylose 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 hydroxyl 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 Cl− ion. The (CH3)2S+‐OH cation increases the dispersion power of DMSO by favoring the formation of those hydrogen bonds involved in the helix.