Cereal Chem 45:140 - 153.  |  VIEW ARTICLE
The Transition from Helix to Coil at pH 12 for Amylose, Amylopectin, and Glycogen.

S. R. Erlander, R. M. Purvinas, and H. L. Griffin. Copyright 1968 by the American Association of Cereal Chemists, Inc. 

Previous studies have indicated that amylose undergoes a transition from the helix to the random coil conformation at pH 12. Such a transition produces a drop in intrinsic viscosity which corresponds to approximately 40% of the initial viscosity. To examine this phenomenon more fully, the viscosities of amylopectin, glycogen, and dextran as well as that of amylose were obtained at pH 7 and 12. The results show that when sufficient salt is present, there is a drop of approximately 40% in intrinsic viscosity for amylopectin, glycogen, and amylose but not for the alpha-1,6-linked dextran. The helix therefore must also exist in amylopectin and glycogen, but not in dextran. It is concluded from this and other data that the stabilizing force for the helix must be hydrogen bonds between the C-2 and C'-3 hydroxyl groups of adjacent glucose units. In the absence of added salt, the value (eta) increases in going from pH 7 to pH 12. This increase is caused by a stiffening of the chain due to electrostatic repulsion of the ionized hydroxyl groups of amylose. Addition of 32% methanol (theta solvent) to a 0.5N NaOH solution reduces the value of (eta) for amylose to that obtained at pH 12. Hence, the reduction in (eta) at pH 12 in the presence of salt is not due to ion-binding or similar phenomena. Rather, this reduction in (eta) for both linear and branched alpha-1,4-linked polyglucosides is caused by elimination of electrostatic repulsion forces and by destruction of the helix. It is concluded that the helix of different molecules in the same medium may have a variable number of glucose units per helical turn (most likely seven to eight glucose units in water). This number may be fractional rather than integral, because the stability of the helix does not depend on hydrogen- bonding between helical rings.