Investigation of the effect of amylose/iodine complexation on the conformation of amylose in aqueous solution

Using a potato amylose fraction of 8 × 10⁵, molecular-weight viscosity studies were carried out at 25°C on solutions containing 0.176–0.042% polymer, 8.67 mM KI, 1% ethanol, and different concentrations of iodine. By a novel extrapolation method, the intrinsic viscosities of the amylose/iodine complex were determined under various conditions of iodine binding (0–0.133 g I₂/g amylose). Contrary to the view long held in this research area, it was found that the intrinsic viscosity of amylose solutions decreases significantly upon complex formation with iodine. Taking into account the results of our previous kinetic studies, the present findings are interpreted in terms of an amylose model characterized by loose, extended helical regions which are interrupted by short disordered regions. It is proposed that the intrinsic viscosity decrease observed is due to a shortening of the linear dimension of the polymer chain. This conformation change is apparently caused by the contraction of loose helical regions of the amylose macromolecule due to the entrapment of iodine (and perhaps other) atoms inside the helical cavities.