Ligand-induced structural changes in amylose partially complexed with iodine

The influence of complexing agents such as methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, cyclohexanol and 2-octanol on the formation of a blue coloured amylose - iodine complex (pH 4.8), under suboptimum concentrations of iodine and in the absence of potassium iodide, is studied by recording the absorbance at 640 nm. A drop in absorbance at 640 nm accompanied by a blue shift in the spectrum (640-580 nm) was observed at higher concentrations of the complexing agents. This behaviour of amylose partially complexed with iodine appears to be due to ligand-induced structural changes in the amylose chain. The fall in absorbance at 640 nm observed when the temperature of amylose - iodine complex in the presence of complexing agents is raised, and the subsequent regeneration of the absorbance on cooling, indicates the possible helix to random coil transition of the amylose chain in an aqueous system.