The complex of amylose and iodine

Abstract

The organization of polyiodide chains in the amylose-iodine complex was investigated by Raman spectroscopy, by UV/vis, and by second-derivative UV/vis spectroscopies complemented by semiempirical calculations based on a simple structural model. The Raman spectra indicate that the primary substructures of the polyiodide chains are I 3 − and I 5 − sub-units. The second derivatives of the UV/vis spectra reveal four absorption peaks that can be attributed to the four polyiodide species I 9 3−, I 11 3−, I 13 3−, and I 15 3− on the basis of AM1 calculations and a polyiodide structural model. In an amylose-iodine-iodide solution, the polyiodides within the amylose helices exist in equilibrium with iodine and iodide. Changing the iodide concentration causes changes in the relative population of the different polyiodide chains and their substructures. The change in population of the various polyiodide species is accompanied by a corresponding shift in the positions of the UV/vis absorption maxima. The Raman spectra reflect parallel changes in the population of the I 3 − and I 5 − subunits. The proposed polyiodide chain lengths are consistent with previously reported observations concerning changes in the color characteristics of iodine-amylose complexes with the length of the amylose molecule up to a dp between 40 and 50.