The complex of xylan and iodine: the induction and detection of nanoscale order.

The complex of xylan and iodine and its formation in a solution of xylan, CaCl2, and I2+KI was investigated by UV/Vis, second-derivative UV/Vis, and Raman spectroscopy. The complex forms only at very high concentrations of CaCl2, suggesting that when the water available in the solution is not sufficient to fully hydrate the calcium cation the chelation with the hydroxyl groups of the xylan can occur. The electronic spectra indicate that iodine is present in the form of three linear polyiodides I9(3-), I11(3-), and I13(3-) structures, which the Raman spectra show to be linear aggregates of the I3- and I5- substructures. Iodide concentration has a significant influence on the relative population of I9(3-), I11(3-), and I13(3-), as well as I3- and I5-, which lead to changes in both the UV/Vis absorption maxima shifts and changes in the Raman spectra. The key difference between this system of complexes with the linear polyiodide aggregates and that of amylose is that the longest aggregate observed with the amylose system, the I15(3-) polyanion, is not observed with the xylans. This indicates that the ordered arrays in the xylan-iodine complex do not exceed 4 nm in length. It is not possible to conclude at this time whether the ordered segment of the xylan molecule is linear or helical. If it is linear the length of the longest ordered arrays would be eight xylose residues. The number would exceed eight if the xylan molecule were helically wound.