Carbohydrate RESEARCH Carbohydrate Research 340 (2005) 981–988 The complex of xylan and iodine: the induction and detection of nanoscale order Xiaochun Yu a and Rajai H. Atalla a,b, * a Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53726, USA b USDA, Forest Products Laboratory, One Gifford Pinchot Dr., Madison, WI 53726, USA Received 28 October 2004; accepted 25 January 2005 Dedicated to Professor David A. Brant Abstract—The complex of xylan and iodine and its formation in a solution of xylan, CaCl 2 , and I 2 + KI was investigated by UV/Vis, second-derivative UV/Vis, and Raman spectroscopy. The complex forms only at very high concentrations of CaCl 2 , 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 I 9 3 ,I 11 3 , and I 13 3 structures, which the Raman spectra show to be linear aggregates of the I 3  and I 5  substructures. Iodide concentration has a sig- nificant influence on the relative population of I 9 3 ,I 11 3 , and I 13 3 , as well as I 3  and I 5  , 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 I 15 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. Published by Elsevier Ltd. Keywords: Xylan; Polyiodide; Xylan–iodine complex; Nanoscale order 1. Introduction Solutions of iodine and iodides have long been used as stains to facilitate the identification of cellulosic fibers with respect to source and processes of isolation. 1 A number of formulations have been used, either in water or in salt solutions, 1,2 but they all rely on the differential color reaction of the plant cell-wall polysaccharides with iodine. Although the formulations were devised com- pletely empirically, they present very subtle color differ- ences among different fibers. For example, Graff ÔCÕ stain presents fiber color from blue to red to yellow. 2,3 These color variations point to subtle differences in * Corresponding author. Tel.: +1 608 231 9443; fax: +1 608 231 9262; e-mail: rhatalla@facstaff.wisc.edu 0008-6215/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.carres.2005.01.029 structure among different fibers. In an effort to under- stand the underlying mechanisms we have carried out a number of studies of the iodine complexes with poly- saccharides. The first effort was focused on understand- ing the nature of the polyiodide structures responsible for the color 4 and relied on the much investigated amy- lose–iodine complex system. Here we report on exten- sion of our investigation to the complexes of iodine with xylans. Our previously reported studies of the amylose–iodine complex in solution 4 established a foundation for extending the studies of iodine complexes to those with the cellulose and the hemicelluloses. With the amylose system, our investigation focused on the organization of polyiodide chains in the amylose–iodine complex. We relied on complementary observations of Raman spectra, UV/Vis, and second derivative UV/Vis