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

Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53726, USA.
Carbohydrate Research (Impact Factor: 1.97). 05/2005; 340(5):981-8. DOI: 10.1016/j.carres.2005.01.029
Source: PubMed

ABSTRACT 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: At T 250 K, the AC-conductivity is dominated by the combinational contributions of the disordered water network, the mobile Sr ions, the polyiodide charge-transfer interactions and the dehydration process. The evolution of the Raman spectroscopic data with temperature reveals the coexistence of four discrete pentaiodide forms. In form (I) (I 3·I2 ↔ I2·I 3), the occupancy ratio (x/y) of the central I ion differs from 50/50. In form (IIa) (I2·I·I2) x/y = 50/50, whereas in its equivalent form (IIb) (I2·I·I2) as well as in form (III) (I 3·I2), x/y = 100/0 (indicative of full occupancy). Through slow cooling and heating, the inverse transformations (I) → (IIa) and (IIa) → (I) occur, respectively.
    Supramolecular Chemistry 09/2010; 22(9):499-510. DOI:10.1080/10610278.2010.487563 · 2.13 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Current research, basic and applied, assumes that observed recalcitrance of celluloses is an inherent characteristic associated with their state of aggregation in their native state; it is thought that processes of isolation remove other components of plant cell walls leaving the celluloses unchanged, even though elevated temperatures are routinely used during isolation. Since temperature elevation is known to influence the structures of all polymers, it is important to explore its influence on the character of isolated celluloses, almost always assumed to be still in their native state. Deuterium exchange is a measure of accessibility of reactive sites in celluloses. We report significant reduction in accessibility to deuterium and other probe molecules for celluloses isolated at ambient temperature and then exposed to elevated temperatures. Our results indicate that native celluloses, which are highly ordered biological structures, are irreversibly transformed and develop polymeric semi-crystalline character upon isolation at elevated temperatures.
    01/2014; 100:2-8. DOI:10.1016/j.carbpol.2013.06.007
  • [Show abstract] [Hide abstract]
    ABSTRACT: The ac-conductivity and the phase shift of the polycrystalline complex (α-CD)2·Cd0.5·I5·26H2O (α-CD = α-Cyclodextrin) have been investigated over the frequency and temperature ranges of 0–100 kHz and 240–425 K. A Raman spectroscopy study is also accomplished in the temperature ranges of (i) 153–293 K and (ii) 303–383 K. From 276.2 up to 335.6 K – where all the water molecules of the crystal lattice exist – the transformation (H2O)tightlybound → (H2O)easilymovable takes place, resulting in the linear increment of the ac-conductivity in the lnσ vs. 1/T plot with activation energy Ea = 0.51 eV. In the range of 335.6–377.0 K a second linear part with Ea = 0.67 eV is observed attributed to the contribution of Cd2+ ions via the water-net. At T > 377.0 K, the abrupt decrease of the ac-conductivity up to 401.6 K is due to the removal of all the water molecules from the lattice. The phase shift presents a topical minimum at 404.3 K directly related to an order–disorder transition of the I− ions in some pentaiodide units and an abrupt decrease at T > 412.9 K due to the sublimation of iodine molecules. The Raman spectra at room temperature present two bands at 160 and 168 cm− 1 indicating the coexistence of two kinds of pentaiodide units I2·I−·I2 and I3−·I2 ↔ I2·I3−, respectively. Because of the inverse transformation (I3−·I2 ↔ I2·I3−) ↔ (I2·I−·I2) the band at 168 cm− 1 disappears as the temperature decreases whereas the band at 160 cm− 1 disappears during the heating process. The X-ray powder diffraction and the Rietveld analysis revealed a tetragonal crystal form with space group P42212 and lattice parameters that are in good agreement with the theoretical values.
    Solid State Ionics 05/2007; 178(s 11–12):793–799. DOI:10.1016/j.ssi.2007.02.027 · 2.11 Impact Factor

Full-text (2 Sources)

Available from
May 20, 2014