Complex formation of chitosan with iodine and its structure and spectroscopic properties - Molecular assembly and thermal hysteresis behavior

To elucidate the factors responsible for the complexation of chitosan with iodine and to gain insight into the structures and spectroscopic properties of chitosaniodine (CI) complexes, extensive studies were performed on the effects of iodine/ chitosan concentrations and temperature on Cl complexation and its physicochemical properties in acidic solutions containing excess KI by means of various spectroscopic (absorption, CD. etc.) and structural-analysis (SAXS, etc.) measurements and molecular dynamics (MD) simulations, The CI complex exhibited absorption spectra with a peak at around 500 urn, regardless of the iodine/chitosan concentrations and temperature. Correspondingly, the Cl complexes exhibited mutually split CD bands with opposite signs (+, -) at around 500 mn. The Cl complex showed thermal hysteresis, i.e., an irreversible reaction process involved in complexation and color formation. Resonance Raman spectra measurements revealed that the iodine species responsible for the purple coloring of the complexes is primarily I-3(-) ions. Moreover, C-13-NMR spectra measurements inferred that the interaction sites in the D-glucosamine moiety in chitosan chains with bound iodine are the hydroxyl groups on C-1, C-3, and C-4. Structural studies based on analytical ultracentrifugation, and SAXS and SANS measurements indicated that the molecular assembly of extended chitosan chains plays a vital role in Cl complexation. MD calculations predicted that the irreversibility and thermal hysteresis behavior of the CI complexes are due to a crystalline-like extended --> compact folded conformational transition.