COOPERATIVITY AND COMPOSITION OF LINEAR AMYLOSE-IODINE-IODIDE COMPLEX

Adsorption isotherms of iodine-iodide in the helical cavity of amylose (D.P. = 800) have been determined by spectrophotometric measurements of titration curves at various iodide concentrations (10-5 to 2.5 × 10 M KI) and temperatures. A new description of the stoichiometry of the bound species is given in terms of two limiting models: At very low iodide concentrations the predominantly bound species is considered to be I2 whereas at high iodide the bound species is I3- (the theoretical Ising lattice treatment predicts I42- ). Both models apply in the intermediate transition region where competition for binding sites occurs between I2 and I3-. The over-all composition of the bound species can be expressed approximately as I2·Ib- where b varies between zero and unity. Statistical analysis of the isotherms using the exact one-dimensional Ising model (n = 15) with first nearest neighbor interactions (between -1.6 and - 3.0 kcal/mol) shows that the degree of cooperativity is moderate. The main source of the stability of the complex originates from the intrinsic binding of a species by one turn of the amylose helix. The enthalpy and entropy of binding have been determined at 10-4 M KI. Factors affecting the position of λmax of the blue band are attributed to the aggregation of partially or completely filled amylose helices.