The interaction of carboxymethylamylose and diethylaminoethylamylose with iodine

Spectrophotometric titrations of slightly substituted carboxymethylamylose (CM-Amy) and diethylaminoethylamylose (DEAE-Amy) with iodine in the presence of iodide (I₂/I^?) were carried out as a function of iodide concentration, temperature, and polymeric charge. Binding isotherms for the polymer-I₂/I^? complex are reported in terms of an apparent binding constant (K_a) plotted versus degree of saturation of the complex (θ). The dependence of K_a upon polymeric charge is interpreted as evidence for the negatively charged character of the bound species. The cooperative nature of the binding process is evident in the positive slope of K_a vs (θ). Whereas the apparent binding constants and binding cooperativities for the derivatives are smaller than for the amylose-I₂/I^? complex, the binding enthalpies deduced from the temperature dependence of K_a at θ = 0.5 appear to be the same for amylose and CM-Amy. A viscometric titration of fully charged CM-Amy with I₂/I^?, conducted at dialysis equilibrium between the CM-Amy-I₂/I^? solution and the polymer-free solvent phase, disclosed a maximum in the plot of intrinsic viscosity (η]) vs θ. The increase in η] at small θ was interpreted as a reflection of polyelectrolyte expansion provoked by absorption of the negatively charged bound species; the subsequent decline in η] is attributed to stabilization by I₂/I^? of compact helical sequences or to the formation at higher θ of intermolecular aggregates.