Role of cations in the conformation of iota and kappa carrageenan

Abstract

Cation binding to iota and kappa carrageenan has been investigated by i.r. spectroscopy, differential scanning calorimetry, optical rotation, intrinsic viscosity and light scattering. From measurements of the salt dependence of the midpoint of the thermally induced order–disorder transition the increase in the number of cations associated with each disaccharide unit was estimated. I.r. studies of sulphate stretching frequencies for the ordered conformation indicate that association of tetramethylammonium ions to carrageenan is by simple atmospheric binding. With K⁺, by contrast, there is evidence of ion-pair formation, with stronger binding to kappa carrageenan than to iota. Enthalpy changes for the order–disorder transition support these conclusions. Comparison of intrinsic viscosity at high temperature in sodium or potassium chloride solution with values obtained in tetramethylammonium chloride solution indicate a less expanded conformation, consistent with a reduction in charge density by site binding of Na⁺ and K⁺, but not Me₄N⁺. Binding constants were estimated from the variation in optical rotation of the disordered form with [Na⁺] and [K⁺]. Light-scattering studies for segmented iota carrageenan in the high-temperature (disordered) conformation give radii of gyration for the Na⁺ and K⁺ salt forms which are close to the value calculated for an unperturbed random coil, while in tetramethylammonium chloride the radius of gyration is appreciably higher.