Counterion dynamics of κ- and ι-carrageenan aqueous solutions investigated by the dielectric properties

The temperature dependence of the dielectric properties of κ- and ι-carrageenan aqueous solutions with various counterion species, K, Ca, and Na, were investigated over the frequency range 10^-2-10⁶ Hz and the temperature range 2.0-80.0 °C. In the case of κ-carrageenan solutions, just below the coil-to-helix transition temperature, T_CH, the dc conductivity sharply decreases within a few degrees of temperature. In contrast, the sharp decrease was not observed for ι-carrageenan solutions. The dielectric relaxation process with the relaxation time ∼100 μs, which can be assigned to the counterion fluctuation in the parallel direction to the helical axis, arises below T_CH. Just below T_CH, the relaxation strength increases sharply with decreasing temperature for both the carrageenan type with all the counterion species exhibiting the coil-to-helix transition, reaching ∼10³ at the temperatures far below T_CH. These findings indicate that the counterions are tightly bound to helical molecules due to an increase of the charge density along the helical axis during the coil-helix transition. The relaxation time reflecting the fluctuation distance increases sharply in the initial stage of gelation and gradually reaches a constant value. We think that the formation of the high charge density region, which is connected with the aggregated region of helices, is determined by which process, a growth of the length of the helices or the aggregation of the helices, precedes in the initial stage of gelation.