Effect of monovalent cations on calcium-induced assemblies of kappa carrageenan

Abstract

The effect of Na+, K+ and Ca2+ cations on the thermal stability and aggregation of kappa carrageenan double helices has been explored by differential scanning calorimetry (DSC). Previous studies have shown that kappa carrageenan helices bind K+ cations, but not Na+. The kappa carrageenan used in this work was therefore in the Na+ salt form, to avoid complications from site-bound counterions to the polymer, and was studied at a fixed concentration of 1.0 wt % (∼25 mN w.r.t. sulfate groups). Na+, K+ and Ca2+ cations were added as chloride salts. Values of peak-maximum temperature (Tmax) in DSC cooling and heating scans (0.5 °C/min) increased progressively with increasing salt concentration, following the order Na+< Ca2+< K+, but greatest thermal hysteresis was seen with Ca2+. Our proposed interpretation is that Ca2+ cations "cement" the carrageenan helices together by binding directly between them, giving greater thermal stability, and thus greater hysteresis, than K+ cations which act indirectly by suppressing charge. On progressive addition of NaCl or KCl to solutions incorporating Ca2+ at concentrations of 5 mM or 12.5 mM (stoichiometric equivalence) the values of Tmax moved asymptotically towards those seen for the same concentrations of the monovalent cations in the absence of calcium, suggesting progressive displacement of site-bound Ca2+. Thus Tmax for the order–disorder transition was increased by KCl but reduced by NaCl, with the strange consequence that addition of NaCl lowered the transition temperature rather than raising it.