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

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 % (similar to 25 mN w.r.t. sulfate groups). Na+, K+ and Ca2+ cations were added as chloride salts. Values of peak-maximum temperature (T-max) in DSC cooling and heating scans (0.5 degrees 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+ thorn cations which act indirectly by suppressing charge. On progressive addition of NaCl or KCl to solutions incorporating Ca2+ at concentrations of 5mM or 12.5mM (stoichiometric equivalence) the values of T-max 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 T-max 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. (C) 2018 Elsevier Ltd. All rights reserved.