Anion effects on equilibria and kinetics of the disorder–order transition of κ‐carrageenan

The effect of a number of tetramethylammonium salts on the equilibria and kinetics of the disorder to order transition in the polysaccharide κ‐carrageenan have been investigated. Data from the temperature dependence of optical rotation show that anion stabilization of the ordered form follows the lyotropic series I− > Br− > NO > Cl− > F−. Stopped‐flow polarimetry was used to study the kinetics of conformational ordering following a rapid increase in salt concentration. The transition to the new equilibrium position was shown to be biphasic for all of the tetramethylammonium salts studied. The rate equation for the fast phase and the temperature dependence of the observed forward rate constant accord with a cooperative dimerization process. Activation parameters for helix nucleation, ΔH* and ΔS*, vary with both salt concentration and (at constant ionic strength) the anion type, increasing through the lyotropic series from I− to F−. The slow phase shows second‐order kinetics, and is interpreted as further stabilization of the ordered form either through limited aggregation or annealing. The rate constant for the slow phase also follows the lyotropic series. Thus we have shown that both the growth and nucleation processes are anion dependent.