Dynamics of cation-induced conformational ordering in solutions of segmented iota carrageenan

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Abstract
Optical-rotation and stopped-flow polarimetric studies of the salt-induced ordering of iota carrageenan segments have been carried out in tetramethylammonium, sodium and potassium chloride solutions over a range of temperatures and salt concentrations. Equilibrium and kinetic data accord with a cooperative dimerisation process, consistent with double-helix formation. The transition mechanism was further characterised by comparing the kinetics of reaction of fully disordered and partially ordered samples. The activation parameters for helix nucleation show that variations in ΔG* 298 are small, but changes in ΔH* and ΔS* with salt form and concentration (c) are substantial. The following values hold at the low and high extremes of concentration studied. [graphic omitted]. Changes in the activation parameters at high [Na+] and [K+] correlate with increased flexibility of the disordered form due to a reduction in interchain electrostatic repulsion on ion-pair formation.

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