Equilibrium and dynamic studies of the disorder–order transition of kappa carrageenan

Abstract

The equilibria and dynamics of the transition from disorder to order in the polysaccharide kappa carrageenan have been investigated over a range of KCl concentrations and temperatures. Kinetic observations can be explained by the following sequential mechanism for ordering of disaccharide residues following a salt-concentration jump: conformational change in disordered form C°→ C (II) dimerisation 2C ⇌ H₂(III) aggregate formation xC +yH₂⇌(H₂)_(½x+y)(IV) where C indicates a random coil and H a helix residue. Reaction (III) was investigated quantitatively using a stopped-flow polarimeter, and the rate equation shown to be that of a second-order forward, first-order reverse equilibration, consistent with double-helix formation. Forward rate constants extrapolate to zero at the transition midpoint, as expected for a cooperative transition.

Rate and activation parameters for helix nucleation in 0.50 mol dm^–3 KCl are k₂(298 K)=(9±1)× 10³ dm³ mol^–1 s^–1, ΔH^*= 5±3 kJ mol^–1 and ΔS^*=–150±10J mol^–1 K^–1. Parallels are drawn with polynucleotide double-helix formation, and in particular the low value of ΔH^* is consistent with a nucleation length of 1 or 2 disaccharides. A lower bound of 36±3 disaccharides is estimated for the cooperative length from equilibrium and d.s.c. data for the transition.