Anomalous stiffening and ion-induced coil-helix transition of carrageenans under monovalent salt conditions.

The macromolecular conformations of anionic polysaccharides with decreasing linear charge densities­lambda, iota, and kappa carrageenan­, at varying NaCl concentrations, are studied by single-chain statistical analysis of high-resolution atomic force microscopy (AFM) images. Lambda remains in the random coil conformation, whereas iota and kappa undergo ion-induced coil-helix transitions, with a 2-3-fold increase in chain rigidity. At low ionic strengths, I, the polymer chains sequester Na⁺, leading to a greater flexibility, and beyond a critical I to the formation of an intramolecular single helix. The persistence length exhibits a sublinear dependence on the Debye screening length, κ⁻¹, L(p)(e) ∼ κ(-y) (with 0 < y < 1), deviating from the classical polyelectrolyte behavior expressed by Odijk-Skolnick-Fixman or Barrat-Joanny models. Above a certain I, the L(p) shows an upturn, resulting in polymer stiffening and nonmonotonic behavior. This phenomenon is inferred from specific ion-polymer interactions and/or nonlinear electrostatic physics involving ion-ion correlations.