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Anomalous Stiffening and Ion-Induced Coil–Helix Transition of Carrageenans under Monovalent Salt Conditions

Schefer, Larissa, Usov, Ivan, Mezzenga, Raffaele
Biomacromolecules 2015 v.16 no.3 pp. 985-991
atomic force microscopy, electrolytes, ionic strength, kappa carrageenan, models, physics, polymers, screening, sodium, sodium chloride, statistical analysis
The macromolecular conformations of anionic polysaccharides with decreasing linear charge densities − lambda, 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, Lₚᵉ ∼ 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 Lₚ 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.