HELIX COIL TRANSITIONS OF IONIC POLYSACCHARIDES ANALYZED WITHIN THE POISSON-BOLTZMANN CELL MODEL .3. SOLVENT EFFECTS

The effect of mixed mono- and divalent counterions on the helix-coil transition of k- and -carrageenan (two charged polysaccharides) in formamide has been studied experimentally and theoretically, and the results are compared with our previously reported results obtained with water as the solvent. In either solvent, the addition of 1:1 electrolyte to a system containing divalent counterions may either stabilize or destabilize the helix conformation, depending on the exact conditions. However, this destabilizing effect appears in a more narrow range of salt concentrations in formamide as compared to the aqueous case. Furthermore, the ability of divalent cations to promote helix formation at comparatively low concentrations is less pronounced in formamide than in water. Theoretically, the electrostatic interactions are analyzed within the full Poisson-Boltzmann equation in a cylindrical cell model, and it is found that this model can explain the differences in the salt effects in the two solvents in a semiquantitative way by only taking their differing dielectric constants into account. In addition to the above purely electrostatic effects, some new observations regarding the cation specificity of the carrageenan helix-coil transition in dimethyl sulfoxide and formamide are reported. © 1990, American Chemical Society. All rights reserved.