Thermodynamics of the conformational transition of biopolyelectrolytes: The case of specific affinity of counterions

A formal development of the Counterion Condensation theory (CC) of linear p olyelectrolytes has been performed to include specific (chemical) affinity of condensed counterions, for polyelectrolyte charge density values larger than the critical value of condensation. It has been conventionally assumed that each condensed counterion exhibits an affinity free-energy difference for the polymer (Delta G(aff)). Moreover, the model assumes that the entha lpic and entropic contributions to Delta G(aff) i.e., Delta H-aff and Delta S-aff., both independent of temperature, ionic strength and polymer concen tration. Equations have been derived relative to the case of the thermally induced, ionic strength dependent, conformational transition of a biopolyel ectrolyte between two conformations for which chemical affinity is supposed to take place. The experimental data of the intramolecular conformational transition of the ionic polysaccharide K-carrageenan in dimethylsulfoxide ( DMSO) have been successfully compared with the theoretical predictions. Thi s novel approach provides the enthalpic and entropic affinity values for bo th conformations, together with the corresponding thermodynamic functions o f nonpolyelectrolytic origin pertaining to the biopolymer backbone change p er se, Le., Delta H-n.pol and Delta S-n.pol according to a treatment previo usly shown to be successful for lower values of the biopolyelectrolyte line ar charge density. The ratio of Delta H-n.pol to Delta S-n.pol was found to be remarkably constant independent of the value of the dielectric constant of the soh,ent, from formamide to water to DMSO, pointing to the identity of the underlying conformational process. (C) 1999 John Wiley & Sons, Inc.