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 polyelectrolytes 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, (DeltaG(aff)). Moreover, the model assumes that the enthalpic and entropic contributions to DeltaG(aff), i.e., DeltaH(aff) and DeltaS(aff), are both independent of temperature, ionic strength and polymer concentration. Equations have been derived relative to the case of the thermally induced, ionic strength dependent, conformational transition of a biopolyelectrolyte between two conformations for which chemical affinity is supposed to take place. The experimental data of the intramolecular conformational transition of the ionic polysaccharide kappa-carrageenan in dimethylsulfoxide (DMSO) have been successfully compared with the theoretical predictions. This novel approach provides the enthalpic and entropic affinity values for both conformations, together with the corresponding thermodynamic functions of nonpolyelectrolytic origin pertaining to the biopolymer backbone change per se, i.e., DeltaH(n.pol) and DeltaS(n.pol), according to a treatment previously shown to be successful for lower values of the biopolyelectrolyte linear charge density. The ratio of DeltaH(n.pol) to DeltaS(n.pol) was found to be remarkably constant independent of the value of the dielectric constant of the solvent, from formamide to water to DMSO, pointing to the identity of the underlying conformational process.
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Dipartimento di Biochimica, Biofisica e Chimica delle Macromolecole, Università di Trieste, via L. Giorgieri 1, I-34127 Trieste, Italy. paolese@bbcm.univ.trieste.it
Benegas
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JC
Pantano
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Vetere
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D001704:Biopolymers / Q000737:chemistry*; D002351:Carrageenan / Q000737:chemistry; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D004573:Electrolytes / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation; D011487:Protein Conformation; D013816:Thermodynamics
D001704:Biopolymers; D004573:Electrolytes; D002351:Carrageenan; D004121:Dimethyl Sulfoxide