Studies of Polyelectrolyte Solutions VI. Effects of Counterion Binding by Dextran Sulfate and Dextran Phosphate in Aqueous/Organic Solvents

Abstract

ABSTRACT:The influence of the solvent on the degree of counterion binding of dextran sulfate and dextran phosphate is studied by activity measurements, dye-spectroscopy, and static light scattering. The solvents used are binary mixtures of water and organic solvents such as acetone, alcohol, and formamide. The mixing ratio is varied over a wide range in order to obtain different solvent dielectric constants. The results of investigations are as follows. First of all, in pure water the alkali ions are bound by dextran sulfate in the order Cs+ > K + >Na+> Li+, i.e., the degree of counterion binding for Li+ is lowest while that for Cs+ is largest. The inverse ion sequence is observed for dextran phosphate. In mixed solvents the ion sequence depends on the mixture used. Secondly, there exists no universal correlation between the dielectric constant of the solvent and the degree of counterion binding. However, it is common to all aqueous/organic solvent mixtures that the degree of counterion binding decreases as the fraction of the organic compound in the solvent increases. Thirdly, condensation theory is not valid in mixed solvents, while the Poisson-Boltzmann approach may be applied. Taking into account the size of the solvated small ions we find that the type of counterion binding of a divalent counterion, such as Mg 2 +, depends on the kind of the univalent counterions added in excess. For water/n-propanol it seems that in the presence of Li+, Mg2+ is bound delocalized, while in the presence of K +, Mg 2 + is bound by site binding. Fourthly, in water/formamide and water/dimethyl sulfoxide (DMSO) the organic solvents are adsorbed preferably by dextran sulfate. As a consequence, the dielectric constant at the polyion surface differs from that of the bulk solvent. Nevertheless, if these changes are taken into account, the predictions of condensation theory remain contradictory to the results of measurements.KEY WORDS Dextran Sulfate / Dextran Phosphate / Counterion Binding / Mixed Solvents / Activity Coefficients / Dielectric Constants / Dye Spectroscopy / Static Light Scattering / There has been great interest in studying the interactions between polyion and simple electrolytes, as this information provides a key to the understanding of biological processes since many biopolymers are polyelectrolytes. A well established result has been that an extra amount of counterions will bind on the polyion of almost all bulk counterions. For instance, in previous reports, 1 -3 we have shown that the polyion charge density and the kind of charged groups play an important role for the degree of counterion binding, but it is also expected that the solvent, in which the polyelectrolyte chains are dissolved, have some influence. Studies in water and aqueous/organic solvent mixtures are therefore very important. The reasons are:I. A certain property of the solvent such as a define value of the dielectric constant, s,, or a particular value of the refractive index, n, can be more precisely adjusted in mixed than in single solvents. ...

Experimental and Monte Carlo Simulation Studies on the Competitive Binding of Li+, Na+, and K+ Ions to DNA in Oriented DNA Fibers

“…Neither the A nor the B form of DNA exhibits selectivity for Na + or K + in mixtures of KCl and NaCl in ethanol/water solutions. Our data on the DNA affinities for the alkali metal ions obtained for DNA oriented fibers in ethanol/water mixtures are in general agreement with the results of the studies of DNA 61 and other phosphate polyelectrolyte solutions and ion exchangers ,,− obtained in water and in water/organic solvents. These studies report that an increase of the organic component in the eluent leads to a change of the metal ion selectivity order.…”

“…Experimental studies of competitive binding of K + , Na + , and Li + to DNA in oriented fibers equilibrated in ethanol/water solutions show that the affinity of DNA to these cations decreases in the order Na ≈ K > Li, which is opposite the sequence determined for DNA and other polyelectrolytes with phosphate cation exchange groups in aqueous solution. ,, Li + ions placed in ethanol/water medium lose their ability to bind specifically to DNA in the form of oriented fibers. Li + −DNA interactions can be qualitatively described by the electrostatic model in this case (in this work, by the GCMC simulation method).…”

The effect of polyelectrolyte counterion specificity, charge density, and conformation on polyelectrolyte-amphiphile interaction: The carrageenan/furcellaran-amitriptyline system

“…This affinity sequence coincide with that found for the DxS-amitriptyline system and the polysterenesulfonate (PSS)-amitriptyline system. 1 Moreover, in the DxS and the PSS cases the affinity sequence was found to correlate well with the sequence found for the mean activity coefficients of alkali ions in aqueous DxS 17,18 and PSS 19 solutions. Reports of the mean activity coefficients of Na ϩ and K ϩ for -carrageenan have indicated the same sequence 20 as indicated above but also the opposite trend has been reported.…”

How potassium came to be the dominant biological cation: of metabolism, chemiosmosis, and cation selectivity since the beginnings of life

“…It is well-known that the sulfonate group, −SO 3 − , is slightly selective to K + . [107,108] This means that in the extracellular liquids, rich in sodium, negative charges of these polyelectrolytes are less shielded by Na + and therefore are more active than if they were performed in the K + -rich cytoplasm.…”