TRANSPORT OF PROBE IONS IN SOLUTIONS OF BIOLOGICAL POLYELECTROLYTES

Transport of monovalent cations was studied in solutions of biological polyelectrolytes, the sodium (or potassium) salts of anionic polysacc harides iota-, kappa-, lambda-carrageenan, dextran sulfate, and chondr oitin sulfate, by steady-state voltammetric reduction bf the probe ion s Tl+ and Hf at mercury film and platinum disc microelectrodes, respec tively. Diffusion coefficients of the electroactive probe ion are dete rmined rapidly and precisely from steady-state, transport-limited curr ent at microelectrodes in solutions with large excess and without supp orting electrolyte over a wide range of polyelectrolyte and probe ion concentrations. Electrostatic interactions between polysaccharide anio n and probe ion decrease the value of the diffusion coefficient of the probe ion with respect to the value without the polyelectrolyte, with the most pronounced effect in solutions without electrolyte. These in teractions are quantified by the transport ratio, D/D-0, of the diffus ion coefficient of the probe ion with polyelectrolyte, D, to that in s olution without polyelectrolyte, D-0. The values of D/D-0 without supp orting electrolyte for thallium ion are 0.59, 0.67, 0.50, 0.35, and 0. 70, for iota-, kappa-, lambda-carrageenan, dextran sulfate, and chondr oitin sulfate, respectively. For the hydrogen probe ion these transpor t ratios are 0.52, 0.79, 0.57, and 0.26, for iota-, kappa-, lambda-car rageenan and dextran sulfate, respectively. Experimental results are c ompared with theoretical predictions based on Manning's linear charge model and the Poisson-Bolzmann cylindrical cell model. According to bo th theories the dimensionless transport of the counterion in deionized solution is related to the charge separation in the polyelectrolyte: the higher the linear charge density, the greater the interaction and the smaller the transport ratio. Interactions in solutions of these bi ological polyelectrolytes are in accord with both theories, to a reaso nable degree of accuracy, considering the uncertainty in the distance between charges.