A dye spectrophotometric method for binding studies of Zn2+ and Mn2+ by biopolyelectrolytes

A dual-wavelength dye spectrophotometric method is reported for measuring zinc and manganese activities using the dyes tetramethylmurexide (TMMX) and murexide (MX) respectively. The method is applied to the measurement of the activities of these metal ions in solutions of the polyelectrolyte dextransulfate with added sodium chloride. Polyion concentrations, Cp (expressed as moles sulfate ion litre ) of 0.001 and 0.002 are studied at total ionic strengths 0.005, 0.0075, 0.01, 0.02 and 0.03 mole/1. Divalent metal ion concentrations are varied between 0 and 1.0 Cp. The results for the metal ion activities are expressed in the form of binding isotherms, theta2 versus C2/Cp (theta2 = C2b/Cp; C2b = bound divalent metal ion concentration) and Scatchard plots, K2 versus theta2/(C2-C2b), at different ionic strengths. The experimental data are correlated with the "two-variable theory" eveloped for these mixed counterion systems by Manning. This comparison shows that the observed decrease in theta2 and K2 with ionic strength at fixed C2 and Cp is generally well predicted by the two-variable theory. Both Zn and Mn bind to the same extent to dextransulfate. This observation, and the reasonable agreement of the data with the "two-variable theory" may be interpreted as indicating a delocalized form of binding of these metal ions to the polymer.