Binding of Calcium and Sodium Ions to Chondroitin Sulfate in Aqueous Solution

Abstract

The amounts of both Ca²⁺ and Na⁺, x and y, respectively, bound to C-type chondroitin sulfate (Chs) from an aqueous solution of CaCl₂ mixed with NaCl were obtained by the membrane equilibrium method. To estimate x and y under the effect of the Donnan equilibrium, [Ca²⁺] and [Cl^-] for both sides of the dialysing tube and [Chs] for the inner side were analysed. The binding isotherm for Ca²⁺ was of the Langmuir type, and both x and k_Ca²⁺, the binding constant for Ca²⁺, decreased with ionic strength, J. On the other hand, the binding isotherm for Na⁺ was not of the Langmuir type but its shape was explained as the competitive adsorption of the two components, Na⁺ and Ca²⁺, because Ca²⁺ binds more strongly to Chs than Na⁺ and, therefore, the effect of coexistence of Ca²⁺ with Na⁺ cannot be neglected when the Na⁺ binding is examined. It was also suggested that y and k_Na⁺ decreases with J when the amount of the occupied site by the Ca²⁺ is kept constant at x. The degree of the ionization, z, of Chs was found to be 0.40-0.41 for Na₂Chs and 0.20-0.21 for CaChs by this membrane equilibrium method assuming x+y+z=1. These values coincide with those obtained from the vapour pressure osmometry by the authors and with the literature values calculated with considering the Manning theory.