Kinetic study of the proton association-dissociation of the functional group on carboxymethyl sephadex C-25 by the pressure-jump method with conductivity detection

Abstract

A kinetic study of the proton association—dissociation reaction on a Sephadex C-25 surface was made taking into account its surface potentials. The Sephadex C-25 itself had a ζ-potential of −32 mV in 3 m M NaCI solution. In order to explain this, the amount of negatively charged groups which the Sephadex originally had was determined by HCl titration. Then, the sum of the amount determined by this titration (0.82 mmol/g) and the amount determined by the usual NaOH titration (4.3 mmol/g) was used as the total amount of functional groups in calculating the acid dissociation constant, K a. The surface potential of the Sephadex was estimated from the K a. In parallel, the rate of the reaction was measured by a pressure-jump method with conductivity detection. A single relaxation was observed in the time scale of 0.1 s. The relaxation time became fast with an increase in pH. On the basis of the pH dependences of the relaxation time and the above surface potential, the relaxation was attributed to the proton association—dissociation reaction of functional groups (predominantly the carboxymethyl group) of the Sephadex.