Diffusion in buffered protein solutions: combined Nernst–Planck and multicomponent Fick equations

Abstract

The Taylor dispersion method has been used to measure diffusion in aqueous solutions of bovine serum albumin (BSA) buffered with H₃PO₄–KH₂PO₄(pH 2.2), KH₂citrate–K₂Hcitrate (pH 4.5), or K₂Hcitrate–K₃citrate (pH 5.8). Analysis of the refractive-index profiles across the eluted solute peaks shows that diffusing BSA produces large coupled flows of the buffer electrolytes. In the 0.100 mol dm^–3 H₃PO₄–0.100 mol dm^–3 KH₂PO₄ buffer, for example, each mole of BSA counter-transports 1500 mol H₃PO₄ and co-transports 850 mol KH₂PO₄. Nernst–Planck and multicomponent Fick equations are used to show that the coupled flows of the buffer electrolytes are driven by the electric field which is generated by the diffusion of BSA and its counterions. Counterintuitively, coupled diffusion is not eliminated by using high concentrations of supporting electrolytes.