Counterion-accelerated diffusion of aqueous serum albumin

A modified Harned conductimetric procedure has been used to measure binary diffusion coefficients at 25°C for aqueous sodium and potassium salts of the protein bovine serum albumin (M_nBSA; M=Na⁺ or K⁺; n=2 to 23). The condition of electroneutrality requires that the BSA ions diffuse at the same speed as the relatively mobile sodium or potassium counterions. Consequently, the protein ions diffuse 3 to 8 times more rapidly than the isoionic protein diffuses. An approximate analysis of counterion-accelerated diffusion of high molecular weight solutes is presented which shows that the increase in diffusivity with the number of counterions is a thermodynamic effect brought about by an increase in the free energy gradient of the solutes.