The net electric charge of proteins. A comparison of determinations by Donnan potential measurements and by gel electrophoresis.

We compare a new method for the determination of the net charge of proteins based on Donnan potential measurements, as described briefly by Ojteg, G., Nygren, K. and Wolgast, M. (1987) Acta Physiol. Scand. 129, 277-286, with a conventional method using polyacrylamide gel electrophoresis. The new technique utilizes the Donnan potential, which develops over a semipermeable membrane that separates the non-permeating protein from the surrounding bath of the same ionic composition as the protein solution, to determine the net valency. The advantages of this method, besides its simplicity, are that it can determine the charge of, e.g., a protein in a free-fluid phase and that the pH and ionic composition of the bathing fluid can be varied over a broad range. The Donnan potential decreased to half its original value when the ionic strength was doubled. Usually a protein concentration of 1-10 mg.ml-1 must be used. The Donnan potential method was applied to determine the net charges of a series of proteins with different isoelectric points. The values showed close agreement with the data obtained by gel electrophoresis.