DETERMINATION OF THE EFFECTIVE CHARGE OF A PROTEIN IN SOLUTION BY CAPILLARY ELECTROPHORESIS

This paper describes two methods to estimate the effective charge of a protein in solution by capillary electrophoresis and demonstrates the se methods by using representative proteins. In one method, a ''charge ladder''-a series of derivatives of a protein differing by known incr ements of charge but differing only minimally in hydrodynamic drag-is generated by covalent modification of the epsilon-amino groups of lysi nes with 4-sulfophenyl isothiocyanate or acetic anhydride. In the seco nd method, the equivalent of a charge ladder is produced by noncovalen t association of a protein with differently charged ligands. Analysis of the electrophoretic mobilities of the protein and its derivatives a s a function of added charge allows the effective charge to be estimat ed for the unmodified protein. This type of analysis permits estimatio n of the effective charge of a protein without knowing its composition , structure, or amino acid sequence.