Electrophoretic mobility and zeta potential of lysozyme crystals

Using free-solution capillary electrophoresis, the electrophoretic mobility of micro m-sized lysozyme crystals in their growth solution at 283 K, 1.5%(w/v) NaC1, and over a range of pH values between 3.59 and 5.70 has been measured. Under these conditions, the mobility is independent of crystal size, while the calculated zeta potential increases from +8 to +24 mV as the pH decreases. Since the pH dependence of the zeta potential mirrors the pH dependence of charge on the free molecule, as determined by acid titration, it is concluded that the charge on the crystal is a result of H(+) adsorption from solution.