Quantitative evaluation of shift of slipping plane and counterion binding to lysozyme by electrophoresis method

Measurement and analysis of electrophoretic mobility (EPM) are widely used to investigate electric charging properties of proteins. However, the proper way of analysis for EPM of protein has not yet been fully consolidated. In this study, the EPM of hen-egg-white lysozyme (LSZ) was measured as a function of pH at different concentrations of KCl solutions. The obtained experimental EPMs are compared to theoretical EPMs which were calculated from charge amount from proton titration. Theoretical EPMs were calculated by a set of models for a small rigid particle including Poisson-Boltzmann model and the effect of double-layer relaxation and by that for a soft particle neglecting the relaxation effect. The results of comparisons show that one can analyze the EPM of LSZ as a small rigid particle. Nevertheless, all analyses overestimate experimental data. We presume that these discrepancies are caused by the shift of slipping plane from the surface and/or by binding of counterion to LSZ. Therefore, we examined these two effects on the analyses of EPM. Our analyses demonstrate that introducing the 0.5-2 nm shift of slipping plane or the 40-80 % reduction of effective charge generates the quantitative agreement between theoretical EPMs and experimental data. We find the required amount of reduced charge is 4-5 elementary charges per LSZ irrespective of pH and ionic strength below pH 7.