Influence of pH on lysozyme conformation revealed by dielectric spectroscopy

In this paper we report permittivity measurements in the frequency range 10(5)-10(8) Hz, at the fixed temperature of 20 degrees C, on lysozyme dissolved in water and in a water-ethanol mixture (ethanol concentration 0.1 molar fraction) varying the pH of the solutions from acid (congruent to pH 2) to basic (congruent to pH 10). The experimental data were fitted with the Cole-Cole equation and from the dispersion parameters the effective hydrodynamic radius and the electric dipole moment of the protein in different conditions of pH were calculated. The results confirm a conformational effect induced on lysozyme by ethanol, reported in our previous work. For pH in the range 4-6, where the enzymatic activity of lysozyme reaches its maximum, our results indicate a stable conformation of the protein. Out of this pH interval aggregation and expansion processes are present.