A Comparative Study of the Interaction between Nafcillin and Catalase by Equilibrium Dialysis and ζ-Potential Measurements

The characterization of the interactions of the penicillin nafcillin with the protein catalase in aqueous solution at pH 3.2, 7.4, and 10.0 at 25 °C has been determined using a combination of equilibrium dialysis and ζ-potential measurements. At pH 3.2, the ζ-potential of catalase tends to reverse charge from positive to negative values as the negatively charged penicillin bind to it. At pH 7.4 and 10.0, this ζ-potential becomes more negative as the nafcillin concentration increases. The number of drug molecules bound to catalase per protein molecule (υ) was obtained from equilibrium dialysis and compared with the ζ-potential data. Nafcillin binds largely to catalase to various extents ranging from ∼750 (pH 3.2) to ∼10 000 (pH 7.4 and 10.0) drug molecules per catalase molecule as the free concentration approaches the critical micelle concentration (cmc) of the free drug. Gibbs energies per drug molecule bound (Δ G υ ) were obtained as a function of υ from equilibrium dialysis using the Wyman potential ( J. Mol. Biol. 1965, 11, 631). Δ G υ is large and negative at low values of penicillin concentration and becomes less negative as the drug concentration increases. Δ G υ against drug concentration plots obtained from the interpretation of the variation of ζ-potential measurements were of the form and magnitude similar to those obtained from direct binding measurements using the equilibrium dialysis technique.