Electrochemical behaviour of hen-egg-white lysozyme at the polarised water/1, 2-dichloroethane interface

The electrochemical behaviour of hen-egg-white lysozyme (HEWL) was studied at the polarized water/1,2-dichloroethane interface. The voltammetric ion-transfer response was found to be dependent on the pH and ionic strength of the aqueous phase solution and also on the nature of the organic phase electrolyte anion. The current–pH behaviour of HEWL was dominated by the charge of the biomolecule at each pH, as indicated by the close relationship between the experimental peak currents and the theoretical curve for HEWL based on its known acid–base chemistry. Three organic electrolyte anions of differing hydrophobicities were investigated (TFPB−, TPBCl− and TPB−) and it was found that the ion transfer voltammetric peaks occurred at successively higher potentials in the order of increasing hydrophobicity, ΔϕTPB < ΔϕTPBCl < ΔϕTPBF. The voltammetric response was time dependent during multi-cyclic voltammetry experiments, with the formation of a white film of precipitate at the interface. A pre-peak consistent with adsorption of the HEWL ion transfer product at the liquid/liquid interface was also observed. The results suggest that an adsorption or re-arrangement of HEWL molecules with time at the interface is taking place. A mechanism for the response on application of a triangular potential waveform with cyclic voltammetry is proposed based on an i-C-i mechanism. Our results indicate that HEWL is electroactive at the polarized liquid/liquid interface and that such electrochemical methods may provide an approach to the label-free detection and characterization of protein molecules.