Brdička-type processes of cysteine and cysteine-containing peptides on silver amalgam electrodes

Silver solid amalgam electrode (AgSAE) was used for differential pulse voltammetric (DPV) measurements of cysteine and cysteine-containing peptides, glutathione, γ-Glu-Cys-Gly and phytochelatin (γ-Glu-Cys)3-Gly (PC3), in the presence of Co(II) ions. It had been established earlier that cysteine-containing peptides and proteins catalyze hydrogen evolution at mercury electrodes in presence of cobalt salts; these processes are known as the Brdička reaction. DPV signals measured with the AgSAE, the surfaces of which had been modified by mercury meniscus or mercury film, were qualitatively the same as those obtained with the hanging mercury drop electrode (HMDE). With these electrodes the number and the intensity of Brdička signals of cysteine, glutathione and PC3 differed, making a distinction among them possible. On the other hand, with the polished silver solid amalgam electrode (the surface of which was completely free of liquid mercury) all three compounds produced only one but strikingly intense peak in the region of Brdička reaction. Using this signal, cysteine, glutathione as well as PC3 could be determined at 10−8 M level, representing sensitivity up to 2 orders of magnitude better than attained with the mercury-modified AgSAEs or HMDE.