Michael Heyrovský

The reactions of cystine at mercury electrodes

Abstract Cystine in aqueous solutions reacts chemically with mercury, forming the surface-bound cysteine mercuric thiolate. When the potential of mercury is made less positive this compound is transformed (at a partially free mercury surface in a currentless process and at an occupied electrode under the passage of current) into cysteine mercurous thiolate. The latter compound is also formed in a reversible interaction of cystine with mercury in the potential range where cystine is adsorbed. At the negative end of this range the electroreduction of cystine occurs by the reduction of cysteine mercurous thiolate. At negatively charged mercury, where cystine is not adsorbed, two electrons are transferred directly to the cystine SS bond in an overall irreversible process. On the positive side the dissolution of mercury into solutions containing cystine starts by anodic formation of slightly soluble mercuric cystinate near the electrode surface.

The anodic reactions at mercury electrodes due to cysteine

Abstract The detailed mechanism of anodic reactions of cysteine on mercury electrodes has been ascertained by means of electrocapillary measurements together with d.c. and a.c. polarography and voltammetry. The reactions consist of electro-oxidation of mercury in two separate steps to cystein mercurous and mercuric thiolates respectively which are both strongly adsorbed at the electrode. At low coverages of the electrode by the mercurous product the mercuric thiolate is formed in the potential region of the second step by a non-faradaic surface process of disproportionation of the adsorbed mercurous thiolate; in consequence, only one faradaic step is then observed. The replacement of mercurous by mercuric thiolate at the electrode surface is accompanied by a pronounced change of electrode capacity. The path of the electrode reaction is determined by the form of mercury thiolates in the adsorbed state which depends strongly on the pH of the solution. In the first step in acidic solutions a compact film is formed of perpendicularly oriented mercurous cysteine thiolate molecules held together by strong lateral interaction, in alkaline solutions cysteine forms chelates with S and N atoms binding monomeric monovalent mercury. At positive potentials preceding the mass dissolution of mercury a direct formation of cysteine mercuric thiolate occurs in the solution, followed by its adsorption at the electrode surface.

Use of Solid Amalgam Electrodes in Nucleic Acid Analysis

Mercury meniscus modified solid amalgam electrodes of silver and of copper proved experimentally as acceptable substitutes for the hanging mercury drop electrode in highly sensitive cathodic stripping voltammetric analyses of nucleic acids and their components. Their surfaces can be simply reactivated electrochemically. The analyses are best done in weakly alkaline solutions, in case of silver amalgam electrode copper(II) ions can be added to the solution.

Electrochemical reactivity of homocysteine at mercury electrodes as compared with cysteine.

Homocysteine differs from cysteine in electrochemical behaviour due to slight differences in hydrophobicity and in structure of their complexes with metals. This shows in adsorptivity, in anodic reactions with mercury, in catalytic reductions of oxygen mediated by mercurous thiolates and of thiol-cobalt(II) complexes, but most markedly in catalytic evolution of hydrogen from solutions containing cobalt ions.

Adsorption effects of electroactive species in d.c. polarography demonstrated in the case of the anodic waves of cysteine

Careful analysis of d.c. polarographic mean and instantaneous currents helps to explain why the anodic electrolytic reaction of cysteine on mercury electrodes proceeds differently in acetate and in borate buffer solutions, even though in both solutions cysteine as well as the product of its electrode reaction are adsorbed at the electrode. In acetate buffer of pH 4.7, cysteine is adsorbed through the interaction of its sulphur atom with mercury, which ultimately leads to a steeper than reversible slope of the first polarographic wave, to a transformation of that wave into an adsorption pseudo-prewave, and to a ready appearance of the second wave. The adsorption of cysteine from borate buffer of pH 9.4, in contrast, causes a marked increase of the electrode capacity but only little interference in the electrode process.

Catalytic Hydrogen Evolution at Mercury Electrodes from Solutions of Peptides and Proteins

Publisher Summary The catalytic evolution of hydrogen at electrodes is catalyzed by a great variety of species, among others, by peptides and proteins; when carried out on small laboratory scale, it can become a useful way for studying physio-chemical properties of the catalysts, as well as for their sensitive quantitative determination. This chapter discusses the aspect of the general electrochemical reaction of peptides and proteins. Peptides and proteins can cause the catalytic evolution of hydrogen on mercury electrodes as catalysts of both categories ((1) organic molecules or ions containing nitrogen atom with a free electron pair, alone or together with thiolic sulfur; and (2) reduced forms of various transition metal complexes with particular ligands), provided they contain catalytically active groups or form catalytically active complexes with appropriate cations. Although the first experience with catalytic evolution of hydrogen on mercury electrodes was gained with the “presodium” current, the great majority of results in research and in practical applications of electrochemical activity of peptides and proteins have been based on hydrogen evolution catalyzed by their complexes with cobalt ions. Only the renewed, successful attempts to catalyze hydrogen evolution by complexes of metals other than cobalt and the recent discoveries based on catalytic hydrogen evolution in absence of cobalt have spurred new development of the specific field of peptide and protein electrochemistry. The inexpensive electrochemical apparatus offers very sensitive methods for the study and analysis of peptides and proteins.

Ac polarography and ac voltammetry of cystine

It has been shown in a recent paper on the reactions of cystine at mercury electrodes [l] that, depending on the potential, cystine undergoes three different kinds of interaction with mercury. First, at the positive limit of polarization, the anodic dissolution of mercury is accelerated by the formation in the solution of mercury cystinate which is subsequently strongly adsorbed at the electrode surface (in the cystinate, mercury is bound to the carboxylic and the amino groups of cystine). Second, in the absence of an applied potential mercury reacts spontaneously with cystine, forming a surface layer of mercuric cysteine thiolate. The potential which mercury thereby acquires lies near the potential of the saturated calomel electrode (SCE) and changes according to the pH of the solution. Finally, in the region around 0.3 V, where cystine is reversibly adsorbed on mercury, a complex equilibrium is established between cystine and mercurous cysteine thiolate. In the present communication we report how the three kinds of interaction can be identified by means of ac techniques. For our measurements we built an apparatus which allows the alternating current in phase with the applied alternating voltage to be distinguished from that out of phase with the voltage [2]. The dropping mercury electrode (DME) and the hanging mercury drop electrode (HMDE) were used as working electrodes for the polarographic and voltammetric studies respectively. The first study of the a.c. polarography of cystine was carried out by Breyer and coworkers [3,4] who used the simple method without phase distinction. They observed two ac peaks; they ascribed the more positive and more prominent peak to the adsorption of cystine and the second peak to a reversible reduction of the adsorbed cystine. Miller [5] and Mairesse-Ducarmois et

The electroreduction of methyl viologen

In electroreduction, after the transfer of the first electron to methyl viologen, MV2+, resulting in formation of the radical cation, MV˙+, the electron transfer to MV˙+ without chemical complications occurs only under special conditions: the species more readily reduced than MV˙+ are the dication or monocation dimers, MV22+ or MV2˙+, formed in reactions in solution near the electrode.

Voltammetry of two single-stranded isomeric end-labeled -SH deoxyoligonucleotides on mercury electrodes.

Voltammetry of isomeric end-labeled -SH deoxyoligonucleotides on a hanging mercury drop electrode depends on the dislocation of the electroactive components along the strand as well as on their adsorptivity with respect to adsorptivity of the other parts of the molecule.

Catalytic and photocatalytic reduction of water by the reduced forms of methylviologen

Polarographic results show that the reaction between water and the methylviologen redox system in the absence of catalyst is due to reduced methylviologen in its molecular form or in the form of its monocation dimer and that it is enhanced by light absorbed by these forms.