Disulfide groups, mercury electrode

Polarographic studies of the reduction of the parent l,2-dithiole-3-thione (27) show two waves.169 The first one corresponds to a two-electron reduction of the thiocarbonyl group to a thiol followed by an oxidation to disulfide 121. The second wave is a catalytic hydrogen wave. The reduction could not be carried out on a preparative scale owing to reactions with the mercury electrode. 

A second mechanism has been proposed for proteins with disulfide groups (Section 2.3.1) and ferredoxins (Section 2.3.2.1) whose concentration dependence reaches a constant level. Therefore only the adsorption layer can be reduced, whereas at a mercury pool electrode the reduction of all molecules in solution takes place. For this reason, a slow exchange in the adsorption layer is very probable and electron hopping may be inhibited because porphyrin rings are absent. 

There have been few direct electrochemical studies of peroxidase and catalase due to the highly irreversible nature of these electrode reactions. Horseradish peroxidase was found to be electroinactive at the dropping mercury electrode. Tarasevich and co-workers observed a cyclic voltam-metric response for the electron transfer of horseradish peroxidase at an amalgamated gold electrode. However, this response was ascribed to the disulfide bonds of the protein at neutral pH and not to the heme group. No response was detected at pyrolytic graphite electrodes. ... 

The upstream electrode acts as a very-low>volume postcolumn reactor. The simplicity of this sdieme is easily appreciated by the fact that previous schemes to detect thiols and disulfides required wet chemical steps to sepa> rate the groups, chemical reduction of the disulfide, and subsequent de > tion of all as thiol using colorimetric coupling reagents. Here the reactor is simply turned on or off by an electronic switch. Preliminary identification of an unknown peak as disulfide may thus rdy on the disappearance of the peak when the reactor is off (Fig. 64). The use of twin mercury-film electrodes is essential to the success of this aiqilication and imparts hi selectivity to the analysis. 

Compared to oxidation, EC reduction has not been not widely used with HPLC. Reducible groups include hydroxylamines, nitrosamines, -oxides, peroxides, quinones, aromatic nitro compounds and disulfides. The antibiotic chloramphenicol can be assayed in blood by EC reduction using a mercury film electrode. ... 

Many sulfur-containing functional groups chemisorb onto metal electrodes (11,25-30). For example, thiols and disulfides spontaneously adsorb on gold. Thiols also chemisorb at silver, copper, platinum, and mercury however, gold is by far the commonly used substrate electrode due to its relative inert nature. Several examples of SAMs that have been reported in the literature are listed in Table 8.2. A detailed discussion of an example (alka-nethiolates on gold) is given below. 

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