Thiols electrolytic reduction

Contrary to catalytic hydrogenation which usually fails in the presence of thiols or thioethers, electrolytic reduction at a mercury cathode takes place without difBculty. Among the thioethers which could be cleaved by electrolytic reduction are the 4-picolyl thioethers. These derivatives are obtained by reacting the free thiol with freshly distilled 4-picolyl chloride in the presence of base. The thioether is completely stable towards acidic... 

The integrated balance of oxidation and reduction which this process makes possible cannot always be achieved. Yet the recycling of the cofactor is obviously crucial. A second, quite separate reaction can be introduced, for example the oxidation of ethanol to acetaldehyde with an alcohol dehydrogenase. However there is some hope that an electrolytic reduction of the NAD could be developed. This is illustrated by the manufacture of glutathione which is an important thiol-containing peptide. The dimeric disulphide is isolated from yeast cells as a copper complex. This is treated with sulphide to remove the copper, and the free peptide is then reduced at acid pH in the catholyte of an electrolytic half cell. The oxidation of the thiol at the anode is prevented by an ionic membrane which separates the two halves of the cell. 

The acetamidomethyl group is recommended for thiol protection in, for example, cysteine it is stable to most peptide coupling conditions, but is readily removed by mercuric ion (Scheme 159). An alternative is the4-picolyl group, which affords protection also to the phenolic group of tyrosine cleavage is achieved by electrolytic reduction. 

These reductively desorbed long-chain thiols (thiolates), which are insoluble in the electrolyte, remain near the surface, probably forming micelles, and do not diffuse in the bulk solution. They can be redeposited back on the surface of the electrode as a monolayer even if there was no thiol in the electrolyte prior to desorption . The amount of material readsorbed depends directly on the solubihty of the thiol. Shorter-chain (more soluble) thiols redeposit to give monolayers with smaller coverage. Redeposition also depends strongly on the pH of solution. Thiols become less soluble at lower pH, and therefore form monolayers with higher coverage under these conditions. 

When S-labelled disulphides are available, the corresponding S-thiol can be readily prepared by electrolytic or H2 reduction (equation 31). j8-Mercaptoacetic acid and cysteine have been obtained in this way . 

The redox electrochemistry of a Fe /Fe couple is easily accomplished on a phthalocyanine-coated electrode with peak separations comparable to that of platinum [141,142,163,164]. Phthalocyanine-coated electrodes are found to be efficient electrocatalysts to catalys catechol, p-benzoquinone and oxalic acid oxidations [120,150]. The electrochemical activity of these electrodes may be due to the high voltage, surface area, high electronic conductivity and redox behaviour of phthalocyanine, vanadium phthalocyanine and other phthalocyanines have been prepared by vapour deposition and show photoelectrochemical responses when dipped in aqueous electrolytes [244-249]. Polymeric phthalocyanines of Co and Fe are coated on active carbon and are shown to give catalytic properties for dioxygen reduction and thiol oxidations. Dioxygen chemisorption and ammonia absorption of metallo... 

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