Disulphides reduction

The thioredoxins appear to have a highly specific relationship with the enzyme carrying out ttieir reduction. Yeast thioredoxin for example is not reduced by the thioredoxin reductase from E. coli. In contrast reduced thioredoxins may donate electrons to a variety of acceptors. Reduced thioredoxin is a good general disulphide reductant. In combination with its reductase a disulphide reductase system is formed which is capable of reducing lipoic acid, oxidized glutathione and other similar structures. In these cases the thioredoxin-disulphide redox system does not appear to require additional enzymatic components. 

Glutathione-mediated disulphide reductions whether enzyme mediated or spontaneous probably proceed through an intermediate mixed disulphide via a thiolate displacement mechanism. 

On the other hand, it has also been found (14) that 1-deamino-1-carba-oxytocin (Ib) is inactivated by liver homogenates, though very much more slowly than oxytocin. This finding demonstrates the existence of mechanisms other than disulphide reduction and aminopeptidase action capable of inactivating oxytocin-like peptides. The biochemical features and physiological significance of these mechanisms remain to be examined. 

By reduction in the degree of polymerisation. To produce processable rubbers the original polymers are masticated with substances such as benzothiazole disulphide and tetramethylthiuram disulphide. The more severe degradation techniques to produce liquid polysulphides are mentioned below. 

The use of triphenylphosphine and 2,2 -bipyridyl disulphide in oxidation-reduction condensations has been extended to the phosphorylation of alcohols and phosphates, and to the preparation of 5 -(2-pyridyl) phosphorothioates (60) which have been used for the synthesis of pyrophosphates (see Chapter 6, Section 1). 

Redox-type reactions show by far the worst performance in meeting the golden atom economical threshold. Three reductions meet this criterion with (AE)min values of 1 hydrogenation of olefins using the Lindlar catalyst (1952), Noyori stereoselective hydrogenation reaction (1985), and Zincke disulphide cleavage reaction (1911) whereas, oxidations... 

Ziegler, D.M. (1985). Role of reversible oxidation-reduction of enzyme thiol-disulphides in metabolic regulation. Annu. Rev. Biochem. 54, 305-329. 

A development reported recently [519] involves reduction of the cystine disulphide bonds in wool with either thioglycolic acid or tetrakis(hydroxymethyl)phosphonium chloride to form thiol groups, followed by crosslinking with bifunctional reactive dyes. This gave improved insect resistance but had adverse effects on physical properties such as strength, shrinkage and stiffness, thus limiting the potential of the process for commercial use. 

Sulphur dyes in the insoluble disulphide form and the Cl Solubilised Sulphur brands are reduced by the dyer as part of the application procedure. In the case of the Cl Leuco Sulphur brands reduction has already been carried out by the manufacturer, so that they are substantially in a form suitable for immediate application (section 1.6.2) The chemistry of the reduction of sulphur dyes is complex, as is the chemistry of the dyes themselves it has been well described elsewhere [204]. It is possible to describe the state of a reduced sulphur dye in alkaline sulphide or polysulphide solution by the general formula 12.42, but there are certain complications. In many cases the chromogen is not itself reduced, but in others, notably reddish browns, blues and navy blues based on indophenols, the chromogenic... 

Reduction of the adhesion level will occur if certain compounding ingredients are not avoided. The acceleration system has a direct effect on the adhesion level dibenzothiazole disulphide (MBTS) gives the highest adhesion levels. If a second accelerator is used, e g., tetramethyl thiuram disulphide (TMTD) or diphenyl guanidine (DPG), then the adhesion is significantly affected. Other ingredients which cause problems are plasticisers and process oils. 

These resemble the vat dyes in certain ways, although they are of indeterminate constitution and usually mixtures of different chemical species (section 6.4). The characteristic disulphide group (D-S-S-D in Scheme 1.2) is always present in the insoluble form of a sulphur dye, which is brought into aqueous solution by reduction to the alkali-soluble (leuco) form (D-S ). The soluble sodium thiolate form of the leuco sulphur dye has substantivity for cellulose. Thus the application of sulphur dyes to cellulosic fibres is a three-stage process (Scheme 1.2) broadly similar to that already outlined for vat dyes. 

The many sulphur dyes synthesised via quinonimine intermediates are polymeric products containing numerous disulphide crosslinkages that can be broken by reduction in aqueous sodium sulphide thioether groups survive the reduction process. The smaller thiolate-containing molecules formed are substantive to cellulose. Although the actual structures of such dyes are complex, their essential features can be illustrated in an idealised model (Scheme 6.23), in which X = S, NH or O, and R indicates substituents or annelated rings. 

On reduction the disulphides take up hydrogen and are reconverted into mercaptans. 

The conversion (19) of thiols to disulphides coupled with reduction of flavin (vitamin B2 family) is a topic of import in connection with coenzyme reactivity in flavoenzymes. Since flavin oxidation of thiols involves nucleophilic attack of thiolate ion in the rate-determining step (Loechler and Hollocher, 1975 Yokoe and Bruice, 1975), this biologically important reaction would be markedly affected by hydrophobic environments. 

Oximes RCH=NOH are produced in the reduction of nitroalkanes RCH2NO2 by carbon disulphide in the presence of triethylamine461 or wet potassium carbonate and... 

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