Oxidation-reduction thiols

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

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

Equilibrium considerations other than those of binding are those of oxidation/reduction potentials to which we drew attention in Section 1.14 considering the elements in the sea. Inside cells certain oxidation/reductions also equilibrate rapidly, especially those of transition metal ions with thiols and -S-S- bonds, while most non-metal oxidation/reduction changes between C/H/N/O compounds are slow and kinetically controlled (see Chapter 2). In the case of fast redox reactions oxidation/reduction potentials are fixed constants. 

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. 

Organochalcogen(II) compounds, 100-102 Organochalcogen(IV) compounds, 100-102 Organoselenium compounds dehalogenation reactions, 96 electrochemical reduction, 113-117 haloperoxidase-like activity, 108-113 with odd number of ligands, 100-102 one-electron oxidation, 117-118 oxidation of thiols, 102-106 redox reactions, 79-80 thioperoxidase-like activity, 108-113 Organotellurium compounds... 

Porphyrin rings containing iron are also a feature of the cytochromes. Several cytochromes are responsible for the latter part of the electron transport chain of oxidative phosphorylation that provides the principal source of ATP for an aerobic cell (see Section 15.1.2). Their function involves alternate oxidation-reduction of the iron between Fe + (reduced form) and Fe + (oxidized form). The individual cytochromes vary structurally, and their classification (a, b, c, etc.) is related to their absorption maxima in the visible spectrum. They contain a haem system that is covalently bound to protein through thiol groups. 

Thiolates reduce nitroprusside stoichiometrically to yield R2S2 and [Fe(CN)s(NO)] , which can then convert to [Fe(CN)4(NO)] with subsequent release of NO. Reduction may take place through thionitrito adduct formation (see below) followed by generation of the redox products and eventually [Fe(CN)e] or [Fe(CN)5L]" or release of the thionitrite ligand with subsequent decay to the same products [7, 54]. If oxygen is present, nitroprusside is regenerated and can catalyze the air oxidation of thiols, such as cysteine to cystine [91]. 

The Smiles rearrangement 5-28 Addition of ArSSCl to alkenes 9-35 Oxidation of thiols 9-54 Reduction of sulfonyl halides... 

The fifth cofactor of the PDH complex, lipoate (Fig. 16-4), has two thiol groups that can undergo reversible oxidation to a disulfide bond (—S—S—), similar to that between two Cys residues in a protein. Because of its capacity to undergo oxidation-reduction reactions, lipoate can serve both as an electron hydrogen carrier and as an acyl carrier, as we shall see. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

Figure 9. An idealized model for the cyclic oxidation-reduction process using thiol-functionalized surfactant, incorporated into DODAC vesicles together with Rh-coated colloidal CdS, for sustained hydrogen generation under visible light irradiation in one part of the cycle.

Ghyczy M, Boros M (2001) Electrophilic methyl groups present in the diet ameliorate pathological states induced by reductive and oxidative stress a hypothesis. Br J Nutr 85 409-414 Gilbert BC, Silvester S (1997) EPR studies of the role of copper in bio-organic free radical reactions. Copper-catalyzed oxidations of thiols with peroxides, especially those involving glutathione. Nukleonika 42 307-322... 

Reactions of Nucleobase Radicals with O2, Oxidants/ Reductants and Thiols... 

D. Ziegler, Role on Reversible Oxidation-Reduction of Enzyme Thiols-Disulfides in Metabolic Regulation, On Rev Biochem 54 (1985) 305-329. 

A second kind of covalent bonding in peptides occurs when a disulfide linkage, RS-SR, is formed between two cysteine residues. As we saw in Section 18.11, disulfide bonds are easily formed by mild oxidation of thiols, RSH, and are easily cleaved by mild reduction. 

Interestingly, the a-acetoxy sulfides (106) obtained from the reaction of 3-keto sulfoxides with acetic anhydride containing sodium acetate undergo in situ oxidation-reduction and acyl transfer, giving a-acetoxy thiol esters (107) as products (Scheme 23)J Yields in this process are generally high. 

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