Cystine ligands

Fig. 14.7. Endohedral functionalization of the inner surface of a wheel-shaped M0154 type nanocluster by cystine ligands (polyhedral representation, carboxylate group in ball-and-stick).

Dichromate anions are readily absorbed under acidic conditions by wool that has been dyed with chrome dyes. The chromium(VI) on the fibre is then gradually reduced by the cystine residues in wool keratin to chromium(III) cations, which react with the dye ligands to form a stable complex. In this way the cystine disulphide bonds are destroyed, resulting in oxidative degradation of the wool fibres [71]. 

The aquated iron(III) ion is an oxidant. Reaction with reducing ligands probably proceeds through complexing. Rapid scan spectrophotometry of the Fe(III)-cysteine system shows a transient blue Fe(lII)-cysteine complex and formation of Fe(II) and cystine. The reduction of Fe(lII) by hydroquinone, in concentrated solution has been probed by stopped-flow linked to x-ray absorption spectrometry. The changing charge on the iron is thereby assessed. In the reaction of Fe(III) with a number of reducing transition metal ions M in acid, the rate law... 

SWV has been applied to study electrode reactions of miscellaneous species capable to form insoluble salts with the mercury electrode such as iodide [141,142], dimethoate pesticide [143], sulphide [133,144], arsenic [145,146], cysteine [134, 147,148], glutathione [149], ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) [150], 6-propyl-2-thiouracil (PTU) [136], 5-fluorouracil (FU) [151], 5-azauracil (AU) [138], 2-thiouracil (TU) [138], xanthine and xanthosine [152], and seleninm (IV) [153]. Verification of the theory has been performed by experiments at a mercury electrode with sulphide ions [133] and TU [138] for the simple first-order reaction, cystine [134] and AU [138] for the second-order reaction, FU for the first-order reaction with adsorption of the ligand [151], and PTU for the second-order reaction with adsorption of the ligand [137]. Figure 2.90 shows typical cathodic stripping voltammograms of TU and PTU on a mercuiy electrode. The order of the... 

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 substitution of H2O ligands coordinated to the molybdenum centres of M0154 by ambivalent and/or multivalent ligands, for example the amino acid cystine can be coordinated to the inner wall of the cluster cavity and hence creates hydrophobic and hydrophilic surroundings with different functionalities (see Figure 14.7) [9]. 

Recently, a related reaction protocol was developed with an amino acid as ligand379. In this protocol various amides were coupled with aryl iodides in the presence of Cul (5 mol%), amino acid (glycine, cystine, cysteine, lysine, arginine, tr-alaninc, /3-alanine) (20 mol%) as ligand and K3PO4 as base in dioxane at 100 °C (Figure 23). 

Platinum compounds have been widely used. These include the PtCl ion or the less reactive cis or trans platinum diaminodichloride compounds. At acid pH they react with methionine, cystine disulphides, N-termini or histidine. In the presence of ammonium sulphate, the chloride ions are rapidly substituted by ammonium ions to form [Pt(NH3)4] which is unreactive. Square planar negatively charged complexes such as Pt(CN)4 have been found to be effective in binding at the nucleotide-binding site in dehydrogenases. The cyanide ligands are firmly bound to the metal and are not displaced by protein atoms. 

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