Chromium, like many transition metal elements, is essential to life at low concentrations yet toxic to many systems at higher concentrations. In addition to the overt symptoms of acute chromium tox...

Mechanisms of Chromium Carcinogenicity and Toxicity

The redox chemistry of chromium(VI) is discussed with respect to the cellular metabolism of the carcinogen chromate in vivo. Possible sites for cellular reduction of chromium(VI) to chromium(III) are considered. The reactions of amino acids, ascorbic acid, carboxylic acids, thiol-containing mole-cules and other small molecules with chromate under physiological conditions are presented. In general only ascorbate and those molecules containing sulfhydryl groups are capable of easily reducing chromate at pH 7.4. Thus, in the cytoplasm, glutathione, cysteine and ascorbate are likely candidates to react with chromate. While most proteins are unreactive toward chromate, certain redox proteins are active in reducing chromate. The heme proteins hemoglobin and cytochrome P-450 possess chromate-reductase activity, whereas cytochrome c and myoglobin are inactive. The NADPH-dependent flavoenzymes glutathione reductase and NADPH-cytochrome P-450 reductase also possess chromate-reductase activity. However, the NAD(P)H enzymes, isocitrate dehydrogenase, glutamate dehyrogenase and malate dehydrogenase do not reduce chromate. Both microsomes and mitochondria possess chromate-reductase activity. The microsomal activity is accounted for by the NADPH-cytochrome P-450 reductase/cytochrome P-450 system. The enzyme(s) responsible for the mitochondrial reduction of chromate have not been identified. Chromium(VI) and its metabolite chromium(III) inhibit the normal activities of enzymes which bind chromium(III) or reduce chromate. The metabolism of chromate involves the generation of reactive intermediates which ultimately bind to cellular constituents and damage their function in the cell.

Metabolism of the carcinogen chromate by cellular constituents

Publisher Summary In this chapter, one attempts to present a review giving emphasis to these new fields, and to this end as much material as possible has been subdivided under the headings of the various techniques and types of information. As a result of the chosen subdivision, certain aspects, such as the nature of the metal-to-ligand bonding are discussed in more than one section. Little mention will be made of the analytical uses of these ligands, which have recently been reviewed separately (711), of the synthesis of ring-substituted derivatives of the parent ligands, although a potentially useful method using transition-metal complexes has been reported; or of related species, such as 4,4‘-bipyridyl. The coverage of the literature is complete up to the end of 1967, and usually to the end of April 1968, and although many publications have regretfully not been mentioned, one have attempted to discuss the great majority of those papers which one believe to be of special importance.

The Chemistry of Complexes Containing 2,2′-Bipyridyl, 1, 10-Phenanthroline, or 2,2′,6′,2”-Terpyridyl as Ligands

The Chemistry of 1,4,7‐Triazacyclononane and Related Tridentate Macrocyclic Compounds

Reduction of Pd ions by hydroquinone in the presence of gold nanoparticles and polyvinylpyrrolidone resulted in the formation of nanoflowers with a Au core and Pd petals. Addition of HCl to the synthesis halted the reduction by hydroquinone and enabled the acquisition of snapshots of the nanoflowers at different stages of growth. TEM images of the reaction after 10 s show that the nanoflower morphology resulted from the homogeneous nucleation of Pd clusters in solution and their subsequent attachment to gold seeds coated with a thin (0.8 ± 0.1 nm) shell of Pd. UV-visible spectra also indicate Pd clusters formed in the early stages of the reaction and disappeared as the nanoflowers grew. The speed at which this reaction can be halted is useful not only for producing a variety of bimetallic nanostructures with precisely controlled dimensions and morphologies but also for understanding the growth mechanism of these structures. The ability of the AuPd core-shell structure to catalyze the Suzuki coupling reaction of iodobenzene to phenylboronic acid was probed and compared against the activity of Pd nanocubes and thin-shelled AuPd core-shell nanoparticles. The results of this study suggest that Suzuki coupling was not affected by the surface structure or subsurface composition of the nanoparticles, but instead was primarily catalyzed by molecular Pd species that leached from the nanostructures.

https://people.duke.edu/~bjw24/Publication50.pdf

Synthesis and Catalytic Properties of Au–Pd Nanoflowers

The reactions of the outer-sphere electron transfer reagent, Ni(9-aneN3)23+, (bis(1,4,7-triazacyclononane)nickel(III) ion) with ascorbic acid, hydroquinone, catechol, and resorcinol have been inves...

Kinetics and mechanism of the oxidation of benzenediols and ascorbic acid by bis(1,4,7-triazacyciononane)nickel(III) in aqueous perchlorate media

The cerium(IV) oxidation of thiourea, NN′-dimethyl-,NN′-diethyl-, and ethylene-thioureas and thiosemicarbazide has been examined at [H+]= 0·5M over the range 5–25° using a stopped-flow technique. The rates are consistent with a mechanism involving the formation of free radicals which subsequently dimerise. The thermo-dynamic parameters for the reactions are: thiourea Ea= 7·96 ± 0·4 kcal./mole (ΔS*=–32·7 ± 3 e.u.), dimethyl-Ea= 9·1 ± 0·5 kcal./mole (ΔS*=–29·3 ± 5 e.u.), diethyl-Ea= 8·9 ± 0·5 kcal./mole(ΔS*=–28·5 ± 4 e.u.), ethylene-Ea= 9·6 ± 0·3 kcal/mole (ΔS*=–25·4 ± 3 e.u.), and thiosemicarbazide Ea= 9·8 ± 0·5 kcal./mole (ΔS*=–28·8 ± 3 e.u.). The overall heats of each reaction have been determined calorimetrically. Linear free-energy relationships obtained are discussed.

Metal-ion oxidations in solution. Part V. Cerium(IV) oxidation of thiourea and its N-substituted derivatives

The stopped-flow technique has been used to study the reactions between [HCrO4]– and L-cysteine, H2L, with the latter in excess, at [HClO4]= 0·020–0·100M and I= 1·00M(NaClO4) in the range 15–35 °C. The transient orange species observed is a 1 : 1 chromate ester [equation (i)] with λmax.= 420 nm, Iµmax.= 1 410 ± 60 l mol–1 cm–1, H2L +[(HO)CrO3]– [graphic omitted] [(HL)CrO3]–+ H2O (i) and K1= 1 030 ± 110 l mol–1 at 25 °C. Evidence regarding sulphur bonding is discussed. The kinetics of the formation reaction conform to the rate law (ii) and, at 25 °C, kf′= 130 ± 20 l2 mol2 s–1 and kf= 2 ± 1 l mol–1 s–1. d[Complex]/dt =(kf′[H+]+kf)[HCrO4–][H2L](ii) For the acid-catalysed path, ΔH‡= 7 ± 3 kcal mol–1 and ΔS‡=–24 ±11 cal K–1 mol–1. For the overall formation reaction, ΔG=–4·1 ± 0·1 kcal mol–1, ΔH=–5·0 ± 0·5 kcal mol–1, and ΔS=–3 ± 2 cal K–1 mol–1. The rate constant kf′ has a value several orders of magnitude smaller than those found for this parameter in comparable reactions. This is interpreted as evidence that the anation mechanism has some associative character. One mole of CrVI oxidises three moles of L-cysteine. The redox-reaction kinetics obey the rate law (iii)–d[CrVI]//dt=K1(k1[H+]+k2[H2L])[H2L][HCrO4–]//1 +K1[H2L](iii) where, at 25 °C, k1=(1·2 ± 0·3)× 10–2 l2 mol2s–1, k2=(9·4 ±0·4)× 10–2 l2 mol2 s–1, ΔH1‡= 12 ± 2 kcal mol–1, ΔS1‡=–27 ± 7 cal K– mol–1, ΔH2‡= 10·8 ± 0·8 kcal mol–1, and ΔS2‡=–27 ± 3 cal K–1 mol–1. The nature of the chromium (III) products of this reaction is described. The significance of second-order reductant terms for reactions of this oxidant is discussed.

Metal-ion oxidations in solution. Part XIII. The reaction of chromium(VI) with L-cysteine in perchlorate media

The cobalt(III) ion oxidations of thiourea, NN′-dimethyl-, NN′-diethyl-, and ethylene-thioureas have been examined over the acidity range [H+]= 0·4–1·0M in the temperature range 0–21° by means of a stopped-flow technique. The rates are consistent with a mechanism which involves the formation of free radicals which subsequently dimerise. The formation of a complex intermediate is considered to be the rate-determining step in each case. Rate constants k1, and k2 for the reactions (1) and (2) [graphic omitted] (where RSH+= a thiourea) have been evaluated for each system. The thermodynamic parameters for the reaction are: thiourea, Ea(1)= 8·6 ± 1·5 kcal./mole, ΔS*(1)=–29 ± 5 e.u., Ea(2)= 9·0 ± 0·5 kcal./mole, ΔS*(2)=–22 ± 3 e.u.; diethyl-, Ea(1)= 7·8 ± 1·6 kcal./mole, ΔS*(1)=–29 ± 5 e.u., Ea(2)= 10·6 ± 0·4 kcal./mole, ΔS*(2)=–12 ± 3 e.u.; dimethyl-, Ea(1)= 7·6 ± 1·5 kcal./mole, ΔS*(1)=–26 ± 4 e.u., Ea(2)= 9·8 ± 0·4 kcal./mole, ΔS*(2)=–14 ± 3 e.u.; ethylene-, Ea(1)= 8·2 ± 1·4 kcal./mole, ΔS*(1)=–30 ± 4 e.u., Ea(2)= 10·9 ± 0·5 kcal./mole, ΔS*(2)=–19 ± 3 e.u. The mechanisms of the reactions are discussed.

Metal-ion oxidations in solution. Part VI. Oxidation of thiourea and its N-substituted derivatives by cobalt(III)

Reduction de plusieurs especes de Ni(III) et d'un complexe de Ni(IV)-oxime. Caractere sphere externe de ces reactions

A. McAuley

Papers

Kinetics and mechanism of the oxidation of benzenediols and ascorbic acid by bis(1,4,7-triazacyciononane)nickel(III) in aqueous perchlorate media

Metal-ion oxidations in solution. Part V. Cerium(IV) oxidation of thiourea and its N-substituted derivatives

Metal-ion oxidations in solution. Part XIII. The reaction of chromium(VI) with L-cysteine in perchlorate media

Metal-ion oxidations in solution. Part VI. Oxidation of thiourea and its N-substituted derivatives by cobalt(III)

Kinetics and mechanism of reduction of nickel(III) complexes by titanium(III) in aqueous media