Thiols metals

For transition and precious metals, thiols have been successfully employed as the stabilizing reagent (capping reagent) of metal nanoparticles [6]. In such cases, various functionalities can be added to the particles and the obtained nanoparticles may be very unique. It is well known that thiols provide good self-assembled monolayers (SAM) on various metal surfaces. When this SAM technique is applied to the nanoparticle preparation, nanoparticles can be covered constantly by functionalized moieties, which are connected to the terminal of thiol compounds. 

Although there is little doubt that the electron transfer reaction (Reaction 2) is involved in the over-all reaction (21), the suggestion that quantitative yields of disulfide (13) arise from the dimerization of thiyl radicals is inconsistent with the observed behavior of other free radicals (24). It seems preferable to suggest that some kind of coordination occurs as a prerequisite to the transfer of electrons (12,15). In this case, metal-thiol complexes should be formed as intermediates in the oxidation, in which the metal acts not only as an electron acceptor but also to locate the resultant thiyl entities in close proximity, thereby favoring dimerization reactions and producing disulfide. The electrons gained by the metal may then be passed on to an oxygen molecule. The over-all reaction may be represented as... 

Some evidence to support this scheme has been obtained. Thus the catalytic activity of metals has been found to be associated with the formation of soluble metal-thiol complexes (13), and the geometric configuration of thiols has been found to affect the over-all rate of oxidation,... 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

In the light of the above discussion, it is necessary to redefine the criteria useful for describing catalytic activity. The coordination atmosphere of any given metal may be expected to affect the catalytic activity by influencing the solubility of the metal. If the metal complex, added to the reactant solution, can be replaced by thiyl entities, colored metal-thiol complexes may be produced, and the rate of reaction in all cases should correspond to Case 3 for adding simple metal salts (Table II). If the metal complex cannot be replaced, the rate of reaction may be quite different and will depend on the ease with which an electron... 

Imidazole also acts as a substrate-competitive inhibitor, forming both binary complexes with LADH, and ternary complexes in the presence of coenzyme. X-Ray studies show that imidazole also binds to the. catalytic zinc by displacing the water molecule.1361 The presence of imidazole at the active site also enhances the rate of carboxymethylation14658 of Cys-46 with both iodoacetate and iodoacetamide.1420 This enhancement of alkylation has become known as the promotion effect .1421 Imidazole promotion also improves the specificity of the alkylation.1422 Since Cys-46 is thought to be alkylated as a metal-thiol complex, imidazole, on binding the active site metal, could enhance the reactivity by donating a electrons to the metal atom, which distributes the increased electron density further to the other ligands in the coordination sphere. The increased nucleophilicity of the sulfur results in promoted alkylation.1409... 

F7. Fishman, W. H., and Ghosh, N. K., Influence of reagents reacting with metal, thiol and amino sites on catalytic activity and L-phenylalanine inhibition of rat intestinal alkaline phosphatase. Biochem. J. (1967) (in press). 

Early work on the catalytic autoxidation of carboxythiols confirmed the effectiveness of manganese, iron, cobalt, copper, and arsenic, but the first major assault on the mechanism of the reaction was due to Michaelis and Barron [123,124]. The oxidation of cysteine at pH 7—8 was found to be zero order in cysteine and to involve metal—cysteine complexes as active intermediates. Several studies of metal—thiol complexes have been... 

Legare D, Richard D, Mukopadhyay R, Stier-HOF Y-D, Rosen BP, Haimeur A, Papandopou-Lou B and Quellette (2001) The Leishmania ATP-binding cassette protein PGPA is an intracellular metal-thiol transporter ATPase conjugate. 

By coating the internal pore surfaces of MCM-41 with functionalized self-assembled monolayers it is possible to impart excellent chemical selectivity for specific toxic metallic species (see Figure 2). For example, installation of a thiol-terminated monolayer provides unprecedented mercury sequestering capability [7,8]. Thiol-SAMMS also is effective for removing other soft heavy metals, such as Cd, Ag ami Au. In fact, the metallated thiols themselves (e.g. Hg and Ag) are also excellent sorbents for sequestering soft anions (e.g. radioiodide) as weU [9]. 

The operation of such a cycle is consistent with observations that while relatively low concentrations of gold are present during chrysotherapy (10-50 /zM Au), the changes in tissue levels of metals, thiols, proteins, etc. in responding patients are much larger than can be accounted for on a stoichiometric basis. 

Arsenic is a semi-metal or metalloid that has both metallic and nonmetallic properties. In contrast to the other gene products of the operon, ArsB does not use metal-thiol chemistry for catalysis. There is only a single cysteine in ArsB, and this residue is not required for arsenite transport (55). 

Y Chen, S Dey, BP Rosen. Soft metal thiol chemistry is not involved in the transport of arsenite by the Ars pump. J Bacteriol 178 911-913, 1996. 

Thyolysis reaction Analogous to hydrolysis, thiolysis involves reaction of H2S, in lieu of H2O, to form metal thiols, which subsequently condense to form metal sulfides TMB Trimethylbenzene TMCS Trimethylchlorosilane... 

Other excellent example of metal ion trapping by SAMs has been proposed by Kolb et al. In this case, SAMs of N-containing thiols were used to trap Pd(ll) ions from solution. The Pd(II) ions are then reduced electrochemically to produce a monolayer of metallic Pd onto the organic monolayer. This approach was used as an alternative to metallize thiol SAMs from vapor phase, where the diffusion of metal atoms through SAM defects destroys the metal-thiol-metal device. 

In the concept promoted by Taggart, the formation of a hydrophobic surface is governed by the solubility of the metal thiol compounds. In order to overcome the difficulty that flotation occurs at collector concentrations that are orders of magnitude less than those at which one would expect a metal thiol compound to deposit from solubility considerations based on bulk species, Taggart and Hassialis considered that the concentration of the metal ion be taken as that in the lattice of the mineral. Sutherland and Wark pointed out the fallacy of this reasoning and reaffirmed the conviction that, to explain the floatability at low collector concentrations, an adsorption process must be operative. 

Thus, three theories had been developed and each had difficulties formation of metal thiols did not fit thermodynamics, adsorption of ions did not fit charge requirements, and adsorption of thiol acids did not fit stability criteria. 

Nixon also suggested that prominent theories of flotation could be reconciled by the electrochemical approach. This is illustrated in Fig. 1, in which the requirement of a neutral surface species in Cook and Nixon s model can be met in different ways. The anodic reaction can be adsorption as proposed by Wark and Cox (Fig. la). Also, it can be the formation of a metal thiol compound as proposed by Taggart and co-workers. The latter process can occur as a single step as in Fig. lb or through separate surface oxidation and ion-exchange processes as shown in Figs. Ic and Id. In addition, the anodic process can be the formation of the dithiolate as suggested by Nixon and illustrated in Fig. le. 

Mielczarski, Suoninen, and their co-workers utilized XPS to provide information on the structure of adsorbed xanthates and dithiophos-phate on metals and metal sulfides. They interpreted their results in terms of the initial distribution of the adsorbed thiol without any special orientation, followed by island formation as the coverage increased and, eventually, a well-ordered monolayer. In subsequent multilayer formation by the metal thiol compound, the orientation of the thiol was considered to become more random. Interestingly, it was noted that other ions preadsorbed on the surface, such as hydroxyl and carbonate, were gradually removed as the xanthate monolayer developed. Only one sulfur environment was observed for surface xanthate species, supporting the conclusion that xanthate is bonded to metal atoms in the surface layer through both sulfur atoms (see Section VII.2). No differences were reported between the binding energies of the thiol in the initial monolayer and those of the bulk thiol compound. 

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