Formation constants thiolate

Thioethers have also been shown to form strong complexes with Hg(II) in aqueous solution (47, 104, 105, 127, 137). The chelate effect has been utilized in several of these thioether compounds in efforts to improve chelation therapy in Hg(II) poisoning (13). Several of the formation constants of ligand systems based on thioethers are in Table IX. In contrast to thiolates, only weak Hg(II)-thioether complexes are formed in highly donating solvents such as dimethyl sulfoxide and pyridine (213). 

The ready availability of thiolate ligands in biological systems implies that thiolate complexes dominate Hg(II) coordination in biological systems. Despite some controversy regarding the accuracy of formation constants for Hg-thiolate complexes, extremely large formation constants are consistently reported in the literature. Recent results suggest that Hg(II) ap-... 

As previously discussed, the stepwise formation constant for the addition of a third thiol or thiolate ligand to Hg(SR)2 compounds range from —10 to 10 (Tables VIII and IX). In thiol buffered solutions of micromolar protein concentrations, MerR has been shown to bind Hg(II) at least 10 times better than the competing buffer thiols (82, 171, 210). A combination... 

Regular pyrimidines are less effective ligands for Ni11 ions. They may use, inter alia, their C=0 donor to yield monodentate coordination.1835 Insertion of a sulfur atom into a pyrimidine moiety increases considerably its binding ability.1836 Thiolation of uridine at C(2) or C(4) results in formation of a quite effective S,N3 four-membered chelate in the complexes with Ni11. Thiolation of purine at C(6) increases the stability constant by 3.5 orders of magnitude. 

Peroxynitrite reacts with heme proteins such as prostacycline synthase (PGI2), microperoxidase, and the heme thiolate protein P450 to form a ferryl nitrogen dioxide complex as an intermediate [120]. Peroxynitrite also reacts with acetaldehyde with the rate constant of 680 1 mol 1 s" 1 forming a hypothetical adduct, which is decomposed into acetate, formate, and methyl radicals [121]. The oxidation of NADH and NADPH by peroxynitrite most certainly occurs by free radical mechanism [122,123], Kirsch and de Groot [122] concluded that peroxynitrite oxidized NADH by a one-electron transfer mechanism to form NAD and superoxide ... 

The inactivation of enzymes containing the zinc-thiolate moieties by peroxynitrite may initiate an important pathophysiological process. In 1995, Crow et al. [129] showed that peroxynitrite disrupts the zinc-thiolate center of yeast alcohol dehydrogenase with the rate constant of 3.9 + 1.3 x 1051 mol-1 s-1, yielding the zinc release and enzyme inactivation. Later on, it has been shown [130] that only one zinc atom from the two present in the alcohol dehydrogenase monomer is released in the reaction with peroxynitrite. Recently, Zou et al. [131] reported the same reaction of peroxynitrite with endothelial NO synthase, which is accompanied by the zinc release from the zinc-thiolate cluster and probably the formation of disulfide bonds between enzyme monomers. The destruction of zinc-thiolate cluster resulted in a decrease in NO synthesis and an increase in superoxide production. It has been proposed that such a process might be the mechanism of vascular disease development, which is enhanced by diabetes mellitus. 

The intrinsic rate constants for thiolate ion addition to 76-Cr and 76-W are substantially larger than those for alkoxide ion addition. This is similar to the previously mentioned higher intrinsic reactivity of thiolate ions compared to amine nucleophiles for the addition to a-nitrostilbene and p-nitrostyrene. It can be understood in terms of the soft-soft interaction of the thiolate ion with the carbene complex which is more advanced than C S bond formation at the transition state.184... 

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