Cysteine complex formation

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

Metal ion catalyzed autoxidation reactions of glutathione were found to be very similar to that of cysteine (76,77). In a systematic study, catalytic activity was found with Cu(II), Fe(II) and to a much lesser extent with Cu(I) and Ni(I). The reaction produces hydrogen peroxide, the amount of which strongly depends on the presence of various chelating molecules. It was noted that the catalysis requires some sort of complex formation between the catalyst and substrate. The formation of a radical intermediate was not ruled out, but a radical initiated chain mechanism was not necessary for the interpretation of the results (76). 

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... 

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

Quantitative conversion of the iron in succinate dehydrogenase to this form is possible if additional cysteine is added to the reaction mixture. It is probable that not enough cysteinyl sulfur ligands are available for complex formation without addition of the extra cysteine some of the nitrosyl complex does form without any cysteine addition in these systems. 

In contrast, addition of cysteine to Cu11 solutions gives only a fleeting appearance of a violet colour before complete and rapid reduction to Cu1 occurs, with the latter stabilized by complex formation, giving the stoichiometry shown in equation (7). 

In response to the presence of detrimental Cd +, Hg +, Pb +, and other heavy metal ions, the human hver and kidneys synthesize more metallothionein, an unusual small protein in which approximately one-third of the 61 amino acid residues are cysteine see Metallothioneins). The frequency and juxtaposition of sulfhydryl groups provide strong binding sites for several heavy metal ions. Though not as profusely as metallothionein, many proteins contain sulfhydryl groups that may become metalated by toxic heavy metal ions such as Cd +, Hg +, and Pb +, and it is widely believed that this complex formation explains the toxicity of these metal ions. The exact proteins where the most consequential damage occurs remain uncertain. 

In addition to fluorescent CdS nanocrystals, cysteine represents an excellent ligand for ZnS nanocrystals. Again, inorganic sulfide is incorporated into a zinc cysteine complex to initiate cluster formation. An examination of reaction temperatmes suggests optimal formation occurred at 45 °C. Once formed, the clusters were highly fluorescent and gave... 

This procedure assumes bis complex formation. It is recommended that a Job s plot be performed with cysteine (A250 see Experiment 3.6). 

MeHg readily crosses the blood-brain barrier. The rapid uptake of MeHg in the brain has been proposed to be due to lipid solubility, but evidence in rats suggests that the transport is due to the formation of MeHg-cysteine complexes. 

As the knowledge of protein structure increases, we may expect to know more about the mechanisms of enzyme action. Already information on the amino acid sequences around the active centres of quite a few enzymes has been obtained, and it has been observed that there are striking similarities amongst the active centres of various different enzymes within the same general class, such as phosphatases and esterases. Most notably, nearly all of these enzymes seem to possess at their active centres one or more serines whilst other classes of enzyme make use of the sulphur-containing amino acid cysteine, suggesting that —SH groups play a role in complex formation in these cases. It may well be that... 

The ruthenium complexes were attached to the specified cysteine by formation of a thioether linkage between the sulfur atom of cysteine and the methylene carbon of one of the bipyridine ligands. The reaction makes use of complexes that contain 4-bromomethyl-4 -methylbipyridine, as indicated. 

MeHg" " is distributed throughout the body, and easily penetrates the blood-brain and blood-placental barriers (Clarkson 1993, Hansen et al. 1989, Suzuki et al. 1984). The transport of MeHg" " into tissues is mediated by the formation of a MeHg-cysteine complex (Aschner and Aschner 1990, Tanaka et al. 1991, Kerper et al. 1992). Soon after application, McHg" " is found in the blood, predominantly in the red cells. In humans, the ratio of MeHg in red blood cells to serum is approximately 20 1 (Kershaw et al. 1980). Short-chain alkyl-mercury compounds such as methylmercury or ethylmercury are rather stable in the body, whereas long-chain alkylmercury or arylmercury compounds such as phenylmercury may be metabolized relatively quickly to Hg " " ions (Roberts et al. 1979) and, therefore, show similar behavior to the Hg " " ion (Pfab et al. 

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