Mercaptide bounds

Investigation of the kinetics of the reaction of 4-chloro-2-pentene, an allylic chloride model for the unstable moiety of polyfvinyl chloride), with several thermal stabilizers for the polymer has led to a better understanding of the stabilization mechanism. One general feature of the mechanism is complexing of the labile chlorine atom by the metal atom of the stabilizer. A second general feature is substitution of the complexed chlorine atom by a ligand (either carboxylate or mercaptide) bound to the metal. Stabilization requires that the new allylic substituent (ester or sulfide) be more thermally stable than the allylic chlorine. The isolation of products from stabilizer-model compound reactions supports the substitution hypothesis of poly(vinyl chloride) stabilization. 

The metals are bound to the peptide by mercaptide bounds and are arranged in two distinct clusters a four-metal cluster called the a domain and a three-metal cluster called cluster (3, at the C terminal of the protein. The a cluster is an obligate zinc cluster, whereas the zinc in cluster 3 may be replaced by copper or by cadmium. 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Analytical data on the soluble products isolated from chloroform are in excellent agreement with the composition 1 Ni+2 1 monoalkylated ligand 1 I or Br. The magnetic moment of this methylated complex was found to be 1.89 Bohr magnetons per nickel (II). The molar conductivities of the methylated and benzylated complexes in methanol at 25° C. are 75.4 and 68.4 ohm-1, respectively. These values approximate those expected for uni-univalent electrolytes in this solvent. The formulation of these alkylated compounds as dimeric electrolytes (structure VII) does not appear to be totally consistent with their physical properties. One or both halide ions may be bound to the metal ion. These results lead to the easily understood generalization that terminal sulfur atoms alkylate more readily than bridged mercaptide groups. 

Figure 6-6 shows a proposed reaction cycle of cytochrome P-450 (9). In contrast to Hb and Mb, its Fe center is axially bound to a mercaptide sulfur... 

In these interactions, R is protein-bound and represents an alkyl group. In addition, mercaptide (10) formation (in which X represents the inhibitor), as depicted in Figure 7.2 occurs with various metals, notably mercury and silver, and with cysteine (9). 

The active site of cytochrome P-450 is an iron protoporphyrin with the iron center axially bound to the mercaptide sulfur of a cysteinyl residue. The axial Fe-S linkage is retained throughout its reaction cycle shown in Fig. -6. Recent X-ray analysis " on cytochrome P-450ca (1) state) confirmed this struclure. The term P-450 was derived from the position of the Sorei band at 450 nm of its CO adduct. 

A. In vitro Metallothionein/Na AuTM Reactions. Highly purified equine renal MT was allowed—to react with various amounts of Na2AuTM and then fractionated on Sephadex 6-50 to separate the protefn-bound and free metals. Because the protein does not have any UV-visible chromophores independent of the metal mercaptide linkages, the protein was "quantitated" by its cadmium content (measured by atomic absorption spectroscopy) and results are reported for various Au/Cd ratios of the reactants. Aliquots of a reaction mixture having a 10/1 Au/Cd ratio were fractionated after 2, 24 and 48 hours incubation time to insure complete reaction. 

The third step in the vulcanization sequence is considered to be the formation of polysulphide cross-links. The most probable route for the conversion of the rubber-bound intermediate into cross-linked polysulphides is by disproportionation reactions, involving cleavage of S—S bonds these reactions may be initiated by mercaptide ions (XS ) derived from the zinc mercaptide (cf., Section 17.2) ... 

Despite these successes serious side reactions can result from the action of alkali on cysteine or cystine-containing peptides. Open-chain unsymmetrical cystine derivatives [92, 103, 166] and some cyclic molecules including oxytocin [153] and calcitonin [24, 25] are known to undergo rapid base-catalyzed disulphide interchange yielding symmetrical disulphides or mixtures of polymer and parallel and antiparallel dimers [107, 153]. Furthermore the possibility of /3-elimination of trityl, benzhydryl, and benzyl mercaptide ion from the corresponding peptide bound cysteine residue is known [154] the extent of... 

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