Disulfide anion

The effect shown in Fig. 9 is a result of the bond-bond interaction which is a characteristic feature for chains and rings of two-valent chalcogen atoms. It can also be recognized from the relatively large bond interaction force constants fir of such compounds. The stretching force constants /r(SS) of polysulfur compounds depend on the SS bond distances as shown in Fig. 10. The data used in this figure include several excited electronic states of the S2 molecule as well as the disulfide anion and a number of sulfur homocycles [77]. 

Vvs SCE) one obtains equal quantities of di-, tri-, and tetrasulfides, at less negative potentials (—0.9 V), tri-and tetrasulfides are obtained almost exclusively [253]. Substituted monohalo-benzenes such as 4-bromobenzophenone and 2-chloronitrobenzene give corresponding diaryldisulfides [254-256]. The addition of Mel in the last case, resulting in methyl-(2-nitrophenyl)disulfide, allowed to suppose the process to occur via an intermediate 2-nitrophenyl disulfide anion. 

PCP-phenolic radical that possesses an absorption spectrum in H2O at 440 nm and decays via second-order kineties with k = 9. x 10 /M/s, as evidenced by pulse radiolytie studies. The PCP-phenolic radical can attach covalently to the C8-site of dG to generate the C8-OPCP adductIn the presence of GSH, redox cyeling of the phenoxyl radieal with thiyl radieal generation will yield a GSH disulfide anion radieal that ean reduetively aetivate O2 to generate the superoxide radieal anion (02 ) that ean generate free ferrous iron Free Fe " " may... 

Another point worth mentioning is the reduction of substituted nitro-benzenes by sodium disulfide in aqueous methanolic solution. This reaction was shown to be largely influenced by the presence of electron- donating or electron-withdrawing substituents (ref.8). The effect of substituents fits the Hammett equation well, the slope p-value being + 3.55, thus leading the authors to propose the disulfide anion as the reductive species. 2- and... 

The well-known instability of the disulfide anion radicals, (R SR2)-, is apparently explained by the antibonding electron population, presumably in the framework of the disulfide bond. In some cases, however, these anion radicals turned out to be more or less stable (Breitzer et al. 2001). Two examples in Schemes 3-22 and 3-23 deserve to be distinguished. Firstly, one-electron reduction of naphthalene-1,8-disulfide using sodium in dimethoxyethane generates the corresponding anion radical, Scheme 3-22. Second, the oxidation of a [l,n]-dithiol by Ti(III)-H202 at pH 7 produces the cyclic disulfide according to Scheme 3-23 ... 

Surdhar PS, Armstrong DA. 1987. Reduction potentials and exchange reactions of thiyl radicals and disulfide anion radicals. J Phys Chem 91 6532-6537. 

Mezyk SP, Armstrong DA (1999) Disulfide anion radical equilibria effects of -NH3+, -CO2 and -CH3 groups. J Chem Soc Perkin Trans 2 1411-1419... 

The stabilization of a disulfide anion through formation of a sandwich-type structure involving heterocyclic substituents providing the possibility for interaction was observed in bis-[l-(2, 3, 5 -tri-0-acetylribosyl)uracilyl-4-yl] disulfide (USSU) (Chart 10) by means of pulse radiolysis. ... 

The butyl derivative - but not the phenyl derivative - undergoes an internal redox reaction below 20 to form dihutyl ditellurium and disulfide anion. ... 

Attachment to Sulfur Groups. Of all the amino acids, the two most reactive in solution toward the electron are cystine and cysteine. Reaction with the former leads to the disulfide anion radical,... 

ESR studies of the amino acids in glasses confirm the formation of the disulfide anion (20), which is especially stable, and the methyl radical (20). Similar reactions may be expected to occur directly or indirectly on peptides. 

The crystal structure shows a marked difference in the copper environments in the two monomers. In one of the subunits two electron density maxima are observed riding the cuprous ion in place of the usual density expected for the weakly bound water molecule. This is a two-center density that can be modeled with two water molecules at full occupancy, each being at coordinating distance from the metal ion, resulting in the imusual geometry shown in Fig. 11. The electron density could not be unambiguously interpreted because it can be modeled as well by a disulfide anion at about half occupancy. Such an anion may have been originated from the dithionite used for copper reduction. 

The disulfide anion (of reaction sequence 1) may then eliminate sulfur to form mercaptide ion. In addition, the dehydroalanine intermediate is a very reactive species. It may react with any nucleophilic species present, such as... 

As shown in Scheme 8.4, the resulting disulfide anion radical dissociates into a thiolate ion R-S and a thiyl radical R-S. Proton transfer from the tryptophan cation radical to the thiolate ion leads to the tryptophan radical Trp and the thiol RSH. The final stage of the process is governed by radical coupling, which may result in sulfenylation of the Trp moiety yielding Trp-S-R, or in inter-molecular cross-linking, i.e. in the formation of enzyme dimers or trimers. 

The nucleophilicity of sulfur anions, for example, the hydrosulfide anion (HS ), the hydrogen disulfide anion (HS2 ), and thiolate anions (RS ), appears to be significantly superior to the corresponding oxygen derivatives (i.e., the hydroxide anion [OH ], the hydrogen peroxide anion [HO2 ], and alkoxide anions [RO ]) under similar conditions, and both the polarizability of sulfur and the extent of solvation of the respective oxyanions (but not the thioanions) have been implicated. 

Bassindale and Iley have considered the possible involvement of sulfur 3d orbitals in the accommodation of the additional electron in RSSR disulfide anions [1]. They point out that the ESR spectra of the.se ions are not consistent with this. Analysis of the proton coupling constants and the hyperfine coupling indicates that the unpaired electron is shared equally between the two sulfurs and that the s character of the orbital containing it is small, ca. 3%. The overall conclusion from ESR studies up to 1990 is that the unpaired electron resides in a a orbital formed by the 3p orbitals of the two S atoms [ I ]. 

The reduction potentials of thiyl radicals and disulfide aniones in Table 7 are of interest because of the importance of sulfhydryls in biological systems and the involvement of glutathione (GSH) as a cellular protective agent [72]. Of primary... 

The existence of this thiol ionization equilibrium means that the RS radical reduction potential is pH dependent at pHs up to the which is near 8 to 10 for aliphatic thiols and 6 to 7 for aromatic ones. In addition to this, alipahtic thiyl radicals form disulfide anion radicals with the parent anions in equilibrium (25) ... 

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