Disulfide radicals

The yellow disulfide radical anion and the briUiant blue trisulfide radical anion often occur together for what reason some authors of the older Hterature (prior to 1975) got mixed up with their identification. Today, both species are well known by their E8R, infrared, resonance Raman, UV-Vis, and photoelectron spectra, some of which have been recorded both in solutions and in solid matrices. In solution these radical species are formed by the ho-molytic dissociation of polysulfide dianions according to Eqs. (7) and (8). 8ince these dissociation reactions are of course endothermic the radical formation is promoted by heating as well as by dilution. Furthermore, solvents of lower polarity than that of water also favor the homolytic dissociation. However, in solutions at 20 °C the equilibria at Eqs. (7) and (8) are usually on the left side (excepting extremely dilute systems) and only the very high sensitivity of E8R, UV-Vis and resonance Raman spectroscopy made it possible to detect the radical anions in liquid and solid solutions see above. 

Selected entries from Methods in Enzymology [vol, page(s)] Acetylthiocholine as substrate, 251, 101-102 assay by ESR, 251, 102-105 inhibitors, 251, 103 modification by symmetrical disulfide radical, 251, 100 thioester substrate, 248, 16 transition state and multisubstrate analogues, 249, 305 enzyme receptor, similarity to collagen, 245, 3. 

Before proceeding to describe the mechanistic features of disulfide reduction, it is useful to first describe the effect of substituents on the stability of thiyl radicals and disulfide radical anions as well as the consequences, from a theoretical viewpoint, of the unpaired electron on both the bond energy and the S—S bond length of disulfides. 

Unlike radical anions of most aryl-containing molecules, the radical anion of PhSSPh, like dialkyl disulfide radical anions, is characterized by localization of the odd electron mostly on the S—S bond. These data suggest that dialkyl, PhSSPh, and probably PhSSR disulfide radical anions are shortlived but well-defined a -type radical anions. 

The resulting disulfide radical is strongly reducing and readily reduces oxygen to form superoxide (Koppenol, 1993). 

The thiol-dependent release of nitric oxide may well involve metal catalysis from trace contamination of copper or iron in buffers. The disulfide radical may also reduce another nitrosothiol to produce more nitric oxide. 

An H-transfer may also occurs intramolecularly such as in DDT via a five-membered transition state [reaction (33) Akhlaq and von Sonntag 1986]. In the given case, an H2S forming chain reaction is induced [cf. reaction (34) followed by the (slow) H-abstraction of the thus-formed radical from DTT] which comes to a halt when the thiyl radical is complexed as the disulfide radical anion at higher pH values [cf. reaction (40)]. 

Sulfur free-radical chemistry is largely governed by the ability of sulfur to form three-electron bonded intermediates. A case in point is the complexation of a thiyl radical with a thiolate ion (for an analogy with the halide and other pseudohalide systems, see Chap. 5.2). These disulfide radical anions are characterized by strong absorptions in the UV-Vis (Adams et al. 1967). Complexation can occur both intermolecularly as well as intramolecularly. For GSH, for example, the stability constant of the disulfide radical anion is 2900 dm3 mol1 (Mezyk 1996a). The protonated disulfide radical anion is not stable, but such intermediates are known in the cases of the intramolecular complexes [reactions (39) and (40) Akhlaq and von Sonntag 1987]. 

Table 7.4. Absorption maxima and molar absorption coefficients of cyclic disulfide radical anions and their protonated forms. According to von Sonntag (1990).The lipoic acid value has been taken from Hoffman and Hayon (1972). Note that the eguilibria (cf. apparent pKa values) include the ring-open forms which are not detectable...

Metallothioneins are small ubiquitous oligopeptides containing a high proportion of cysteine residues but no disulfide bonds (Tsunoo et al. 1978 Suzuki and Maitani 1983). Mammalian metallothionins are made up of 61 amino acids, 20 ofwhich are cysteine (Hamer 1986 for their protective effects see Chap. 12.10). Despite this, disulfide radical anion formation is mostly bimolecular (Fang et al. 1995). 

The disulfide radical anions are also formed upon the reduction of the disulfide by C02- [reaction (41)] and the thymine radical anion (Willson 1970 Elliot et al. 1984). For example, the reducing hydroxymethyl radical can reduce the disulfide (Anderson et al. 1986), but the rate is only fast when the hydroxymethyl radical is deprotonated (Akhlaq et al. 1989) and thus its redox potential decreased. 

Disulfide radical anions are rather strong reductants (E = -1.7 V Wardman 1989), and it has hence been proposed that in cellular systems these intermediates may contribute to the repair of DNA radicals (Priitz 1989). 

Addition to the disulfide function is also observed with OH (Bonifacic et al. 1975b). But here, in addition to the substitution reaction (45), generation of a disulfide radical cation and a hydroxide ion is also observed [reaction (46)]. The high solvation energy of the latter certainly provides additional driving force. There is EPR evidence that RSO- is also formed [reaction (47) Gilbert et al. 1975]. In dithiodipropionic acid and in cystine, the thiol is indeed a major product (30-40% Elliot et al. 1981). 

Obviously, these sulfur-centered radical cations are good sinks for 02, and, for example, disulfide radical cations give rise to sulfoxides in a very fast reaction [reaction (53), k = 1.6 x 1010 dm3 mol1 s Bonifacic et al. 2000b]. 

Fang X, Wu J, Wei G, Schuchmann H-P, von Sonntag C (1995) Generation and reactions of the disulfide radical anion derived from metallothionein a pulse radiolytic study. Int J Radiat Biol 68 459-466... 

Lawrence, C. C., Bennati, M., Obias, H. V., Bar, G., Griffin, R. G., and Stubbe, J., 1999, High-field EPR detection of a disulfide radical anion in the reduction of cytidine 5 -diphosphate by the E441Q R1 mutant of Escherichia coli ribonucleotide reductase. Proc. Natl. Acad. Sci. USA 96 8979ii8984. 

A third possibility, in which a highly favorable outer-sphere oxidation of disulfide radical anions to neutral disulfides, Eq. (38), becomes ratedetermining, will yield... 

If the concentration of DTT is high enough to provide a sufficient amount of disulfide radical anions (RSSR) , reaction (9) becomes important ... 

Most of the studies concerning the formation and decay mechanisms of (RS.-. SR)- radical anions have been performed on aliphatic disulfides in aqueous solutions. Since the solubility of aromatic disulfides (ArS-SAr) is very poor in water, their radical anions (ArS.-. SAr)- can be easily investigated in organic solvents. A dissociation mechanism of the S-S bond in the a,a -dinapthyl disulfide radical anion (NpS.. SNp)" in dimethylformamide (DMF) solutions was investigated by pulse radiolysis and in methyltetrahydrofuran (MTHF) rigid glasses by y-radiolysis. ... 

Yamaji N, Tojo S, Takehira K, Tobita S, Fujitsuka M, Majima T. (2006) S-S Bond mesolysis in a,a -dinapthyl disulfide radical anion generated during y-radiolysis and pulse radiolysis in organic solution./P/tyr Chem A 110 13487-13491. 

Disulfide radical anions might play an important role in oxidative stress. In cellular media, they can be formed by oxidation of protein thiol functions (by OH radicals, for instance. Chapter 1), followed by dimerization ... 

These disulfide radical anions are believed to be strong reductants. Thus they might counterbalance the action of oxidizing free radicals. 

Figure 3 Hen egg white lysozyme has 4 disulfide bridges. However, only one of them is easily reduced A marked with an arrow. The detail of its structure Is shown In BJ The disulfide radical anion is stabilized by interaction with the charged end of an Arginine residue (In green). In white the distances (In A) between the central carbon atom and the sulphur atoms. In red the polypeptidic chain.

Pulse radiolysis studies demonstrated that all disulfide radicals do not have the same chemical properties. An example is given by the study of protonation equilibrium (Scheme 1). 

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