Disulfide radical cations

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]. 

Disulfide radical cations have been prepared and studied. Some EPR spectroscopic parameters for these 7r-radical cations [439] are listed in Table 12. 

Table 12. EPR spectroscopic parameters for disulfide radical cations...

Several reactions have been reported for disulfide radical cations generated in water by pulse radiolysis techniques. These species are oxidants and undergo one-electron transfer with Fe(CN) at near diffusion controlled rates (ca. 1010 M 1s 1) [456,457], with Fe2+ with rate constants [456] the order of 106 M 1s 1 with 1 [458] and with RS [484]. Disulfide radical cations, like other sulfur radical cations, do not react with 02 [399]. On the basis of the second order decomposition of these radical cations in acidic and neutral aqueous solutions, their disproportionation to the corresponding dication as shown in Eq. (55) has been suggested [456] ... 

In the absence of suitable redox partners sulfide and disulfide radicals cations decay mainly by disproportionation or deprotonation. Considering their positive charge they are also prone for nucleophilic attack. Examples for the latter are the reaction of R2S with OH , leading to sulfuranyl radicals R2S (0H), or with halide ions, yielding sulfur-halide coupled radicals. Both these product radical species will be dealt with in more detail in separate sections. 

Disproportionation has been assumed primarily from the observed second order decay kinetics. This applies, in particular, to most disulfide radical cations and to those sulfide radical cations which do not deprotonate at all or not fast enough. Little is known so far on mechanistic details and products as result of the disproportionation, except that the R2S product dication seems to account for most of the sulfoxide formed upon the OH-induced oxidation of sulfides in aqueous solution. 3 97... 

Disulfide radical cations are an example for a 5-electron bond. As depicted in eq. 46, removal of a sulfur-p-electron in the initial oxidation step formally leads to a radical cation site at one of the sulfurs. This situation stabilizes by electron, spin and charge sharing, and accommodation of the unpaired electron as well as the lone p-electron pair of the other sulfur in the sulfur-sulfur bridge.55... 

Analogues to the five-electron bonded 2a/27t/l7t disulfide radical cations in nitrogen-based systems, namely, hydrazine radical cations could be generated by one-electron reduction of trialkyl diazenium salts (eq. 49) and subsequent equilibration of the neutral product radical with its protonated form. The pK of the R=t-butyl substituted species, incidentally, is 7.0, 2.6 units below that of the parent hydrazine compound. The results emphasize the importance of structural parameters, in particular those which control orbital orientation and overlap. >46,147 Jhe alternative possibility to generate such... 

OH free radicals react with almost all amino-acids. For aliphatic residues, rate constants are correlated with the strength of the X-H bond(X = S, C or N) (1). Thus the reaction is relatively slow with glycine (k = 1.7 x 10 mol 1 s ) and fast with the -SH function of cysteine (k = 1.9 x 10 mol M s i). The thiyl radical formed upon oxidation of cysteine, whose spectral properties are in table 3, is formed but a carbon-centered radical is also present (50, 51). In the presence of oxygen, thiyl radical fixes O2 giving peroxy radicals (52). These radicals may photoisomerize into sulfonyl radicals RS02 (53). In small molecules, disulfide groups can also be oxidized. This reaction was not demonstrated in proteins, but cannot be neglected. A disulfide radical cation is formed (54). Final compounds are not known. 

Whether this can be rationalized directly by reaction (11) yielding sulfinyl radicals besides thiols, or involves a more complex reaction scheme still remains an open question. There seems little doubt about the production of the sulfinyl radicals, as they could unambiguously be identified in complementary ESR experiments in which the hydroxyl radicals were generated via Fenton chemistry in acid solutions [23]. One mechanistic suggestion put forward [22] discusses the intermediacy of an OH-adduct radical, R-S (OH)-S-R which in basic solution may suffer OH attack to yield RS together with the hydrated form of the sulfinyl radical, RS(OH)2 . In acid solution, on the other hand, a secondary oxidation of the sulfenic acid (formed in reaction (10)) by disulfide radical cations (also formed upon OH reaction with disulfides, vide infra), that is reaction (12) has been brought into the discussion. 

Such adduct intermediates have been identified for R and R being methyl and/or the substituted aliphatic constituents of cysteine, cysteamine, and penicillamine. The same kind of intermediate also appears to be formed in the oxidation of the cysteamine thiolate, CyaS", by the disulfide radical cation of lipoic acid, Lip(SS). Identification of the adduct is based on its optical absorption ( -max ca. 380 nm, e ca. (3-4) x 10 M" cm" ) and facilitated by relatively long lifetimes (> 100 ps). In fact, the adduct formation is most appropriately formulated as a reversible process reaction (34b) ... 

Disulfide radical cations are reasonably good oxidants. Some representative reduction potentials, measured for the dimethyldisulfide and lipoic acid systems [162] are listed in Table 9. 

Table 9 Reduction potentials of some selected disulfide radical cations.

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