Theoretical and Electrochemical Analysis of Dissociative Electron Transfers Proceeding through Formation of Loose Radical Anion Species:  Reduction of Symmetrical and Unsymmetrical Disulfides

Abstract

The dissociative reduction of a series of symmetrical (RSSR, R = H, Me, t-Bu, Ph) and unsymmetrical disulfides (RSSR', R = H, R' = Me and R = Ph, R' = Me, t-Bu) was studied theoretically, by MO ab initio calculations and, for five of them, also experimentally, by convolution voltammetry in N,N-dimethylformamide. The reduction is dissociative but proceeds by a stepwise mechanism entailing the formation of the radical anion species. The electrochemical data led to estimated large intrinsic barriers, in agreement with an unusually large structural modification undergone by the disulfide molecules upon electron transfer. The theoretical results refer to MP2/3-21G*//MP2/3-21G*, MP2/3-21*G*//MP2/3-21G*, CBS-4M, and G2(MP2), the latter approach being used only for the molecules of small molecular complexity. A loose radical-anion intermediate was localized and the dissociation pattern for the relevant bonds analyzed. For all compounds, the best fragmentation pathway in solution is cleavage of the S-S bond. In addition, S-S bond elongation is the major structural modification undergone by the disulfide molecule on its way to the radical anion and eventually to the fragmentation products. The calculated energy of activation for the initial electron transfer was estimated from the crossing of the energy profiles of the neutral molecule and its radical anion (in the form of Morse-like potentials) as a function of the S-S bond length coordinate. The inner intrinsic barrier obtained in this way is in good agreement with that determined by convolution voltammetry, once the solvent effect is taken into account.

Spectroscopic and theoretical approaches for studying radical reactions in class I ribonucleotide reductase

“…Equilibrium constants for this association reaction are in the range 10 2 -10 4 M -1 (Mezyk and Armstrong, 1999). Direct evidence for the reaction of the thiyl radical and thiolate to the disulfide radical anion was recognized by EPR in the enzymatic process, at least for the E441Q R1 mutant (Lawrence et al, 1999 Figure 13 Step (4) as suggested by Cerqueira et al (2004b) and by Pelmenschikov et al (2004). of disulfides are in the range of -1.9 to -2.3 V SCE for dialkyl derivatives (Mezyk and Armstrong, 1999;Antonello et al, 2002); hence, disulfide radical anions are strong reducing agents that can reduce carbonyl derivatives, in particular if these are involved in hydrogen bonding (Oelgemö ller et al, 2002).…”

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

“…[20] On top of this, our knowledge on the mechanistic details of the SÀS bond fission has been limited by the fact that most of these attempts were based on symmetric model systems, in which the two substituents attached to the SÀS bridge were identical. Actually, to the best of our knowledge, only Antonello et al [21] have addressed the ECD in RSSR' unsymmetrical disulfides, where the R and R' substituents were alkyl or phenyl groups.…”

Electrochemical Behavior of Diphenyl Disulfide and Thiophenol on Glassy Carbon and Gold Electrodes in Aprotic Media

“…The study of the electrochemical behavior of DDS and C 6 H 5 SH on GC electrode allows to clarify the nature of the ET process in aprotic solvent (ACN), and the overall electrochemical reduction mechanism. The S À S bond free energy was calculated using E8 values obtained by a convolution method [21,22,23].…”