# Thiol–disulfide exchange
Thiol–disulfide exchange (thiol–disulfide interchange) is the reaction in which a thiolate anion attacks the sulfur atom of a disulfide, breaking one S–S bond and forming a new one, so that the disulfide linkage is transferred from one pair of sulfur atoms to another. Disulfides are the major products of thiol oxidation, a process with primary roles in defense against oxidative stress and in redox regulation of cell signaling.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3613173/) This article covers the mechanism, thermodynamics, kinetics, and redox potentials of small-molecule thiol–disulfide exchange; protein disulfide bonds and polymer chemistry are outside its scope.
| Key fact | Value | Conditions / note |
|---|---|---|
| Cysteine thiol pKa | 8.4 | macroscopic, aqueous[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| Glutathione thiol pKa | 8.7 | macroscopic, aqueous[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| Cystine/cysteine redox potential | −0.22 V | macroscopic standard[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| GSH/GSSG redox potential | −0.26 V | macroscopic standard[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| Second-order rate constants | 10²–10⁷ M⁻¹ s⁻¹ | thiol–disulfide interchange overall[3](https://doi.org/10.1002/ijch.198500126) |
| Intrinsic Marcus barrier | 11.6 kcal/mol (plus 4.0 kcal work term) | thiol–disulfide interchange[3](https://doi.org/10.1002/ijch.198500126) |
| Equilibrium log K vs thiol pKa slope | 1.21 | aryl thiolates, 2-hydroxyethyl disulfide[3](https://doi.org/10.1002/ijch.198500126) |
## Mechanism of thiolate attack on disulfides
The reacting nucleophile is the thiolate anion, not the neutral thiol. Only the deprotonated thiolate species is directly oxidizable or exchange-active, and the deprotonated fraction depends strongly on pH, typically across the 6–11 range.[4](https://www.nature.com/articles/srep37596) Because common thiol pKa values are 8.4–8.7 for cysteine and glutathione,[2](https://www.mdpi.com/2076-3921/13/9/1053) only a small percentage of each thiol is in the reactive form at pH 7, and the pH-rate profile of interchange largely tracks thiol ionization.
The exchange follows an SN2-at-sulfur pathway. High-level CCSD and DFT calculations show that degenerate exchange of a thiolate with dimethyl disulfide passes through an SN2-like transition structure that lies very close in energy to a trisulfur anionic intermediate.[5](https://pubs.acs.org/doi/abs/10.1021/jo702051f) QM/MM metadynamics places the transition state at S–S distances of 2.7 Å with an S–S–S angle of 165°, a nearly linear, symmetric, trisulfide-like arrangement.[6](https://pubs.rsc.org/en/content/articlelanding/2018/cp/c8cp01325j) The charge density in the transition state is concentrated on the attacking and leaving sulfur atoms, consistent with an uncomplicated SN2 process.[7](https://doi.org/10.1002/chem.200305343)
For the sterically demanding t-butyl mercaptide/di-t-butyl disulfide exchange, the SN2 displacement barrier is 9.8 kcal/mol, and COSMO solvent corrections implicate the SN2 mechanism in both polar and nonpolar solvents.[5](https://pubs.acs.org/doi/abs/10.1021/jo702051f) With a model peptide substituent, the trisulfur intermediate sits 10.7 kcal/mol below the isolated reactants, deep enough to allow conformational adjustment before the new disulfide bond forms.[5](https://pubs.acs.org/doi/abs/10.1021/jo702051f)
An exception worth noting is that water-mediated attack by the neutral thiol is calculated to be almost kinetically competitive with thiolate attack, and can become competitive under some conditions.[7](https://doi.org/10.1002/chem.200305343) Nevertheless, the practical rate law at accessible pH is dominated by the thiolate term (see Kinetics).
## Thermodynamics and driving forces
The equilibrium constant of an interchange between thiolate RS⁻ and disulfide R′SSR′ is governed largely by the relative acidities of the two thiols. For aryl thiolate reactions with 2-hydroxyethyl disulfide, log K plotted against thiol pKa has a slope of 1.21.[3](https://doi.org/10.1002/ijch.198500126)
Redox potentials provide the second thermodynamic language. Aqueous thiol–disulfide potentials cannot be measured directly at electrodes because stable metal–thiolate complexes form at electrode surfaces; they must instead be derived indirectly from equilibrium constants against reference redox systems.[4](https://www.nature.com/articles/srep37596) This is how the species-specific, pH-independent standard potentials of glutathione were obtained by comparison with 1-methylnicotinamide, and those of cysteamine, cysteine, homocysteine, penicillamine, and ovothiol from microscopic equilibrium constants with glutathione, yielding a set of 30 microscopic standard redox potential values.[4](https://www.nature.com/articles/srep37596)
Apparent (conditional) potentials differ from these species-specific values. An apparent potential is a pH-dependent, concentration-weighted resultant of several protonation forms, whereas the underlying species-specific potentials are pH-independent.[2](https://www.mdpi.com/2076-3921/13/9/1053) Comparing potentials across thiols therefore requires stating which quantity, at which pH, is meant.
Solvent environment also matters. Hydrophobic environments catalyze thiol/disulfide exchange because the transition structures distribute charge more widely than the reactants, and exchange in the active site of ribonucleotide reductase is estimated to be accelerated 10³-fold relative to water on this basis.[7](https://doi.org/10.1002/chem.200305343)
## Kinetics and structure–reactivity
The rate law for thiol–disulfide interchange contains only the term for the thiol anion attacking the disulfide, with a Brønsted coefficient \(\beta_{\mathrm{nuc}}\) of about 0.5 for the attacking thiol and about −0.3 for the central thiol at 25 °C and ionic strength 1.0.[3](https://doi.org/10.1002/ijch.198500126)
Second-order rate constants span roughly 10² to 10⁷ M⁻¹ s⁻¹ across thiolate/disulfide pairs.[3](https://doi.org/10.1002/ijch.198500126) Measured values for aryl thiolates reacting with 2-hydroxyethyl disulfide run from 7.24 × 10¹ M⁻¹ s⁻¹ for C₆H₅S⁻ up to 2.04 × 10⁵ M⁻¹ s⁻¹ for substituted aryl thiolates.[3](https://doi.org/10.1002/ijch.198500126) Aromatic entering or leaving thiols react about six-fold faster than their aliphatic counterparts, a hard–soft acid–base effect, while charging the entering, central, or leaving thiol in uncharged systems changes the rate by up to a factor of 2.5.[3](https://doi.org/10.1002/ijch.198500126)
In peptides and proteins, electrostatics can dominate. Sixteen model peptides carrying a single cysteine (pKa 7.4–9.1) gave Brønsted coefficients of 0.5 with cystine and 0.3 with 2-hydroxyethyl disulfide, but 0 and 0.8 with the charged reagents oxidized glutathione and cystamine; with charged disulfides, electrostatic attraction or repulsion can override thiol basicity as the rate-determining factor.[8](https://doi.org/10.1021/bi973101r) [Thiol](https://www.edgepedia.org/thiol) pKa, local electrostatic environment, molecular strain, and entropy jointly set exchange rates and stabilities, and oxidoreductase enzymes lower the activation barrier further.[9](https://liebertpub.com/doi/10.1089/ars.2008.2297) An intrinsic Marcus barrier of 11.6 kcal/mol, plus a 4.0 kcal work term, characterizes the baseline reaction.[3](https://doi.org/10.1002/ijch.198500126)
## Key redox couples and potentials
The glutathione couple (GSH/GSSG) is regarded as the 'gold standard' of thiol–disulfide biochemistry, the reference against which thiol-containing antioxidants are compared.[4](https://www.nature.com/articles/srep37596) Its macroscopic standard potential is −0.26 V, with thiol pKa 8.7; cysteine/cystine sits at −0.22 V with pKa 8.4.[2](https://www.mdpi.com/2076-3921/13/9/1053)
Because electrodes cannot be used, potentials for other couples come from equilibrium measurements. The glutathione/dithiothreitol interchange was characterized this way: equilibrium and kinetic constants were determined at several pH values and temperatures by approaching equilibrium from both directions, with pH-quench or methyl methanethiosulfonate quenching to freeze the composition.[10](https://www.pnas.org/doi/abs/10.1073/pnas.89.17.7944)
A practical compilation now exists for prediction: across 31 thiolates, species-specific standard redox potentials correlate linearly with log k basicity values (slope −0.0605, intercept 0.1683, adj. R² = 0.9910), so a thiolate's potential can be estimated from its basicity alone.[2](https://www.mdpi.com/2076-3921/13/9/1053) The microscopic potentials themselves correlate with thiolate basicities that span roughly 1–10 log units in protonation constants.[4](https://www.nature.com/articles/srep37596) For selenolates the analogous line is offset by approximately 250 mV relative to thiolates, quantifying selenium's easier oxidation.[2](https://www.mdpi.com/2076-3921/13/9/1053)
## By the numbers
| Quantity | Value | System and conditions |
|---|---|---|
| Thiol pKa | 8.4 | cysteine, macroscopic[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| Thiol pKa | 8.7 | glutathione, macroscopic[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| E°′ | −0.22 V | cysteine/cystine[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| E°′ | −0.26 V | GSH/GSSG[2](https://www.mdpi.com/2076-3921/13/9/1053) |
| k (second-order) | 7.24 × 10¹ to 2.04 × 10⁵ M⁻¹ s⁻¹ | aryl thiolates + 2-hydroxyethyl disulfide, 25 °C[3](https://doi.org/10.1002/ijch.198500126) |
| k range | 10²–10⁷ M⁻¹ s⁻¹ | thiol–disulfide interchange overall[3](https://doi.org/10.1002/ijch.198500126) |
| Activation barriers | 9.8 kcal/mol (t-Bu exchange)[5](https://pubs.acs.org/doi/abs/10.1021/jo702051f); 11.9 kcal/mol (first step of SO₂F₂-mediated formation)[11](https://preview-www.nature.com/articles/s41467-024-52606-w); 11.6 kcal/mol intrinsic Marcus barrier[3](https://doi.org/10.1002/ijch.198500126) | calculated / kinetic |
| Transition-state geometry | S–S 2.7 Å; S–S–S angle 165° | QM/MM metadynamics[6](https://pubs.rsc.org/en/content/articlelanding/2018/cp/c8cp01325j) |
The numbers carry a consistent message: barriers cluster near 10–12 kcal/mol, so modest changes in pKa, charge, or solvent shift rates by many orders of magnitude.
## Recent developments: new exchange chemistries
Classical thiolate interchange now has new, mechanistically distinct companions. A 2024 PNAS study showed that hydroxy(tosyloxy)iodobenzene (HTIB) initiates clean, bidirectional disulfide metathesis through a radical mechanism (validated by control experiments and EPR) under biocompatible conditions, driven forward by an excess of one disulfide or, for intramolecular cyclic five- and eight-membered disulfides, without excess reagent.[12](https://doi.org/10.1073/pnas.2405337121) This radical route enables selective modification of carbohydrates, drugs, native amino acids, and proteins.[12](https://doi.org/10.1073/pnas.2405337121)
Also in 2024, a [Nature Communications](https://www.edgepedia.org/nature-communications) study reported an SO₂F₂-mediated 'click' reaction converting thiols to disulfides with a calculated thermodynamic driving force of 64.4 kcal/mol and an 11.9 kcal/mol barrier for the first nucleophilic substitution step.[11](https://preview-www.nature.com/articles/s41467-024-52606-w) Under aqueous conditions, mPEG-SH was converted to its disulfide in 98% yield with SO₂F₂/Na₂CO₃, and at physiological pH in borax buffer, boc-L-cysteine and glutathione gave disulfides in 85% and 98% yields with SO₂F₂/Et₃N.[11](https://preview-www.nature.com/articles/s41467-024-52606-w)
In 2026, a visible-light strategy using cerium salts as catalysts achieved oxidant-free dehydrogenative coupling of thiols to disulfides, compatible with thiophenols, alkanethiols, and peptides and scalable by microchannel continuous flow.[13](https://pubs.rsc.org/en/content/articlelanding/2026/ob/d6ob00450d) These methods expand the toolbox beyond the thiolate SN2 pathway while leaving its mechanism and thermodynamics unchanged.
## References
1. Kinetics and Mechanisms of Thiol–Disulfide Exchange Covering Direct Substitution and Thiol Oxidation-Mediated Pathways. Antioxidants & Redox Signaling. https://pmc.ncbi.nlm.nih.gov/articles/PMC3613173/
2. Prediction of Antioxidant Capacity of Thiolate–Disulfide Systems Using Species-Specific Basicity Values. Antioxidants, 2024. https://www.mdpi.com/2076-3921/13/9/1053
3. A Demonstration of the Reactivity–Selectivity Principle for the Thiol–Disulfide Interchange Reaction. https://doi.org/10.1002/ijch.198500126
4. Species-Specific Standard Redox Potential of Thiol-Disulfide Systems: A Key Parameter to Develop Agents against Oxidative Stress. Scientific Reports. https://www.nature.com/articles/srep37596
5. Mechanism of Thiolate−Disulfide Interchange Reactions in Biochemistry. Journal of Organic Chemistry. https://pubs.acs.org/doi/abs/10.1021/jo702051f
6. On the mechanism of spontaneous thiol–disulfide exchange in proteins. Phys. Chem. Chem. Phys., 2018. https://pubs.rsc.org/en/content/articlelanding/2018/cp/c8cp01325j
7. Theoretical Insights into the Mechanism for Thiol/Disulfide Exchange. Chemistry—A European Journal. https://doi.org/10.1002/chem.200305343
8. Ionization−Reactivity Relationships for Cysteine Thiols in Polypeptides. Biochemistry. https://doi.org/10.1021/bi973101r
9. Kinetic and Thermodynamic Aspects of Cellular Thiol–Disulfide Redox Regulation. Antioxidants & Redox Signaling, 2009. https://liebertpub.com/doi/10.1089/ars.2008.2297
10. Equilibrium and kinetic constants for the thiol-disulfide interchange reaction between glutathione and dithiothreitol. PNAS. https://www.pnas.org/doi/abs/10.1073/pnas.89.17.7944
11. SO2F2 mediated click chemistry enables modular disulfide formation in diverse reaction media. Nature Communications, 2024. https://preview-www.nature.com/articles/s41467-024-52606-w
12. An innovative strategy for radical-mediated, bidirectional controlled disulfide exchange. PNAS, 2024. https://doi.org/10.1073/pnas.2405337121
13. Visible-light-induced oxidant-free thiol–disulfide transformation. Organic & Biomolecular Chemistry, 2026. https://pubs.rsc.org/en/content/articlelanding/2026/ob/d6ob00450d
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Alcohols, ethers, and organooxygen groups › Organosulfur, selenium, and heavier main-group organo derivatives › Organosulfur, selenium, and tellurium analogues › Thiols and mercaptans › Thiol–disulfide exchange and redox chemistry*
*Initially written Sep 17, 2026 · Edited: Oct 11, 2026; Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license