Short answer: A disulfide bond is a covalent sulfur–sulfur link, R–S–S–R′. In peptides and proteins it forms between the thiol (–SH) side chains of two cysteine residues. Mild oxidation joins them and mild reduction splits them apart again [1][2]. A disulfide between two chains ties those chains together, and a disulfide within one chain closes a loop [2].
This article is educational chemistry for laboratory readers. It does not describe biological effects, and it is not guidance for use in people or animals.
What a disulfide bond is
The chemistry starts with the thiol group of cysteine:
- The building block is cysteine. IUPAC names cysteine 2-amino-3-mercaptopropanoic acid, HS-CH₂-CH(NH₂)-COOH, with the symbols Cys and C [6]. Its side chain ends in a sulfhydryl (SH) group [5].
- Oxidation makes the bond. Thiols can be oxidized by bromine or iodine to give disulfides (R–S–S–R′) [1]. Going from two reduced sulfhydryl groups (S–H) to the oxidized cystine (S–S) is a two-electron oxidation [8].
- Reduction reverses it. The reaction is easily reversed: a disulfide can be reduced back to two thiols, for example with zinc and acid [1].
- Cysteine vs cystine. "Cysteine" is the residue with a free thiol. "Cystine" is the name for two cysteines joined through their sulfur atoms [8].
Disulfide bond vs disulfide bridge
The two terms describe the same S–S link between cysteine residues. OpenStax uses "disulfide linkage," "disulfide bond" and "disulfide bridges" in a single passage on peptides [2], and Wiedemann and colleagues use "disulfide bridges" and "disulfide bonds" side by side in their 2020 review [8].
Geometry of the S–S link
A disulfide has a characteristic shape:
- Bond length: the disulfide linkage in a cystine has a typical bond length of about 2.04 Å [8].
- Two handed forms: the lone electron pairs on the two sulfur atoms repel the neighboring groups. That leaves two energetically favorable, mirror-image conformations for the Cβ–Sγ–Sγ–Cβ torsion angle [8].
- Typical angles: a study of 1,505 native disulfide bonds found average torsion values of about −87° (left-handed) and +97° (right-handed) [8].
- Unusual for a side chain: most other amino-acid side-chain torsions sit mainly near trans (180°) or gauche (±60°), so these values stand out [8].
Interchain and intrachain disulfides
Where the two cysteines sit decides what the bond does to the molecule's architecture:
- Between chains (interchain): a disulfide between cysteines in different chains links the otherwise separate chains together [2].
- Within one chain (intrachain): a disulfide between cysteines in the same chain forms a loop [2].
- Insulin as the textbook case: insulin is made of two chains totaling 51 amino acids, linked by two cysteine disulfide bridges [2][4]. In OpenStax Biology's insulin figure, the A chain is 21 amino acids long and the B chain 30. Two disulfide bonds connect the A and B chains, and a third, within the A chain, helps it fold into the correct shape [5].
- Naming the ring: IUPAC calls a cyclic peptide heterodetic when its ring linkages are not solely ordinary peptide (eupeptide) bonds, and one or more is an isopeptide, disulfide, ester or other bond [7]. A peptide ring closed through a disulfide falls in this group.
How common are they? In reviewed UniProt records, 83% of proteins contain at least one cysteine, and cysteine accounts for 1.38% of all amino acids [8]. Bachem notes that cysteine rarely occurs in bioactive peptides, but where it does, its thiol is important for stabilizing structure through disulfide bonds, formed either within one molecule or between molecules [9].
How disulfides are formed in synthetic peptides
In chemical synthesis, the thiol is too reactive to leave unprotected while the chain is being built:
- Protection is mandatory. Bachem states that protecting the cysteine thiol during peptide synthesis is mandatory, which is why specifically cleavable thiol protecting groups were developed [9].
- Common protecting groups: in Fmoc-based solid-phase synthesis, trityl (Trt) is the preferred thiol protection and is removed with TFA in the presence of scavengers such as EDT. Mob, the very acid-labile Mmt, and the orthogonal Acm are frequent choices for more complex cysteine peptides [9]. Acm is removed in a separate step and works with both Fmoc and Boc chemistry [9].
- One disulfide: when both cysteines are added as Fmoc-Cys(Trt)-OH, TFA/scavenger cleavage gives the crude, deprotected linear peptide. It can then be cyclized directly by atmospheric oxygen or other mild oxidants such as potassium hexacyanoferrate(III) [9]. The oxidation is usually run in very dilute solution to favor the intramolecular reaction [9].
- Air oxidation conditions (Bachem): the peptide is dissolved at 10⁻³ to 10⁻⁴ M in dilute acetic acid or 0.05 M ammonium acetate, held at pH 7.5–8 and stirred at room temperature open to air. Additives such as 1–10% DMSO or 3% H₂O₂ can speed things up, but H₂O₂ must not be used if the peptide contains methionine. Progress is followed by HPLC, and the solution is acidified with acetic acid once conversion is complete [9].
- Two or more disulfides: the S-protection may have to be varied so the bridges can be formed one after another, giving an unambiguous structure [9].
- Racemization risk: cysteine derivatives are notorious for base-catalyzed racemization during activation and coupling; Cys(Acm) derivatives show a lower tendency to racemize [9].
How disulfide bonds are broken
Because a disulfide is cleaved by mild reduction [2], laboratories use reducing agents to open it:
- TCEP: Thermo Fisher describes TCEP-HCl as an odorless, thiol-free reducing agent that reduces disulfide bonds as effectively as dithiothreitol (DTT). Unlike DTT and other thiol-containing reducing agents, it does not have to be removed before certain sulfhydryl-reactive crosslinking reactions [10].
- TCEP practicalities: it is stable in aqueous, acidic and basic solutions, and dissolving it directly in water gives a pH of about 2.5. It is not particularly stable in phosphate buffers, especially at neutral or alkaline pH [10].
- Before amino-acid analysis: a peptide is prepared by reducing all disulfide bonds, capping the cysteine –SH groups by S_N2 reaction with iodoacetic acid, and then hydrolyzing the amide bonds in aqueous 6 M HCl at 110 °C for 24 hours [3].
How disulfides are detected and measured
Several methods report whether cysteines are free or paired:
- Ellman's reagent (DTNB): DTNB reacts with free (reduced) sulfhydryl groups to give a colored product, 2-nitro-5-thiobenzoic acid (TNB). It is used to measure reduced cysteines and other free sulfhydryls in solution [11]. TNB absorbs at λmax 412 nm with an extinction coefficient of 14,150 M⁻¹cm⁻¹; Ellman's original 1959 value was 13,600 M⁻¹cm⁻¹ at pH 8.0 [11]. The reagent targets the thiolate (R–S⁻), so the rate depends on pH, the thiol's pKa, and steric and electrostatic effects [11].
- NMR: the disulfide bridge itself is invisible in NMR. Its conformation is worked out from constraints on the neighboring NMR-active nuclei [8].
- Mass spectrometry: Unimod lists "half of a disulfide bridge" (Dehydro, interim name Cystine) as a loss of one hydrogen per cysteine: −1.007825 Da monoisotopic, −1.0079 Da average [12]. One disulfide therefore makes a peptide 2.01565 Da (monoisotopic) lighter than its fully reduced form. Alkylating a free cysteine with iodoacetamide (carbamidomethyl) adds 57.021464 Da monoisotopic, 57.0513 Da average [13].
- With one disulfide formed: 1,500.0000 − 2 × 1.007825 = 1,497.9843 Da [12].
- Reduced, then both cysteines alkylated with iodoacetamide: 1,500.0000 + 2 × 57.021464 = 1,614.0429 Da [13].
Practical notes for lab readers
- Check the sequence for Cys. A cysteine thiol can be oxidized to a disulfide and reduced back again, so it matters which form a document describes [1][2].
- Know which form a mass refers to. Each disulfide shifts the monoisotopic mass by −2.01565 Da relative to the reduced peptide [12].
- Thiol assays count free –SH only. Ellman's reagent measures reduced cysteines and other free sulfhydryls [11].
- Read the documentation. See how to read a peptide COA for where identity and mass data appear, and N-terminus vs C-terminus for how residue positions are numbered.
Frequently Asked Questions
What is a disulfide bond?
A covalent S–S link (RS–SR) between two thiol groups. In peptides it forms between two cysteine residues by mild oxidation and is cleaved by mild reduction [1][2].
Is a disulfide bond covalent?
Yes. OpenStax describes it as a second kind of covalent bonding in peptides, alongside the amide bonds of the backbone [2].
What is the difference between a disulfide bond and a peptide bond?
A peptide bond is the amide bond that links residues along the backbone. A disulfide bond links two cysteine side chains through their sulfur atoms, either between chains or within one chain [2].
Which amino acid forms disulfide bonds?
Cysteine, through the sulfhydryl (SH) group on its side chain [2][5][6].
What is the difference between cysteine and cystine?
Cysteine has a free thiol; cystine is the oxidized, S–S-linked pair of two cysteines [8].
How do you break a disulfide bond?
By reduction. In the lab, reducing agents such as TCEP or DTT are used [10]; textbook organic chemistry also uses zinc and acid [1].
How long is a disulfide bond?
About 2.04 Å in cystine [8].
Bottom line
A disulfide bond is a reversible, covalent S–S link between two cysteine thiols [1][2][8]. Between chains it ties them together; within a chain it closes a loop [2]. Synthetic chemists protect cysteine thiols during assembly and form the bridges deliberately afterwards [9]. Analysts read them through reduction chemistry, thiol assays and a predictable mass shift [3][10][11][12][13].
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