What a disulfide bridge does
Two cysteine thiols oxidise to form a covalent sulfur to sulfur bond, losing two hydrogens in the process. The result is a crosslink that ties two points of the chain together, and it is the single most common way a short peptide is given a defined shape.
The mass change is exact and diagnostic: 2.02 daltons lighter per bridge. A mass spectrum showing a peptide two daltons below the calculated linear mass is showing you a formed disulfide, and one at the calculated mass is showing you a reduced peptide.
Why the number of possible pairings matters
With four cysteines there are three ways to pair them all. With six there are fifteen, with eight a hundred and five, and the count grows as the double factorial. Only one arrangement is normally the biologically correct one, and the others are misfolded isomers with the same molecular weight.
That identical weight is the problem. Mass spectrometry cannot distinguish correctly folded from misfolded material when the connectivity differs but the composition does not. Separating them requires chromatography, and identifying which is which requires proteolytic mapping or a functional assay.
- •2 cysteines: 1 pairing
- •4 cysteines: 3 pairings
- •6 cysteines: 15 pairings
- •8 cysteines: 105 pairings
- •10 cysteines: 945 pairings
Intramolecular and intermolecular bridges
A bridge can form within one molecule or between two, and the second case produces a dimer at roughly twice the expected mass. Dilute conditions favour intramolecular formation because the two ends of one chain find each other more readily than two separate molecules do, which is why controlled oxidative folding is performed at low concentration.
An odd number of cysteines guarantees at least one free thiol, which is a reactive group looking for a partner. Free thiols are why some peptides dimerise on storage, and why reducing agents are added to solutions where the free form is wanted.
Keeping bridges in the state you want
Reduced peptides are kept reduced with a reducing agent, typically DTT or TCEP, and oxidised ones are kept oxidised by excluding reductants and, where necessary, buffering above neutral pH where thiol exchange is slower to reshuffle.
Copper ions catalyse thiol oxidation, so a trace of copper in a buffer can quietly convert a reduced peptide to a bridged one over hours. Chelators such as EDTA are the usual defence.
How the bridge analysis is calculated
Cysteine positions are located, the number of complete pairings is counted combinatorially, and the mass change is applied per bridge. The exhaustive pairing list is capped, because the count grows faster than a browser can draw it.
max bridges = floor( cysteine count / 2 ) possible pairings = (n - 1)!! for n cysteines, n even bridged mass = linear mass - 2.016 x number of bridges
- Locate the cysteines. Positions are numbered from the N-terminus, matching the convention used in certificates and papers, so a bridge described as 1-6 can be checked directly.
- Count the possible complete pairings. The double factorial of one less than the cysteine count. With an odd number, one cysteine is necessarily left unpaired and the count uses the even number below.
- Enumerate them, up to a limit. Every complete pairing is listed for ten cysteines or fewer. Above that the list is suppressed: twelve cysteines is 10,395 arrangements and eighteen is over two million, which no reader can use and no browser should build.
- Apply the mass change. Each bridge removes 2.016 daltons, the mass of two hydrogen atoms. The fully reduced and fully oxidised masses are both reported.
- Accept a known pattern. If you know the correct connectivity, entering it as position pairs highlights that arrangement among the possibilities and uses it for the mass calculation.
What this method cannot tell you
- •It cannot predict which pairing is correct. That requires experimental determination or knowledge of the native structure.
- •It considers intramolecular bridges only. Intermolecular bridges producing dimers and higher oligomers are not enumerated.
- •The exhaustive list stops above ten cysteines. The bridge count and the mass arithmetic still work.
- •It does not model the kinetics of oxidative folding, which is what determines the mixture you actually obtain.
Disulfide bond calculator: frequently asked questions
A covalent bond between the sulfur atoms of two cysteine residues, formed by oxidation, which crosslinks two points of a peptide chain.
It is the most common way a short peptide is given a defined three-dimensional shape.
It removes 2.02 daltons, the mass of the two hydrogen atoms lost when the two thiols oxidise.
A peptide with two bridges is 4.03 daltons lighter than its fully reduced form, which is easily resolved on any modern mass spectrometer.
At most half the number of cysteines, rounded down. Six cysteines can form three bridges; seven can still only form three, with one thiol left over.
The count grows as the double factorial of one less than the cysteine count:
- •2 cysteines: 1 way
- •4 cysteines: 3 ways
- •6 cysteines: 15 ways
- •8 cysteines: 105 ways
- •10 cysteines: 945 ways
Only one is normally the correct one. The rest are misfolded isomers of identical molecular weight.
It can tell you how many bridges have formed, because each one shifts the mass by 2.02 daltons. It cannot tell you which cysteines are paired with which.
Every arrangement with the same number of bridges has the same mass. Determining connectivity requires digesting the peptide and mapping the resulting fragments, or a functional assay.
You get an isomer with the right composition, the right mass and the wrong shape. It is typically inactive or much less active.
Misfolded isomers often separate from the correct one on reverse-phase chromatography, which is why a certificate showing a cluster of closely eluting peaks around the main one is worth asking about.
A cysteine whose thiol has not paired. It is chemically reactive and looks for a partner, which can be a cysteine on another molecule.
That is how dimers form on storage. An odd number of cysteines guarantees at least one free thiol.
Add a reducing agent, typically dithiothreitol or TCEP, and exclude oxidants. Chelate trace metals with EDTA, since copper in particular catalyses thiol oxidation.
TCEP is generally preferred: it is odourless, more stable in solution and effective across a wider pH range than DTT.
By controlled oxidation, usually air oxidation at slightly alkaline pH, or with a redox buffer of oxidised and reduced glutathione.
It is done at low peptide concentration to favour intramolecular over intermolecular bridging: at high concentration two separate molecules find each other more easily than the two ends of one chain do.
Cysteine is the free amino acid with a thiol side chain. Cystine is two cysteines joined by a disulfide bridge.
The distinction matters for extinction coefficients, where the 125 M⁻¹cm⁻¹ contribution belongs to the cystine bridge rather than to each cysteine.
As a pair of residue numbers counting from the N-terminus. Oxytocin's bridge is written 1-6 because its cysteines are the first and sixth residues.
Multiple bridges are listed separated by commas, as in the 1-15 and 3-11 pattern of endothelin-1.
Because it listed a bridge that does not exist within that chain. The A chain has cysteines at 6, 7, 11 and 20, but only 6-11 is intra-chain.
A7 and A20 bridge to the B chain, so describing 7-20 as an internal bond of the A chain was wrong. The preset now lists 6-11 only.
They generally increase it, by holding the chain in a compact conformation that is harder for proteases to unfold and attack.
They also create a new failure mode, since a reducing environment breaks them and the peptide unfolds. Intracellular conditions are reducing, which is why disulfide-stabilised peptides are far more common outside cells than inside them.
Because the number of arrangements grows faster than any use for the list. Ten cysteines gives 945; twelve gives 10,395; eighteen gives over two million.
Above ten cysteines the count and the mass arithmetic are still reported. Only the exhaustive enumeration stands down.
Yes. Thiol-disulfide exchange lets bridges reshuffle, and it is fastest under mildly alkaline conditions with a trace of free thiol present.
Storing bridged peptides at slightly acidic pH slows the exchange considerably, which is one reason many peptide formulations are acidic.
Oxytocin and vasopressin each have one, somatostatin has one, insulin has three, and endothelin-1 has two.
In each case the bridge closes a ring, and the ring is what gives a short chain a defined shape that the linear sequence alone would not produce.
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