An analyst checks a new lot of a side-chain-cyclized peptide by mass spectrometry. The observed mass sits exactly 18.01 daltons below the calculated mass of the linear precursor, which is what ring closure by amide formation should produce. The result looks like confirmation. It is only partial confirmation. Several different outcomes of a cyclization reaction produce the same mass change, and a laboratory reviewing data on a cyclic peptide needs to know which of them the evidence actually rules out.
This article explains how rings are formed in peptides, what the constraint changes chemically, and why cyclic compounds ask more of an analytical package than linear ones.
Three common ways to close a ring
| Ring type | Bond formed | Mass change from linear precursor | Opened by reducing agents? | Same-mass lookalikes |
|---|---|---|---|---|
| Disulfide bridge | S–S bond between two cysteines | -2.016 Da | Yes | Wrong cysteine pairing; intermolecular dimer at twice the mass |
| Lactam bridge | Amide between a side-chain amine and a side-chain carboxyl | -18.011 Da | No | Aspartimide side product; cyclodimer at twice the mass |
| Head-to-tail | Amide joining the N- and C-termini | -18.011 Da | No | Cyclodimer and other oligomers |
Disulfide bridges are the mildest route, formed by oxidizing two cysteine thiols, commonly with air, DMSO or iodine. The reaction is usually run at high dilution so that each chain bonds with itself rather than with a neighbor. Where a peptide has more than one cysteine pair, chemists use protecting groups that can be removed selectively, such as Acm, Trt or Mmt, so that each bridge forms between its intended partners. Get the sequence wrong and the product is a disulfide isomer with the correct mass and the wrong structure. Disulfides are reversible by design: reducing agents such as DTT, TCEP and 2-mercaptoethanol open them.
Lactam bridges link a side-chain amine, typically lysine or ornithine, to a side-chain carboxyl, typically aspartate or glutamate, usually on the synthesis resin using orthogonal side-chain protection. The result is an ordinary amide bond, unaffected by reducing agents. The spacing of the two residues is a design choice; an i to i+4 bridge spans one turn of an alpha helix, i to i+7 spans two. Melanotan II is a familiar lactam-cyclized example, discussed in the Melanotan II overview.
Head-to-tail cyclization joins the N-terminal amine to the C-terminal carboxyl, removing both free ends. It is the most constraining and the hardest to make, because the intended reaction competes with cyclodimerization and oligomerization. High dilution and turn-inducing residues that pre-organize the chain are standard remedies, and very short rings of five residues or fewer are notoriously difficult for geometric reasons.
Other ring-forming chemistries
Several less common strategies appear in research compounds. Hydrocarbon staples are installed by ring-closing metathesis between two alkene-bearing residues and cannot be reduced. Side-chain-to-terminus rings tie one end of the chain to a side chain, leaving the other end free for further modification. Click chemistry joins an azide and an alkyne into a triazole, a linker that is chemically inert and easy to install but bulkier and more rigid than an amide.
What the constraint changes
A ring removes free ends, and free termini are what exopeptidases act on, so cyclic peptides are generally more resistant to that route of enzymatic breakdown than their linear counterparts. The more fundamental change is conformational. A linear peptide in solution samples a very large range of shapes. A ring restricts that range, holding the molecule nearer to a limited set of conformations. That restriction is why cyclic analogs of the same parent sequence can behave very differently from each other in binding experiments, and why ring position and size are regarded as design variables rather than details. Melanotan I, a linear peptide, and the cyclic compounds in the same family make a useful comparison; see the Melanotan I overview and the PT-141 overview.
Why a matching mass is not enough
Return to the opening example. A loss of 18.011 daltons shows that a molecule of water was lost. The intended lactam does that. So does an aspartimide, a cyclic side product that can form at aspartate residues. A cyclodimer, in which two chains close onto each other, loses two waters and appears at twice the mass, but in electrospray its doubly charged ion lands at the same m/z as the singly charged monomer, a trap for anyone reading the spectrum without checking isotope spacing.
Disulfides have the same problem. Every possible pairing of cysteines gives the same -2.016 Da change. Mass shows that bridges formed, not that the right ones did.
Chromatography helps, but with limits. The open-chain precursor has the same amino acid composition and is within a few daltons of the product, and oligomers share the monomer’s composition. These species may elute close to the main peak or co-elute with it, so retention time alone is weak identity evidence and a single-run purity figure may not have resolved them. More on how mass data are used for identity is in mass spectrometry and peptide identity.
Evidence that separates the lookalikes
- Reverse-phase retention. A cyclized peptide is usually more compact than its linear precursor and often elutes earlier. Isomers that share a mass frequently separate here, especially on a shallow gradient.
- Free-thiol measurement. Ellman’s assay quantifies free thiol groups. A fully oxidized disulfide peptide should show essentially none; a measurable result points to incomplete oxidation.
- Size-based separation. Size-exclusion chromatography or native mass spectrometry can distinguish an intramolecular product from a cyclodimer where reverse-phase alone may not.
- Partial reduction with tandem MS. Selective reduction, alkylation and fragmentation reveal which cysteines were actually paired, the definitive approach for peptides with several bridges.
- Structural spectroscopy. Circular dichroism gives a quick view of secondary structure; NMR can define the constrained geometry when the question justifies the effort.
A review checklist for cyclic peptide data
- Is the calculated mass given for the cyclic form, with the lost water or hydrogens stated?
- Is the ring chemistry named, since it determines what the compound tolerates?
- Was the possibility of a cyclodimer considered, including isotope spacing in the spectrum?
- For multiple disulfides, is there evidence of connectivity rather than mass alone?
- Are the HPLC column and gradient stated, so the chance of isomer co-elution can be judged?
Battle Born publishes an independent reverse-phase HPLC result for each product; for a cyclic compound, that result is best read alongside this list.
Questions
Does a correct mass confirm the right cyclization?
No. It confirms that a ring-forming loss occurred. Wrong disulfide pairings, aspartimides and cyclodimers can produce the same mass change or the same m/z.
Which ring types survive reducing agents?
Lactam, head-to-tail and hydrocarbon-stapled rings are not opened by reducing agents. Disulfide bridges are.
Why do cyclic and linear forms sometimes co-elute?
They share composition and differ in mass by only a few daltons. A shallow gradient improves the chance of separating them.
What is a cyclodimer?
Two chains closed onto each other instead of each onto itself, giving a species at twice the mass of the intended ring.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.