Cyclic vs linear peptides: what ring closure changes
Last reviewed 2026-10-10 · Sable Peptides · Research use only
Sable research method explainer. Updated 2026-10-10. Research use only.
Short answer
A linear peptide is a single open chain with a free (or capped) N-terminus and C-terminus. A cyclic peptide has at least one ring, formed by joining the two ends, by linking a side chain to an end, or by linking two side chains. Ring closure restricts how the chain can fold, which affects shape, stability against exopeptidases and analytical behaviour. It does not change the amino-acid building blocks themselves.
Ways a peptide can be cyclised
- Head-to-tail: an amide bond joins the N-terminal amine to the C-terminal carboxyl, so there are no free ends.
- Side-chain to side-chain lactam: an amide bond joins, for example, an aspartate or glutamate side chain to a lysine side chain. Melanotan II and PT-141 share such a lactam-bridged ring: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys], amidated in Melanotan II and a free acid in PT-141.
- Disulfide: two cysteine thiols are oxidised to an S-S bond. Oxytocin is the textbook example, with a disulfide between Cys1 and Cys6 that closes a six-residue ring, followed by a three-residue tail.
- Other chemistries, such as thioether links and hydrocarbon staples, are used in research but are not represented in the Sable catalogue.
What changes when a peptide is cyclised
Conformation. A ring removes much of the flexibility of a short chain, so the peptide spends more time in a narrower set of shapes. Zorzi, Deyle and Heinis review how this conformational constraint is a main reason cyclic peptides are studied as binders (Zorzi et al., 2017).
Stability to exopeptidases. Enzymes that trim from free ends cannot act on a ring with no free terminus. Internal cleavage by endopeptidases is still possible.
Mass. Head-to-tail and lactam cyclisation each remove one water molecule (about 18 g/mol) compared with the open-chain form. Disulfide formation removes two hydrogen atoms (about 2 g/mol). These small differences are exactly what mass spectrometry on a certificate of analysis has to resolve.
Chromatography. A cyclic peptide and its linear precursor usually elute at different HPLC retention times, so incomplete cyclisation shows up as a related impurity.
How cyclic peptides are made
Most are assembled as a linear chain by solid-phase synthesis (Merrifield, 1963) and then cyclised, either while still attached to the resin or in dilute solution after cleavage. Dilution favours the intramolecular ring over intermolecular dimers. Disulfides are formed by controlled oxidation; peptides with more than one disulfide need orthogonal cysteine protection so that the correct pairs form (Góngora-Benítez et al., 2014).
Reading a /ref record
On Sable /ref pages, the cyclo[ ] notation marks the residues inside the ring, and a disulfide is stated in the structure text. The molecular formula and weight given come from PubChem for the cyclised form. When you check a mass-spectrometry result against a /ref page, make sure you are comparing it with the cyclic form, not with the open chain.
Laboratory notes
- Disulfide-containing peptides can scramble or reduce in the presence of thiols or reducing agents, so check buffer compatibility. Lactam rings are chemically robust under ordinary handling. Certificates of analysis for batches are available for every lot at /coa. Sable Peptides has been operating since 2019 and supplies every compound mentioned here as laboratory research material only. Nothing on this page is advice on use in people or animals, and no administration or medical-use information is provided.
Key terms
- Lactam
- A cyclic amide, here an amide bond formed between a side-chain carboxyl and a side-chain amine.
- Head-to-tail cyclisation
- An amide bond joining a peptide's own N-terminus and C-terminus.
- Exopeptidase
- An enzyme that removes residues one at a time from a free end of a chain.
- Endopeptidase
- An enzyme that cuts within a chain.
- Conformational constraint
- Restricting the shapes a molecule can adopt, for example by ring closure.
Common questions
Is a disulfide-bridged peptide a cyclic peptide?
Yes. A disulfide between two cysteines closes a ring, as in oxytocin (Cys1 to Cys6).
Does cyclisation change molecular weight?
Slightly. Lactam or head-to-tail closure removes one water molecule (about 18 g/mol), and a disulfide removes two hydrogen atoms (about 2 g/mol).
Which Sable compounds are cyclic?
Examples on the reference pages include Melanotan II and PT-141 (lactam ring) and oxytocin (disulfide ring).
Related explainers
Related reference pages
Sources
- Zorzi A, Deyle K, Heinis C. Cyclic peptide therapeutics: past, present and future. Curr Opin Chem Biol. 2017;38:24-29. https://doi.org/10.1016/j.cbpa.2017.02.006
- Góngora-Benítez M, Tulla-Puche J, Albericio F. Multifaceted roles of disulfide bonds. Peptides as therapeutics. Chem Rev. 2014;114(2):901-926. https://doi.org/10.1021/cr400031z
- Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. J Am Chem Soc. 1963;85:2149-2154. https://doi.org/10.1021/ja00897a025
Questions
Is a disulfide-bridged peptide a cyclic peptide?
Yes. A disulfide between two cysteines closes a ring, as in oxytocin (Cys1 to Cys6).
Does cyclisation change molecular weight?
Slightly. Lactam or head-to-tail closure removes one water molecule (about 18 g/mol), and a disulfide removes two hydrogen atoms (about 2 g/mol).
Which Sable compounds are cyclic?
Examples on the reference pages include Melanotan II and PT-141 (lactam ring) and oxytocin (disulfide ring).
Sources
CoA for every lot — request yours and we email it within 2 hours.
Research use only. Not for human or veterinary use. No health claims.