Which catalogue peptides are cyclic, and how are their rings formed?

Several catalogue peptides are cyclic. Oxytocin, AOD 9604 and cagrilintide are closed by a disulfide between two cysteines. Melanotan II and PT-141 are closed by a lactam, an amide bond between an Asp side chain and a Lys side chain. Rings constrain shape, change protease sensitivity and change the expected mass.

Which compounds are cyclic, and how?

Compound	Ring type	Ring residues	PubChem MW
Oxytocin	Disulfide Cys1–Cys6	6	1007.2 g/mol
AOD 9604	Disulfide (two Cys)	C-terminal loop	1815.1 g/mol
Cagrilintide	Disulfide in N-terminal ring	Amylin-type ring	4409.0 g/mol
Melanotan II	Lactam Asp–Lys	cyclo[Asp-His-D-Phe-Arg-Trp-Lys]	1024.2 g/mol
PT-141	Lactam Asp–Lys	Same ring as MT-II; C-terminal acid	1025.2 g/mol

Source: PubChem CID 439302: Oxytocin; PubChem CID 92432: Melanotan II; PubChem CID 9941379: PT-141

What is the difference between a disulfide and a lactam ring?

A disulfide (S–S) forms by oxidising two cysteine thiols. It loses 2 Da against the open form and can be reduced back. A lactam is an amide bond made during synthesis between side-chain carboxyl and amine groups. It loses 18 Da (water) and is chemically stable. Medicinal-chemistry reviews compare these and other ring-closing strategies.

Source: A role for ring-closing metathesis in medicinal chemistry: mimicking secondary architectures in bioactive peptides, Med Res Rev 2011 (PubMed 20077561); Diverse ring architectures of cyclic peptidomimetics targeting melanocortin receptors, RSC Med Chem 2026 (PubMed 42037742)

How do MT-II and PT-141 differ if they share a ring?

Both are Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]. Melanotan II ends in a C-terminal amide and PT-141 in a free acid, which accounts for the roughly 1 Da mass difference (1024.2 vs 1025.2 g/mol). Structure–function studies on MT-II's lactam and on N-methylated variants show how the ring shapes receptor selectivity.

Source: Structure-function studies on the cyclic peptide MT-II, lactam derivative of alpha-melanotropin, Peptides 1999 (PubMed 10447101); Multiple N-methylation of MT-II backbone amide bonds leads to melanocortin receptor subtype hMC1R selectivity: pharmacological and conformational studies, J Am Chem Soc 2010 (PubMed 20496895); Comprehensive characterization of bremelanotide acetate and its degradants by LC-HRMS/MS and predicting epimerization through computational modelling, Anal Methods 2026 (PubMed 42485063)

Why does cyclisation matter for identity testing?

The open (linear) form and the cyclic form differ in mass by a fixed amount (2 Da for a disulfide, 18 Da for a lactam), so mass spectrometry tells them apart. Disulfide peptides can also form dimers through intermolecular S–S bonds, which appear at twice the mass.

Sourcing note

Sable lists Oxytocin: 5 mg at £23.98; Melanotan II: 10 mg at £12.49; PT-141: 10 mg at £13.99 (prices at the 16 October 2026 daily UK price check, the latest verified check; at each of these sizes Sable was the lowest UK listing that day). Lowest UK price guaranteed, confirmed by daily price checks.

Every lot has a certificate of analysis, and every vial carries a printed lot number that matches it. Supplied for laboratory research use only.

Frequently asked questions

Is PT-141 the same as Melanotan II?

No. They share the lactam ring but PT-141 has a C-terminal acid and MT-II a C-terminal amide.

What is a lactam bridge in a peptide?

An amide bond between side chains, here Asp and Lys, that closes a ring. It forms with loss of water.

Which catalogue peptides contain a disulfide?

Among others, oxytocin, AOD 9604 and cagrilintide.

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

Lactam: −18 Da; disulfide: −2 Da vs open form
PubMed references (verified via NCBI): 20077561, 42037742, 10447101, 20496895, 42485063
PubChem CIDs: 439302, 92432, 9941379
Research use only; no administration amounts or health claims
Updated 2026-10-16