--- title: "Which catalogue peptides are cyclic, and how are their rings formed? | Sable Peptides" description: "A cross-catalogue look at cyclic research peptides: disulfide-bridged (oxytocin, AOD 9604, cagrilintide) and lactam-bridged (Melanotan II, PT-141). Ring types, sizes, PubChem data and why cyclisation matters for identity." canonical: https://www.sablepeptides.co.uk/guides/cyclic-peptides-in-the-catalogue-disulfide-and-lactam-rings dateModified: 2026-10-16 publisher: Sable Peptides Limited (UK company 17488610) note: Research use only. Not for human or veterinary use. --- # 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](https://pubchem.ncbi.nlm.nih.gov/compound/439302); [PubChem CID 92432: Melanotan II](https://pubchem.ncbi.nlm.nih.gov/compound/92432); [PubChem CID 9941379: PT-141](https://pubchem.ncbi.nlm.nih.gov/compound/9941379) ## 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)](https://pubmed.ncbi.nlm.nih.gov/20077561/); [Diverse ring architectures of cyclic peptidomimetics targeting melanocortin receptors, RSC Med Chem 2026 (PubMed 42037742)](https://pubmed.ncbi.nlm.nih.gov/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)](https://pubmed.ncbi.nlm.nih.gov/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)](https://pubmed.ncbi.nlm.nih.gov/20496895/); [Comprehensive characterization of bremelanotide acetate and its degradants by LC-HRMS/MS and predicting epimerization through computational modelling, Anal Methods 2026 (PubMed 42485063)](https://pubmed.ncbi.nlm.nih.gov/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. ## Related - [Oxytocin structure](/guides/oxytocin-structure-disulfide-ring-and-research-background) - [α-MSH analogues](/guides/melanocortin-peptides-alpha-msh-analogues) - [AOD 9604](/guides/aod-9604-and-the-hgh-176-191-fragment) - [PT-141](/products/pt-141) - [Cheapest Melanotan II UK](/cheapest-melanotan-2-uk) 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