Oxytocin Peptide: Research Guide (CYIQNCPLG-NH₂)
By UK Peptide Lab Research Team•19 September 2026•8 min read
Key Takeaways
- Oxytocin is the cyclic nonapeptide CYIQNCPLG-NH₂ with a Cys1-Cys6 disulphide bridge, MW 1007.19 g/mol, CAS 50-56-6. Its structure was proposed and first synthesised by du Vigneaud's group in 1953.
- Oxytocin acts at OXTR, a Gq-coupled class A GPCR; the human receptor structure was solved in 2020 (Waltenspühl et al., Sci Adv).
- The pair bonding literature is built on prairie vole pharmacology (Insel and Hulihan, Behav Neurosci, 1995), while the famous human trust finding (Kosfeld et al., Nature, 2005) has not replicated robustly (Nave et al., Perspect Psychol Sci, 2015).
- Oxytocin is a licensed prescription medicine in injectable formulations; the lyophilised material here is a research reagent only, supplied for in-vitro laboratory use with no dosing information and no human-use claims.
What is Oxytocin?
Oxytocin is a cyclic nonapeptide with the sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂, written CYIQNCPLG-NH₂ in single-letter code. It has the molecular formula C₄₃H₆₆N₁₂O₁₂S₂, a molecular weight of 1007.19 g/mol and the CAS number 50-56-6. The chemistry that defines it is the disulphide bridge between the cysteine residues at positions 1 and 6, which closes the peptide into a twenty-atom ring, plus a C-terminal primary amide. Both features are shared across the vasotocin-oxytocin family of neuropeptides and separate oxytocin chemically from the linear peptides elsewhere in the catalogue. UK Peptide Lab supplies Oxytocin 5mg as lyophilised powder for in-vitro laboratory research only.
Structure and the Disulphide Bridge
Because the ring is closed by a covalent disulphide rather than a salt bridge, oxytocin is conformationally constrained compared with a linear nonapeptide, and the oxidation state of the two cysteine residues is load-bearing: reduction or scrambling of the bridge inactivates the molecule and changes its chromatographic behaviour. The bridge is also the main chemical vulnerability at the bench, since oxidation and disulphide exchange generate the dimer and related species over time in solution. That is why handling guidance centres on minimising air exposure and freeze-thaw cycling, and why intact peptide should dominate the RP-HPLC trace of properly stored material.
The 1950s: Sequence, Synthesis and a Nobel Prize
Oxytocin's history is unusually well documented. Its nine-residue sequence, and the proposal that the two cysteines are disulphide-bridged, came in 1953 from Vincent du Vigneaud's group (du Vigneaud, Ressler and Trippett, J Biol Chem, 1953), and the first synthesis of the peptide was reported later the same year (du Vigneaud et al., J Am Chem Soc, 1953). That made oxytocin the first polypeptide hormone ever synthesised, and the work contributed to du Vigneaud's 1955 Nobel Prize in Chemistry. The practical consequence for modern research is that the structure has been chemically confirmed for seven decades, which is why oxytocin is such a common reference standard and positive control in peptide and receptor work.
Receptor Pharmacology
Oxytocin acts at a single high-affinity receptor, OXTR, a class A G-protein-coupled receptor coupled through Gq/11 to phospholipase C and calcium mobilisation. Gimpl and Fahrenholz (Physiol Rev, 2001) remains the standard catalogue of OXTR expression and regulation, and Jurek and Neumann (Physiol Rev, 2018) updated the field with a focus on how intracellular signalling maps onto behaviour. In 2020 the crystal structure of the human receptor was solved in complex with a ligand (Waltenspühl et al., Sci Adv), giving the interaction an atomic description. Peripheral receptor biology, uterine smooth muscle contraction and mammary myoepithelial cell activation, underpins the licensed medical uses of the hormone, while the same receptor system is expressed across a broad set of brain regions implicated in social behaviour.
Pair Bonding in the Prairie Vole
The canonical behavioural literature rests on the prairie vole, a species that forms lifelong pair bonds. Insel and Hulihan (Behav Neurosci, 1995) reported that oxytocin administered to female prairie voles facilitated partner preference formation, while males relied on vasopressin, establishing the receptor-specific model of pair bonding that Insel and Young (Nat Rev Neurosci, 2001) later generalised into the neurobiology of attachment. Donaldson and Young (Science, 2008) reviewed how variation in the oxytocin and vasopressin receptor systems correlates with sociality across species. These are animal pharmacology findings, and the leap from vole pair bonding to human attachment remains an inference, not a measurement.
Human Social Cognition and the Trust Study
The human literature became famous through a single experiment: Kosfeld and colleagues (Nature, 2005) reported that oxytocin administration increased trust in a monetary game. That finding seeded a decade of social cognition research with mixed results. The sober summary is Nave, Camerer and McCullough (Perspect Psychol Sci, 2015), who reviewed the cumulative evidence and concluded that human trust is not reliably associated with, or caused by, oxytocin across studies. The episode is instructive for experimental design: effect sizes in human social neuroendocrinology are small, heterogeneous and context-sensitive, and the replication record must be weighed before any strong claim is built.
Stress Response Research
A separate strand examines oxytocin within the stress response, where animal work shows OXTR engagement modulating hypothalamic-pituitary-adrenal axis activity in region-specific and sex-specific ways. Grinevich and Neumann (Mol Psychiatry, 2021) provide the clearest modern map of how oxytocinergic circuits identified in rodents relate, and sometimes fail to relate, to psychiatric hypotheses, and their review is the honest entry point to this literature. The same paper is explicit that translating animal circuit findings to human psychiatry has repeatedly overshot what the evidence supports, a caveat that applies broadly across oxytocin research.
Reading the Evidence Honestly
Three points deserve stating plainly. First, oxytocin is a licensed human pharmaceutical; the lyophilised material supplied for research is not that pharmaceutical, carries no marketing authorisation as supplied, and must not be used for human or veterinary administration. Second, the strongest literature is the receptor and animal social behaviour work, while the human social cognition findings are contested and small in effect. Third, this guide deliberately contains no dosing information and no therapeutic claims, because the research framing is the only one the evidence base supports. Oxytocin is best treated as a well-characterised chemical tool with an unusually rich literature, not as a molecule with a settled behavioural story.
Laboratory Handling
Store the lyophilised vial at -20°C, protected from light and moisture. Reconstitute by injecting Bacteriostatic Water slowly down the inner glass wall and swirling gently until the solution is clear and colourless; never shake or vortex. The disulphide bridge is the oxidation-sensitive site, so minimise headspace, re-seal promptly and avoid repeated freeze-thaw cycles by preparing single-use aliquots. Store the reconstituted solution at 2-8°C and use within 4 weeks. See the peptide storage guide for general lyophilised-compound handling principles. Supplied for in-vitro laboratory research use only.
Sourcing in the UK
UK Peptide Lab supplies research-grade Oxytocin 5mg as lyophilised powder, supplier-tested, with the certificate of analysis published on the product page and independent third-party verification pending. Same-day UK dispatch on orders placed before 2pm GMT, free Royal Mail Tracked shipping over £45. For in-vitro laboratory research use only, not for human or veterinary administration, and purchasers must be 18 or over.
Related Products
Disclaimer: This article is for research and educational purposes only. All information provided is not intended as medical advice. UK Peptide Lab products are not for human consumption and are sold strictly for laboratory research use only.
Frequently Asked Questions
What is oxytocin?
Oxytocin is a cyclic nonapeptide with the sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂ (CYIQNCPLG-NH₂), a disulphide bridge between the cysteines at positions 1 and 6, a molecular weight of 1007.19 g/mol and CAS number 50-56-6. It is one of the most heavily studied neuropeptides in existence and is supplied here as lyophilised powder for in-vitro laboratory research only.
Is oxytocin a licensed medicine?
Yes. Injectable oxytocin formulations are licensed human pharmaceuticals and prescription-only medicines in the UK. That status applies to the pharmaceutical products, not to this material: UK Peptide Lab supplies lyophilised oxytocin strictly as an analytical reference material for in-vitro laboratory research, and it is not supplied for, and must not be used for, human or veterinary administration. No therapeutic use is suggested or implied.
Does oxytocin make people more trusting?
The human evidence is contested. Kosfeld and colleagues (Nature, 2005) reported that oxytocin administration increased trust in a monetary game, but the finding has not replicated robustly, and Nave, Camerer and McCullough (Perspect Psychol Sci, 2015) concluded that the cumulative evidence does not show trust to be reliably caused by oxytocin. The animal pair bonding literature is considerably stronger, and the two should not be conflated.
How do I reconstitute oxytocin for laboratory use?
Allow the vial to reach room temperature, inject bacteriostatic water slowly down the inner glass wall, and swirl gently until the solution is clear and colourless. Never shake or vortex, agitation oxidises the Cys1-Cys6 disulphide bridge. Store the reconstituted solution at 2-8°C and use within 4 weeks, keeping aliquots single-use. This material is a research reagent for in-vitro laboratory use only.
References
- [1] The sequence of amino acids in oxytocin, with a proposal for the structure of oxytocin. J Biol Chem (1953). View →
- [2] The synthesis of an octapeptide amide with the hormonal activity of oxytocin. J Am Chem Soc (1953). View →
- [3] The oxytocin receptor system: structure, function, and regulation. Physiol Rev (2001). View →
- [4] A gender-specific mechanism for pair bonding: oxytocin and partner preference formation in monogamous voles. Behav Neurosci (1995). View →
- [5] The neurobiology of attachment. Nat Rev Neurosci (2001). View →
- [6] Oxytocin, vasopressin, and the neurogenetics of sociality. Science (2008). View →
- [7] Oxytocin increases trust in humans. Nature (2005). View →
- [8] Does oxytocin increase trust in humans? A critical review of research. Perspect Psychol Sci (2015). View →
- [9] The oxytocin receptor: from intracellular signaling to behavior. Physiol Rev (2018). View →
- [10] Crystal structure of the human oxytocin receptor. Sci Adv (2020). View →
- [11] Brain oxytocin: how puzzle stones from animal studies translate into psychiatry. Mol Psychiatry (2021). View →