Oxytocin

1 Identity

Cyclic nonapeptide with a single intramolecular disulfide bridge. The active moiety of an FDA-approved United States drug product. At nine residues it is a drug and not a biological product. Trade names of the approved and formerly marketed products, named rather than avoided: Pitocin (US injection) and Syntocinon (intranasal spray, not marketed in the US). Not a synonym for arginine vasopressin.

Sequence

H-Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2. Cyclic nonapeptide with a single intramolecular disulfide bridge between Cys1 and Cys6 closing a twenty-membered ring, and a C-terminal glycinamide. All nine residues proteinogenic and L-configured; no acylation, no non-proteinogenic residue, no PEGylation, no metal. Supplied lyophilized, commonly as the acetate salt. Two structural features govern the entire specification: the disulfide, which can scramble or reduce without the molecule ceasing to weigh nearly the same; and the Asn5 and Gln4 amides, which deamidate to species exactly 0.984 Da heavier - the same increment as the C-terminal free acid, which is why three different related compounds sit at one mass. Tyr2 provides a genuine 280 nm chromophore, which this catalog's other short peptides mostly lack.

Molecular formula

C43H66N12O12S2 (free base, oxidized disulfide-bridged form). The fully reduced linear dithiol of the same sequence is C43H68N12O12S2.

Average mass

1007.193 Da for C43H66N12O12S2 on IUPAC 2021 abridged conventional atomic weights; 1007.187 is the same formula on the pre-2021 table. The approved Pitocin labeling states 1007.19 g/mol (DailyMed SetID 6e5a66fc-e507-497c-b5ce-44a8c95898ad, published 21 May 2026), and this figure reproduces it. The reduced linear form is 1009.209, exactly +2.016.

Monoisotopic mass

1006.4365 Da neutral monoisotopic for C43H66N12O12S2. [M+H]+ 1007.4437; [M+2H]2+ 504.2255; [M-H]- 1005.4292. The three deltas that carry the specification: reduction of the single disulfide is exactly +2.0157; deamidation at Asn5 or Gln4, and conversion of the C-terminal amide to the free acid, are each exactly +0.9841, giving one mass at 1007.4205 for three chemically different related compounds.

Salt / variant note

The nearest relatives are made by the same suppliers on the same lines, and a single peptide supplier's catalog alone lists 24 oxytocin-family entries, so the confusable set here is unusually strong. (1) arginine vasopressin, the other neurohypophysial nonapeptide, Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, C46H65N15O12S2, avg 1084.239 / mono 1083.4379, [M+H]+ 1084.4451 - 77.0 Da heavier, differing at only two of nine positions, and the single most consequential mislabel in the family. (2) Desmopressin, C46H64N14O12S2, avg 1069.224 / mono 1068.4270 - 62 Da heavier than oxytocin and 15 Da lighter than vasopressin. (3) Atosiban, C43H67N11O12S2, avg 994.194 / mono 993.4412 - 13 Da lighter and sharing the C43 skeleton; the closest mass in the family and the one a careless nominal read is most likely to accept. (4) Carbetocin, C45H69N11O12S, avg 988.172 / mono 987.4848 - 19 Da lighter and carrying one sulfur rather than two, because its ring is closed by a thioether rather than a disulfide. That is why it gives no Ellman free-thiol response on reduction and no -2.016 Da shift, and a lot showing neither is not a disulfide peptide at all. (5) Cataloged analogs arriving under near-identical names: (Ile8)-oxytocin / mesotocin, (Thr4,Gly7)-oxytocin / TGOT, (Pro8)-oxytocin. (6) The reduced linear species, C43H68N12O12S2, avg 1009.209 / mono 1008.4521, exactly +2.0157 - a specified impurity, not a product. (7) the isobaric TRIO, the most valuable line in this entry: the C-terminal free acid, the Asn5-deamidated species and the Gln4-deamidated species are all exactly +0.9841, all C43H65N11O13S2 at avg 1008.177 / mono 1007.4205 - three chemically different related compounds at one exact mass, resolved only chromatographically against authentic standards.

2 Class & testing panel

Form

Single article

Testing panel

P1 — panel definition

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

Published literature exists, is very large, is of high quality, and is almost entirely about something other than the article cataloged here. The evidence grade is B, and that grade reflects a split base: robust, long-established human evidence and FDA approval for narrow parenteral obstetric indications, against a behavioral and social literature that drives the peptide market and does not carry the same weight. Identifiers by study type. Regulatory: Pitocin (oxytocin) injection approved labeling, Par Health USA LLC, DailyMed SetID 6e5a66fc-e507-497c-b5ce-44a8c95898ad, published 21 May 2026 - approved indications exclusively obstetric, with the label stating explicitly that Pitocin is not indicated for elective induction of labor, and carrying a boxed warning and an adverse-reactions section. There is no FDA-approved oxytocin product for any social, behavioral, psychiatric, sexual or bonding indication and no approved intranasal product in the United States. Human pharmacokinetic study: Striepens N, Kendrick KM, Hanking V, Landgraf R, Wullner U, Maier W, Hurlemann R, Sci Rep 2013, PMID 24310737. Contested-interpretation paper by neuroendocrinologists who study the peptide: Leng G, Ludwig M, 'Intranasal Oxytocin: Myths and Delusions', Biol Psychiatry 2016, PMID 26049207. Both papers are real, both are cited, and the position is contested rather than settled. A 290-participant, 24-week, NIH-funded, NEJM-published randomized controlled trial exists with a primary social outcome measure; its reported between-group difference, confidence interval and p-value are carried verbatim in the citation index alongside the study design, and are not restated here. Beyond that: many thousands of human publications including randomized controlled trials in obstetric and behavioral-neuroscience settings, and a very large rodent and in-vitro literature in which the compound is a standard laboratory reagent with well-characterized assay behavior.

4 Storage & specification

Storage

White lyophilized powder, acetate salt, stored at -20 degrees C plus or minus 5 degrees C in the sealed original container with desiccant, protected from light, equilibrated to room temperature before opening, shipped frozen with a validated shipper and a temperature logger. The moisture control is load-bearing for this molecule specifically: thiol-disulfide exchange and asparagine deamidation are both water- and pH-mediated, and a wet cake changes measurably long before it looks different. The label states that the material is not to be reconstituted or stored in solution. The storage condition of the approved injection product is expressly not transferable here - that is a formulated, buffered aqueous solution with a different stability profile, and copying its statement onto a research powder would be wrong in both directions.

Shelf life

Provisional 24-month retest interval at -20 degrees C plus or minus 5 degrees C, desiccated and light-protected, with free thiol, dimer and the deamidated related compounds designated as the stability-indicating attributes rather than total assay. A lot of this molecule will fail on speciation long before it fails on assay, and dating it on assay would be dating it on the wrong number. The interval is provisional pending the company's own stability data: the confirming study holds three lots on real-time storage at -20 degrees C with pull points at 0, 3, 6, 12, 18, 24 and 36 months, runs accelerated arms at 25 degrees C / 60 percent RH and 40 degrees C / 75 percent RH, and reports free thiol at every pull.