Oxytocin is the name of a specific nine-residue peptide amide with a single intramolecular disulfide. The purpose of this page is narrow: to set out the sequence, the formula and mass, the closest structural relative it can be confused with, and the analytical data that pin the identity of a sample to the name on the vial. Nothing here describes any action of the molecule, and there are no instructions on preparing, handling or dosing it.
The oxytocin sequence and ring structure
Oxytocin is a nonapeptide:
Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 (one-letter code CYIQNCPLG-NH2)
Three features define it. First, the cysteines at positions 1 and 6 are joined by a disulfide bridge. Counting the atoms around that closed loop — the backbone atoms of residues 1 to 6, the two cysteine side-chain carbons and the two sulfur atoms — gives a 20-membered ring, and the systematic name used by PubChem describes it as a dithia-pentaaza cycloicosane for exactly that reason. Second, the last three residues, Pro-Leu-Gly, hang off the ring as a short linear tail. Third, the C-terminus is not a free acid: glycine 9 is present as glycinamide, a carboxamide. The N-terminal amine of cysteine 1 is free.
PubChem (CID 439302) lists the molecular formula as C43H66N12O12S2, the average molecular weight as 1007.2 g/mol and the monoisotopic mass as about 1006.44 Da. Those numbers refer to the oxidized, cyclic free base. The difference between the average and monoisotopic figures matters when reading a spectrum, as our note on monoisotopic vs average mass explains.
Oxytocin versus vasopressin: two residues apart
Arginine vasopressin shares the same architecture as oxytocin: nine residues, a Cys1–Cys6 disulfide, a proline at position 7 and a C-terminal glycinamide. Its sequence is CYFQNCPRG-NH2. Set side by side, the two peptides differ at only two positions:
- Position 3: isoleucine in oxytocin, phenylalanine in vasopressin.
- Position 8: leucine in oxytocin, arginine in vasopressin.
Seven of nine residues are identical, the ring size is the same, and both compounds carry the same terminal groups. That similarity is why a name on a label is not sufficient evidence on its own. The two substitutions shift the formula to C46H65N15O12S2 for vasopressin, with an average mass near 1084.2 g/mol per PubChem, about 77 daltons above oxytocin. A mass reading therefore separates the two cleanly, and an arginine at position 8 also changes charge state distribution and retention. Any analysis offered as proof of oxytocin identity should include a mass value, not only a chromatographic purity figure.
Oxytocin on reverse-phase HPLC
With isoleucine, leucine, proline and tyrosine on one side and glutamine, asparagine and two amides on the other, oxytocin is moderately hydrophobic. It carries no arginine, lysine or histidine, so under acidic mobile phase its only strongly basic site is the N-terminal amine. Run on C18 silica against an acetonitrile gradient in water with trifluoroacetic acid, it gives a single sharp main peak at a relatively early-to-middle position in a standard peptide gradient.
The only aromatic residue is tyrosine 2, and the molecule has no tryptophan. Absorbance at 280 nm is therefore weak and not useful for judging purity. Purity is integrated at about 214 nm, where the amide bonds absorb and every peptide-related species shows up. Integration windows and baseline handling are covered in our HPLC chromatogram reading guide.
Oxytocin impurity classes and their mass shifts
Most impurities in a synthetic oxytocin sample fall into a handful of recognizable groups, and each one leaves a characteristic mass signature:
- Reduced (linear) form. If the Cys1–Cys6 bridge has not formed or has been cleaved, the two free thiols add two hydrogens: +2 Da, usually at a different retention time.
- Disulfide-linked dimers. Two chains can pair intermolecularly, in parallel or antiparallel orientation, giving species at roughly twice the monomer mass. Our article on disulfide formation and scrambling explains how they arise.
- Deamidation. Asn5 and Gln4 side chains, and the C-terminal glycinamide itself, can hydrolyze to the acid form. Each event adds +0.98 Da, a shift small enough to hide inside the isotope envelope on a low-resolution instrument; see our explainer on the 0.98-dalton deamidation shift.
- Deletion sequences. A missed coupling during synthesis removes one residue, so the mass drops by that residue’s increment, for example about 113 Da for a missing Ile or Leu.
- Trisulfide. An extra sulfur inserted into the bridge gives a species about +32 Da above the parent.
Mass spectrometry expectations for oxytocin
By electrospray, a peptide of about 1 kDa with one strongly basic site typically appears as the singly protonated ion [M+H]+ near m/z 1007.4 (monoisotopic), often with a doubly charged [M+2H]2+ near m/z 504.2 and sodium adducts 22 Da above the protonated ion. A certificate should state whether a reported value is monoisotopic or average and which ion it corresponds to. A deconvoluted value near 1085 instead of 1007 would point to vasopressin; a value 2 higher to the reduced chain. Charge states and adducts are discussed further in our piece on mass spectrometry and peptide identity.
Chromatographic purity and net peptide content are separate measurements. Oxytocin is commonly isolated as a salt, and the counter-ion adds weight to the powder without adding peptide, so a good report names the salt form alongside the purity value.
Confirming oxytocin identity independently
Look up the compound by sequence and formula rather than by name alone. PubChem indexes oxytocin under CID 439302 with the formula C43H66N12O12S2, and the vasopressin entry can be pulled up next to it for comparison. On the published analysis, check three points: that the stated sequence has isoleucine at position 3 and leucine at position 8, that the observed mass matches about 1007 (average) or 1006.4 (monoisotopic) for the cyclic form, and that the main peak is not accompanied by a sizable +2 Da or dimer signal.
How oxytocin is supplied here
Oxytocin 2mg ships as a lyophilized powder inside a sealed vial. An independent laboratory runs reverse-phase HPLC on it, and that result is posted on the product page together with the vial size. No batch or lot number is printed on the vial; the crimp and cap color is what links each one to its published test.
Frequently asked questions
What is the amino acid sequence of oxytocin?
Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 (CYIQNCPLG-NH2), with a disulfide between cysteines 1 and 6 and an amidated C-terminal glycine.
What molecular weight should an oxytocin analysis report?
About 1007.2 g/mol average for the cyclic free base, or roughly 1006.44 Da monoisotopic, per PubChem. Salt forms weigh more per milligram of powder.
How can oxytocin be told apart from vasopressin?
By mass and sequence. Vasopressin has phenylalanine at position 3 and arginine at position 8, and its average mass is roughly 77 daltons higher, near 1084.2 g/mol.
Why is oxytocin purity measured at 214 nm and not at 280 nm?
Its only aromatic chromophore is a single tyrosine, so 280 nm absorbance is weak. The peptide bond absorbs strongly near 214 nm, where all related impurities are also detected.
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