Description
PrymaLab · Research Use Only
Oxytocin 2mg
Two residues away from a different hormone entirely
Oxytocin peptide has a near twin. Vasopressin is also a nine residue hormone, also carries a disulfide ring and a C-terminal amide, and differs from it at exactly two positions out of nine. Almost every difficulty this compound presents in the laboratory traces back to that fact.
Specification Table
| Property | Value |
|---|---|
| Compound | Oxytocin |
| CAS number | 50-56-6 |
| Sequence | Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 |
| Residue count | Nine |
| Molecular formula | C43H66N12O12S2 |
| Molecular weight | 1007.19 |
| Ring | A disulfide between Cys1 and Cys6, enclosing six residues |
| Tail | A three residue tail carrying a C-terminal glycinamide |
| Nearest relative | Arginine vasopressin, also a nonapeptide |
| Difference from vasopressin | Two positions. Isoleucine for phenylalanine at 3, leucine for arginine at 8 |
| Mass separation from vasopressin | Roughly 77 daltons, which is resolvable by mass spectrometry |
| Charge consequence | The vasopressin arginine at position 8 is absent here, so the two differ in net charge |
| Historical note | First chemically synthesised in 1953, the first peptide hormone to be made this way |
| Receptor | The oxytocin receptor, a class A G protein-coupled receptor |
| Vial | 2mg lyophilized |
| Storage | Sealed at minus 20°C. Protect from moisture |
How Close Is Oxytocin Peptide to Vasopressin?
Both are nonapeptides. Both carry a disulfide between the first and sixth residues, enclosing a six residue ring with a three residue tail hanging off it. Both are amidated at the C terminus.
Seven of the nine positions are identical.
Position 3 is isoleucine in oxytocin and phenylalanine in vasopressin. Position 8 is leucine in oxytocin and arginine in vasopressin.
That is the whole difference, and it is enough to direct each peptide predominantly to its own receptor.
The position 8 change is the more consequential of the two, because it removes a positive charge. An arginine side chain is basic and a leucine side chain is not, so the two molecules differ in net charge as well as in shape.
The pair are not unrelated accidents. They arise from an ancient gene duplication and the nonapeptide family they belong to runs across the vertebrates with the same architecture and small substitutions at those same positions.
Understanding the relationship as a family rather than as a coincidence explains why the analytical problems below are structural rather than fixable.
Why Does a Two Residue Difference Cause Problems?
The first oxytocin peptide problem is at the receptor. Oxytocin and vasopressin receptors are closely related class A G protein-coupled receptors, and the ligands cross over.
Oxytocin has measurable affinity at vasopressin receptor subtypes, and vasopressin has measurable affinity at the oxytocin receptor. The selectivity is real but it is a matter of degree rather than of kind.
The consequence is that a concentration high enough to saturate the intended receptor is often high enough to engage the others, and an effect attributed to oxytocin signalling may not be oxytocin receptor signalling.
Manning and colleagues reviewed the analogue chemistry developed specifically to address this, and the existence of that literature is itself the point. A great deal of medicinal chemistry has gone into separating two molecules that differ at two positions.
The second oxytocin peptide problem is analytical. Antibodies raised against one of these peptides commonly recognise the other, because seven of nine residues are shared and the epitope is small.
Any immunoassay applied to a sample containing both should be treated as reporting a combined figure unless the cross-reactivity has been characterised for that specific antibody.
The third problem is chromatographic. The two are similar in size and polarity, and a gradient that has not been developed to separate them may not.
Mass spectrometry resolves the pair cleanly, since roughly 77 daltons separates them, and that is the check worth having when identity actually matters.
Why Does the Oxytocin Peptide End in Glycinamide?
The molecule ends in glycinamide rather than in a free carboxyl group, which is a post-translational modification in the natural hormone and a synthetic choice in a manufactured one.
Amidation removes a negative charge from the C terminus and it is required for activity in this family.
The free acid form, where the terminal amide has been lost, is a different molecule with substantially reduced receptor engagement.
That matters because amide loss is a real degradation route rather than a theoretical one. Hydrolysis of a C-terminal amide is slow at neutral pH and accelerates under acidic or basic conditions and in the warm.
The mass change is one dalton, from the amide to the acid.
One dalton on a 1007 dalton molecule is a tenth of a percent, which is comfortably within the resolution of a modern instrument and comfortably outside what a nominal resolution method will notice.
A chromatographic method may separate the two, since the free acid carries an extra negative charge, but whether it does depends on the gradient and it should be demonstrated rather than assumed.
The practical implication is that a certificate confirming an intact amide is worth having, and that a solution held warm or at an extreme pH is losing something the assay may not report.
There is a second reason the amidation is worth attention on a research article specifically.
A synthetic peptide is amidated by the choice of resin it was assembled on, rather than by an enzyme after the fact.
That is a manufacturing decision taken at the start of a synthesis, and a preparation built on the wrong support gives the free acid throughout rather than as a degradation product.
The two situations look identical on a mass spectrum and are completely different in origin.
A certificate stating the intact amide distinguishes them, and a certificate that omits it leaves a researcher unable to tell a degraded lot from one that was never right.
What Did the 1953 Synthesis Establish?
Oxytocin peptide holds a specific place in the history of the field. It was the first peptide hormone whose structure was determined and then confirmed by total chemical synthesis.
du Vigneaud and colleagues published that synthesis in 1953, and the work earned the Nobel Prize in Chemistry two years later.
The significance is methodological rather than sentimental.
Before this, the structure of a natural hormone was inferred from degradation studies, and the inference was hard to close. Building the proposed structure from scratch and showing that the synthetic material matched the natural one in every assay closed it.
That argument, proof of structure by synthesis, became a standard of the field, and it is the reason a modern certificate can present a synthetic peptide as identical to a natural sequence rather than as an approximation of it.
It also explains why oxytocin analogues are so numerous. The synthetic route was established early, which made systematic substitution possible decades before it was routine for other hormones.
For a researcher today the practical inheritance is a very deep literature. Work on this molecule spans seventy years, which is an advantage for context and a hazard for interpretation, since assay methods and purity standards from the earlier decades are not those of the present one.
One further consequence of that long history is worth naming, because it affects how the older literature should be read.
Purity standards have moved a long way since the 1950s and 1960s.
Figures reported in that era were generated by methods that would not now be considered capable of resolving the related nonapeptides, and a preparation described then as pure could contain material a modern column would separate.
That is not a reason to discount the early work, which established most of what is known about the receptor system.
It is a reason to read quantitative figures from it as approximate, and to prefer a modern replication where a number is going to carry weight in an argument.
What Should an Oxytocin Peptide Certificate Show?
The sequence written out, with the disulfide positions and the C-terminal amide both stated explicitly.
Observed mass against the calculated 1007.19, which is the check that distinguishes it from vasopressin at roughly 77 daltons away.
Confirmation of the intact amide rather than the free acid, which is a one dalton distinction and needs a method capable of seeing it.
Confirmation that the disulfide is formed rather than reduced. A reduced peptide has two free thiols, a two dalton mass difference and different behaviour.
Purity by chromatography with the gradient stated, and ideally with a note on whether the method separates the related nonapeptides.
Net peptide content and salt form, since the vial mass includes counterion and residual water.
Absence of dimer, which two free thiols will form on standing.
Lot number and manufacturing date.
On a 2 milligram vial the net content question is proportionally the same as on a larger one, but the absolute quantity is small enough that weighing errors and residual moisture matter more than usual.
How Should the Vial Be Handled?
Oxytocin peptide is a small disulfide bridged sequence with a terminal amide, and it carries two chemical liabilities that are both manageable.
Sealed lyophilized material holds at minus 20 degrees Celsius, which is where the amide and the disulfide are both at their most stable.
Let the vial reach ambient temperature before it is opened. A 2 milligram fill is a very small quantity of powder, and condensation onto cold glass does proportionally more damage than it would on a larger fill.
Reconstitute gently, adding diluent down the wall and allowing it to stand rather than swirling hard.
Keep prepared solutions close to neutral pH. Amide hydrolysis is slow at neutral and accelerates at both extremes, so an unbuffered dilution that drifts is losing material by a route nothing will report.
Avoid prolonged warmth. Time at temperature is the variable that drives both amide loss and disulfide exchange.
Aliquot on first reconstitution and use low-binding consumables. The molecule is small, and at working dilutions the fraction lost to a tube wall is not negligible.
Record lot, net peptide content, diluent, buffer and pH, storage temperature and date.
Buffer and pH belong on that list here rather than being assumed, because on this molecule the storage pH is a chemistry variable rather than a convenience.
Published Literature
Selected references on the original synthesis, on the receptor system, on the analogue chemistry developed to separate the nonapeptides and on peptide storage generally.
- du Vigneaud V, Ressler C, Swan JM, Roberts CW, Katsoyannis PG, Gordon S. Journal of the American Chemical Society. 1953;75(19):4879-4880. DOI: 10.1021/ja01115a553
- Gimpl G, Fahrenholz F. Physiological Reviews. 2001;81(2):629-683. DOI: 10.1152/physrev.2001.81.2.629
- Manning M, Misicka A, Olma A, Bankowski K, Stoev S, Chini B, et al. Journal of Neuroendocrinology. 2012;24(4):609-628. DOI: 10.1111/j.1365-2826.2012.02303.x
- Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Pharmaceutical Research. 2010;27(4):544-575. DOI: 10.1007/s11095-009-0045-6
Frequently Asked Questions
What is oxytocin?
Oxytocin peptide is a nine residue hormone with a disulfide ring between residues one and six and a C-terminal glycinamide. It carries CAS 50-56-6 and calculates to 1007.19 daltons.
How does it differ from vasopressin?
At two of nine positions. Isoleucine replaces phenylalanine at position 3, and leucine replaces arginine at position 8. The other seven residues are identical.
Why does that closeness matter?
Because it produces receptor cross-talk, antibody cross-reactivity and chromatographic overlap. All three are structural consequences of a shared architecture rather than avoidable errors.
Can mass spectrometry tell them apart?
Yes. Roughly 77 daltons separates the two, which any modern instrument resolves cleanly. It is the check worth running when identity genuinely matters.
Are immunoassays reliable for this pair?
Not without characterisation. Seven of nine residues are shared and the epitope is small, so antibodies raised against one frequently recognise the other.
What does the C-terminal amide do?
It removes a negative charge from the terminus and is required for activity in this family. The free acid form engages the receptor far less well.
How is amide loss detected?
By a one dalton mass shift, from the amide to the free acid. That is a tenth of a percent on this molecule, so it needs a high resolution method rather than a nominal one.
What accelerates amide loss?
Acidic or basic conditions, and warmth. Keeping prepared solutions near neutral pH and cold is the practical control.
Why is the 1953 synthesis significant?
It was the first total chemical synthesis of a peptide hormone, and it confirmed the proposed structure by building it. The work received the Nobel Prize in Chemistry in 1955.
What did that establish for the field?
Proof of structure by synthesis, which is why a synthetic peptide can be treated as identical to a natural sequence rather than as an approximation of it.
What should a certificate confirm?
The sequence, the formed disulfide, the intact amide, observed mass against 1007.19, purity with the method stated, and net peptide content with the salt form.
Does the small vial size change anything?
The proportions are the same but the absolute quantity is small, so weighing error and residual moisture have more influence on a 2 milligram fill than on a larger one.
Compliance Statement
Oxytocin is sold exclusively for laboratory research use. It is not a drug, food, or cosmetic product, and it is not a dietary product of any kind. It is not approved by the FDA or any comparable authority for human or veterinary use, it is closely related to vasopressin and cross-reactivity at receptors and in immunoassays means published findings cannot always be attributed to a single nonapeptide, its literature spans seventy years and the assay conventions of the earlier decades differ from current ones, the C-terminal amide is required for activity and its loss is not visible to a nominal resolution method, and no compound in this range is offered for any human or veterinary purpose. This product is not intended to diagnose, treat, cure, or prevent any disease. It must not be given to humans or animals. Purchase is restricted to qualified researchers and institutions operating within applicable laws. All handling is the responsibility of the purchasing laboratory.
Other formats of Oxytocin
Oxytocin is also stocked as Oxytocin 2mg Nasal Spray and Oxytocin 10mg preloaded 3ml pen. Each listing states its own quantity and concentration, and the pen and vial comparison explains what changes between formats.
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