Ipamorelin is a pentapeptide, which makes it sound simple. It is not. Three of its five residues are things no organism builds, two of them are in the mirror-image configuration, and the chain ends in an amide rather than an acid. Every one of those choices is deliberate, and together they explain both why the molecule behaves the way it does and why confirming a batch takes more than a mass measurement.

Identity on paper

  • CAS number: 170851-70-4
  • Molecular formula: C38H49N9O5
  • Molecular weight: approximately 712 g/mol
  • PubChem CID: 9831659

Five residues, three of them unusual

The sequence is Aib-His-D-2-Nal-D-Phe-Lys-NH2. Reading that from left to right: an α-aminoisobutyric acid residue, a histidine, a D-configured 2-naphthylalanine, a D-phenylalanine, and a lysine whose C-terminus is amidated. (Reference: Raun et al., 1998)

Only histidine and lysine are among the twenty standard amino acids. Aib carries two methyl groups on the α-carbon instead of a side chain and a hydrogen, which stiffens the backbone. 2-naphthylalanine is a bulkier aromatic than anything in the natural set. And the two D residues are mirror images of the forms biology uses, which is a common way to make a short peptide resistant to enzymes that only recognise the L configuration. Our note on amino acids, peptides and proteins covers why the standard twenty are not a limit in synthetic chemistry.

The amide at the end

The C-terminal amide is not a detail. Whether a chain finishes as a free carboxylic acid or as an amide is decided by the linker used to attach the first residue to the resin during solid-phase synthesis, and the two forms differ in mass by about one dalton. They are different compounds, and a specification that names one is making a claim an identity measurement can check.

Length is only one variable

Aib is the case in point. Because it carries two methyl groups where most residues carry one hydrogen, the reactive nitrogen is crowded, and hindered residues of this type couple more slowly and less completely than ordinary ones. A step that does not go to completion leaves a fraction of chains missing that residue, and in a five-residue sequence a single deletion is proportionally a large change, not a subtle one.

The unusual residues are also more expensive than the standard set, and each one has to be supplied already protected in a form compatible with the rest of the synthesis. A short sequence built from four ordinary amino acids and one difficult one is not four-fifths of an ordinary problem.

The receptor was found before its natural ligand

The pharmacology here has an unusual history. Synthetic compounds that released growth hormone were known before anyone knew what the body's own signal was. The receptor they acted on was identified in 1996 and named for the compounds that found it, the growth hormone secretagogue receptor. (Reference: Howard et al., 1996) It sat as an orphan until 1999, when its endogenous ligand, ghrelin, was identified. (Reference: Kojima et al., 1999)

That is why the same receptor goes by two names in the literature, and why ipamorelin is described both as a growth hormone secretagogue and as a ghrelin receptor agonist. Both are correct. For the broader class, see our note on growth hormone secretagogues.

The meaning of selective

Ipamorelin was characterised as the first selective compound of its class, meaning that in the models used it acted on its target pathway with limited activity at other hormone axes that earlier secretagogues also touched. (Reference: Raun et al., 1998) For a research tool that property is the whole value: the cleaner the profile, the more confidently an observation can be attributed to one receptor rather than several. The general distinction between agonists and antagonists is covered separately.

These are preclinical, model-based observations. Ipamorelin is supplied strictly as a research compound, and this article does not describe use in people or animals.

Why a mass measurement is not enough here

This is where the unusual residues come back. A D-amino acid and its L counterpart have the same formula and the same mass. Nothing in a standard mass spectrum separates them, so a batch in which a D residue was accidentally supplied as the L form would return the expected mass and the expected formula while being a different molecule.

They differ in how they behave on a column. Diastereomeric impurities, chains that differ only in the configuration of one residue, generally elute at a slightly different time, so they appear in the chromatographic run as a peak close to the main one. It is a good illustration of why purity and identity are two separate questions and why a certificate that answers only one is incomplete.

Handling and verification

Ipamorelin ships lyophilized, which is the stable form. Keep it dry and cool, and let a cold vial reach room temperature before opening so condensation does not form inside; our guide to storing research peptides covers the general case. Each batch can be checked against its paperwork by batch number before the vial is opened.

One point specific to small peptides: at the masses typically supplied, the freeze-dried material in the vial can be a thin film or a few flecks on the glass instead of a visible powder. A vial that looks empty has usually not lost anything; that is how the drying works.

Research use only

All products sold by Codex Research, including ipamorelin, are strictly for laboratory research and development. They are not for human or animal consumption and are not intended to diagnose, treat, cure, or prevent any disease.

The compound at Codex Research

Ipamorelin, 10 mg per vial · batch-verified, with its certificate of analysis.

Frequently asked questions

What is ipamorelin, chemically?

A synthetic pentapeptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH2, formula C38H49N9O5 and a molecular weight of about 712 g/mol. Its CAS number is 170851-70-4.

Why does ipamorelin contain amino acids that are not among the standard twenty?

Aib stiffens the backbone by carrying two methyl groups on the alpha carbon, and 2-naphthylalanine provides a bulkier aromatic side chain than anything in the natural set. Synthetic chemistry is not restricted to the residues biology uses.

What do the D residues do?

They are mirror images of the forms biology uses. Enzymes that recognise only the L configuration do not act on them, which is a common way to make a short peptide more resistant to degradation.

Is the ghrelin receptor the same as the growth hormone secretagogue receptor?

Yes. The receptor was identified in 1996 through the synthetic compounds that acted on it and named after them. Its natural ligand, ghrelin, was identified in 1999, which is why the same receptor carries both names in the literature.

What does selective mean for ipamorelin?

It was characterised as acting on its target pathway with limited activity at other hormone axes that earlier secretagogues also affected. For a research tool that matters because it narrows the number of receptors an observation could be attributed to.

Can mass spectrometry confirm the D residues in ipamorelin?

No. A D residue and its L counterpart have identical formulas and identical masses. A chain built with the wrong configuration would return the expected mass. What distinguishes them is retention time, since such diastereomers generally separate on a chromatographic column.

References

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