Ipamorelin: sequence, formula and the two molecular weights

Published August 28, 2026 · Artemis Labs

Ipamorelin is a five-residue synthetic peptide with the sequence Aib-His-D-2-Nal-D-Phe-Lys-NH₂, the molecular formula C38H49N9O5, and a molecular weight of 711.9. Its CAS number is 170851-70-4 and its PubChem record is CID 9831659. Two details in that line trip people up constantly. It is a pentapeptide, not a hexapeptide, because Aib counts as one residue even though it is not one of the twenty standard amino acids. And catalogue listings sometimes carry a mass several hundred units higher than 711.9, because those figures describe salt forms rather than the peptide itself.

Key findings

• Verified identity fields, retrieved from PubChem on August 28, 2026: CID 9831659, formula C38H49N9O5, molecular weight 711.9, InChIKey NEHWBYHLYZGBNO-BVEPWEIPSA-N, CAS 170851-70-4, UNII Y9M3S784Z6.
• The founding paper states the sequence directly: “Ipamorelin is a pentapeptide (Aib-His-D-2-Nal-D-Phe-Lys-NH2)” (PMID 9849822).
• Its development code, still a PubChem synonym, is NNC 26-0161 — the compound came out of Novo Nordisk.
• Three of the five residues are unusual: one non-standard residue (Aib) and two D-amino acids (D-2-Nal, D-Phe). That is deliberate chemistry, and it is why the molecule survives in the body long enough to act.

Reading the sequence, one residue at a time

Written out, ipamorelin is Aib-His-D-2-Nal-D-Phe-Lys-NH₂. Each part carries information.

Aib is α-aminoisobutyric acid, also called 2-methylalanine. It is an amino acid that the body does not use to build proteins. Chemists put it at the front of peptides because enzymes that chew through normal protein sequences do not recognize it well, so the chain lasts longer.

His is histidine, a standard amino acid.

D-2-Nal is a naphthylalanine in the D configuration — a bulky, flat, two-ring side chain. D-Phe is phenylalanine, also in the D configuration. Amino acids in living things are almost all L-form; the D forms are mirror images. Enzymes are shaped for the L version and mostly cannot process the D version, so this is the same durability trick applied twice more.

Lys is lysine, and the -NH₂ at the end means the tail is capped as an amide rather than left as a free acid. That cap is part of the molecule’s identity, and it is why the correct full name ends in “-lysinamide.”

So four of the five positions carry a modification whose purpose is stability. This is a molecule built to survive, not a fragment of a natural hormone.

Why “hexapeptide” is wrong

Ipamorelin is regularly described as a hexapeptide — six residues. It has five. The error is easy to make for two reasons: the older compounds it is compared against, GHRP-2 and GHRP-6, genuinely are hexapeptides, and Aib does not look like an amino acid to someone scanning a sequence for familiar three-letter codes.

The primary sources are unambiguous. PubChem’s own name for the compound is “2-Methylalanyl-L-histidyl-3-(2-naphthyl)-D-alanyl-D-phenylalanyl-L-lysinamide” — five residues named in order. The 1998 paper calls it a pentapeptide in its abstract. When a specification sheet says hexapeptide, that sheet was not built from the primary record.

Why two different molecular weights circulate

PubChem lists 711.9 for the compound with formula C38H49N9O5. That is the free peptide — what chemists call the free base.

Peptides with a basic residue like lysine are usually supplied as salts, most often acetate, because the salt is more stable and easier to handle. A salt has extra molecules attached, so it weighs more per unit. That is why a catalogue can honestly list a number several hundred units above 711.9 for the same peptide: it is quoting the salt, not the peptide.

The practical consequence is worth stating plainly. A weight given for a peptide is only meaningful when the form is named alongside it. A number with no form attached cannot be checked against anything, and two documents quoting different numbers for the same vial may both be right.

We do not publish a specific salt mass here, because we could not verify one against a primary source. The free base figure, 711.9, is the one that resolves at PubChem, and that is the one we cite.

What the research does not show

Identity data describes what the molecule is. It says nothing about what the molecule does, and nothing on this page should be read as evidence of an effect. What ipamorelin has and has not been shown to do in people is covered on our human trials page, where the record is two studies and one null result.

A published sequence also does not tell you what is in any particular vial. That is a separate question with its own published answer — forensic laboratories have found ipamorelin analogues carrying an extra residue in seized material, which we cover on our page on product quality findings.

Finally, the figures on this page describe the peptide’s chemistry, not its behavior in a body. The half-life reported in humans is about two hours, from a 1999 study — a fact about disposition, not about structure, and one that comes from that study rather than from the formula.

Frequently asked questions

What is the molecular weight of ipamorelin?

711.9 for the free base, formula C38H49N9O5, per PubChem CID 9831659. Salt forms weigh more, and a stated weight should always name the form.

What is the CAS number for ipamorelin?

170851-70-4. Its UNII, the FDA’s unique ingredient identifier, is Y9M3S784Z6.

What does NNC 26-0161 mean?

It is ipamorelin’s original development code at Novo Nordisk, where the compound was made. It still appears as a synonym on the PubChem record and in several of the 1998 and 1999 papers.

Are D-amino acids unusual in a research peptide?

Not in this family. Two of ipamorelin’s five residues are D-form, and a 2014 method paper describes the whole class as “synthetic met-enkephalin analogues that include unnatural D-amino acids” (PMID 24574167). The purpose is resistance to breakdown.

References

1. PubChem Compound Summary, CID 9831659 (Ipamorelin). National Library of Medicine. Retrieved August 28, 2026. pubchem.ncbi.nlm.nih.gov/compound/9831659.
2. Raun K, et al. Eur J Endocrinol. 1998. PMID 9849822.
3. Timms M, Hall N, Levina V, Vine J, Steel R. Drug Test Anal. 2014. PMID 24574167.
4. Gobburu JV, Agersø H, Jusko WJ, Ynddal L. Pharm Res. 1999. PMID 10496658.

Methodology: every identity field on this page was retrieved directly from PubChem’s chemical record via its programmatic interface on August 28, 2026, and cross-checked against the sequence as published in the 1998 primary paper.

All compounds sold by Artemis Labs are for laboratory research use only. Nothing on this page is medical advice, and no statement has been evaluated by the FDA.