GHK‑Cu vs AHK‑Cu: the two copper tripeptides compared, atom by atom

Line the two sequences up and the difference is a single carbon.

• GHK = glycyl-L-histidyl-L-lysine. Glycine, the first residue, has a hydrogen as its side chain.
• AHK = L-alanyl-L-histidyl-L-lysine. Alanine has a methyl group (CH₃) where glycine has a hydrogen.

That methyl group adds 14 Da (CH₂) to the mass and introduces a chiral center at the first residue.

Everything else is identical: the histidine in the middle, whose imidazole ring is what binds copper, and the lysine at the end. Each tripeptide is then complexed with one copper(II) ion to give GHK‑Cu and AHK‑Cu.

GHK was isolated from plasma in 1973 by Loren Pickart, which is why it has fifty years of chemistry behind it.

AHK is a synthetic analogue that appeared in the cosmetic-ingredient literature in the 2000s. Both are made today by solid-phase synthesis; neither is extracted from anything. Research use only. Not for human or veterinary use.

The copper-content line is worth a second look. In both complexes a single copper atom (63.5 g/mol) accounts for about one sixth of the mass.

A "100 mg GHK‑Cu" vial therefore holds about 16 mg of copper and 84 mg of peptide; a listing that quotes the copper-free tripeptide mass for a copper complex has miscounted by 16 %.

The copper(II) ion in GHK‑Cu is bound by three atoms of the peptide: the nitrogen of the N-terminal amine (on glycine), the deprotonated nitrogen of the first peptide bond, and a nitrogen of the histidine imidazole ring.

A fourth site on the copper is occupied by water or, at higher concentration, by a second GHK molecule.

The geometry is square-planar around the copper, which is the arrangement that gives copper(II) peptide complexes their blue-violet color.

AHK‑Cu binds copper through the same three nitrogens.

The methyl group of alanine sits on the carbon next to the N-terminal amine, outside the coordination plane, so it changes the immediate environment of the copper without changing which atoms bind it.

Two consequences are measurable:

• The color is the same. Both powders are blue-violet, both solutions are blue. Nothing on a bench distinguishes them by eye.
• The mass is different. On a mass spectrum the free tripeptide ions appear at 341.2 (GHK, M+H⁺) and 355.2 (AHK, M+H⁺); the copper dissociates under the usual ionization, so the certificate reports the peptide mass, and the 14 Da gap is the identity check.

Both complexes are stable as dry solids but sensitive to oxygen and light over long periods in solution, which is why the storage line of both catalog entries adds "oxygen" to the usual "light and moisture".

Because the two look identical, the certificate of analysis is the only document that tells a buyer which one is in the vial. Three lines do the work.

Retention time

On a reverse-phase HPLC column the extra methyl group makes AHK slightly more hydrophobic, so AHK‑Cu elutes a little later than GHK‑Cu under the same gradient.

On its own, that shift is too small to identify a compound; it becomes useful when the laboratory has run both standards, and it is the reason a supplier that lists both must show a separate chromatogram for each.

Mass confirmation

The decisive line. An observed mass consistent with 340.4 (or an M+H⁺ at 341.2) means GHK; 354.4 (M+H⁺ at 355.2) means AHK.

A certificate that shows the GHK mass on an AHK‑Cu vial has tested the wrong compound, or the vial holds the wrong compound.

Copper assay

The best certificates add a copper determination (by ICP or a colorimetric method) and report it against the theoretical 15 to 16 %.

A copper reading far below that means the vial holds the free tripeptide, or a complex made with too little copper, and the blue color will be weaker than expected.

Few laboratories include this line by default; ask for it on a large order.

The OXpeptides certificates are published per lot in the COA library.

The remaining fields (laboratory, verifiable test number, lot, date, photo of the labeled vial) are the same for every peptide and are read in how to read a peptide COA; what the HPLC percentage does and does not say is in HPLC purity: what 99 % means.

This is the largest practical difference between the two reagents, and it is a fact about documentation, not about the molecules.

GHK‑Cu has been characterized since 1973.

Its crystal structure, its copper-binding constants, its behavior in cultured-cell models and in gene-expression screens (the Broad Institute Connectivity Map, where it appears by name) are published, reviewed and re-reviewed.

Pickart and Margolina's 2018 review, listed below, is the standard entry point.

AHK‑Cu has a handful of primary papers, most from the cosmetic-ingredient side and most in cultured-cell models, of which Shin et al. (2019) is the one that studies it by name alongside GHK‑Cu.

There is no review dedicated to it. A laboratory that needs a reagent with a deep published baseline chooses GHK‑Cu; one that studies the analogue itself chooses AHK‑Cu, and usually orders both to run them side by side.

What a buyer takes from that: any listing that describes AHK‑Cu with GHK‑Cu's literature is borrowing. The two compounds are one methyl group apart in structure and a few hundred papers apart in documentation.

The catalog offers both as lyophilized powders in sealed 3 mL borosilicate vials with a bromobutyl septum and crimp cap:

GHK‑Cu comes in larger vials because it is produced at industrial scale and costs less per milligram to make; AHK‑Cu is a smaller-volume synthesis and starts at 50 mg.

The live prices, per vial and per milligram, are on the two product pages, and the buying-side questions (supplier checks, what the price includes) are in the GHK‑Cu buying guide and the AHK‑Cu buying guide.

"Copper peptide" on a marketplace can mean either compound, a mixture of the two, the copper-free tripeptide, or a cream containing a fraction of a percent of one of them. Four questions settle it:

1. Is the sequence written out? Gly-His-Lys or Ala-His-Lys. "Copper peptide" alone is a category, not a compound.
2. Is the mass on the certificate the right one? 340.4 for GHK, 354.4 for AHK, on a report from a named laboratory that can be looked up.
3. Is the powder blue-violet, and is the quantity stated in milligrams? A white powder is not a copper complex; a listing in fluid ounces is a cosmetic.
4. Does the page describe the reagent only? Composition, purity documentation, storage, "research use only". A page that describes anything else is a page that can be taken down with an order in transit.

The full seven-point method is in how to check a peptide supplier.