GHK and AHK are tripeptides that differ at one position: glycine in the first, alanine in the second. Both bind copper, and both are sold as the copper complex. A single methyl group is the whole chemical difference, which makes this a useful pair for seeing how much a small change can and cannot do.

The sequences

GHK is glycyl-histidyl-lysine. AHK is alanyl-histidyl-lysine. Alanine is glycine with a methyl group on the alpha carbon, so the two peptides are identical except for that one addition at the N-terminal residue.

Our molecular weight calculator returns C14H24N6O4 for GHK and C15H26N6O4 for AHK: one carbon and two hydrogens apart, about 14 units of mass. Both figures are for the free peptide, without copper.

Where the copper sits

This is the part the substitution does not change, and it is the reason the two behave as a pair instead of as unrelated compounds. In peptides of this shape the copper is held by an amino-terminal binding motif, in which the terminal amine, the following backbone nitrogen and the histidine imidazole converge on the metal. (Reference: Harford & Sarkar, 1997) The histidine at position two is doing the structural work, and it is present in both.

Spectroscopic work on the GHK copper complex established the geometry of that site. (Reference: Freedman et al., 1982) What the glycine-to-alanine swap adds is a methyl group on a carbon adjacent to the site, not a new donor atom. Our note on copper peptides covers the family.

Why "GHK-Cu" is not a formula

The name is a shorthand and the shorthand hides the question that matters on a certificate. GHK is the tripeptide. GHK-Cu is a complex of that tripeptide with copper, with its own formula, its own mass, its own PubChem record and its own CAS number, 89030-95-5.

The two are not interchangeable on paper. A certificate reporting the mass of the free tripeptide for a product sold as the copper complex is reporting the wrong molecule, and it will look entirely normal unless you know which of the two numbers you should be seeing. The same applies to AHK and AHK-Cu, CAS 682809-81-0. You can run either through the check digit validator and then look the number up to read back which substance it returns.

Copper is also an analytical problem

A bound metal changes what the usual methods see. In mass spectrometry copper has two stable isotopes in substantial proportions, so a copper complex gives an isotope pattern noticeably different from that of an organic molecule of the same nominal mass. That pattern is evidence in its own right: it is hard to fake and easy to read.

It also means a mass calculated from the three letters of the sequence will not match a measurement on the complex, and should not. A calculator that works from a letter sequence cannot see a bound metal, which is a limit of the method, not a disagreement about the compound.

What a certificate should show for either

Which species was analysed, stated explicitly: free peptide or copper complex. Purity from RP-HPLC with its conditions written out. Identity from a mass measurement, with observed and theoretical values both given for whichever species the document is about. Copper content, if the product is the complex, because that is the difference between the two products and it is measurable.

Net peptide content matters here too, and for an additional reason. These are short peptides, so the counterion and absorbed water are a larger share of the powder than they would be on a long one. A figure of 99 percent pure by area still says nothing about how much of the weighed mass is peptide. Our note on what else is in the vial covers it.

Short peptides are easier to make cleanly

Three residues means two coupling steps. Each step is an opportunity for an incomplete reaction, so a tripeptide starts from a much better position than a fifteen-mer, and high purity figures on compounds this short are unremarkable. Our note on synthesis explains where the impurities come from.

In practice, on a short peptide purity is the easy number and identity is the one worth reading. A document that gives you 99 percent and no mass has told you the easy half.

What the methyl group does not settle

A one-residue difference reads like a small difference in everything, and that inference does not hold in either direction. A single substitution next to a metal binding site can change how tightly the metal is held, how the complex behaves at a given pH, and how the molecule sits in a solvent, without changing anything you can see in a formula.

It can also change nothing of consequence. Which of the two is the case is a question for measurement, not for reasoning about structures, and it is the kind of question a catalogue page is in no position to answer. What a catalogue can state is the sequence, the species, the mass and the batch, and that is the boundary this page stays inside.

Reading the two certificates side by side

If you hold both documents, the fields that should differ are few and specific: the sequence, the formula and mass, the CAS number, and the copper content if both are complexes. Everything else, from the method conditions to the net content line to the date format, should look like the work of the same laboratory.

A pair of certificates that differ in more than that is worth a question. Two documents from the same supplier for two closely related compounds, produced on different templates with different fields, usually means they came from different places. No accusation is intended: it is an observation anyone can make with both pages open, and it is the sort of thing only a pair makes visible.

What this comparison is not

It is not a claim that one is better, and it says nothing about what either does in an organism. The comparison that belongs here is between what can be documented about each, which is the only honest comparison a seller can make.

Research use only

All products sold by Codex Research 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.

These compounds in the catalog

Frequently asked questions

What is the difference between GHK-Cu and AHK-Cu?

The first residue. GHK is glycyl-histidyl-lysine and AHK is alanyl-histidyl-lysine, and alanine is glycine with an added methyl group. Everything else, including the histidine that organises the copper site, is the same.

Do both bind copper the same way?

Both use an amino-terminal binding motif in which the terminal amine, the next backbone nitrogen and the histidine imidazole converge on the metal. The glycine-to-alanine change adds a methyl group next to that site, not a new donor atom.

Is GHK the same as GHK-Cu?

No. GHK is the free tripeptide; GHK-Cu is its complex with copper, with a different formula, mass, PubChem record and CAS number. A certificate reporting the free peptide mass for a product sold as the complex is describing the wrong species.

Why does copper change the mass spectrum?

Copper has two stable isotopes present in substantial proportions, so a copper complex produces an isotope pattern clearly different from an organic molecule of similar nominal mass. That pattern is useful evidence on its own.

Why can a sequence calculator not give the mass of GHK-Cu?

Because the copper is not part of the letter sequence. A calculator working from three letters returns the mass of the free tripeptide, and the complex necessarily differs.

Is a high purity figure impressive on a tripeptide?

Not particularly. Three residues means two coupling steps, so there are far fewer opportunities for incomplete reactions than on a long peptide. On short compounds purity is the easy number and identity is the one worth reading.

References

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