Copper Peptides Compared: GHK, GHK-Cu and AHK-Cu

GHK, GHK-Cu and AHK-Cu are three closely related but chemically distinct materials: a free tripeptide, its copper(II) coordination complex, and the copper complex of a single-residue variant. The distinction is not cosmetic. The free peptide and the metal complex differ in molecular weight, in colour, in how they behave on a chromatography column, and in what a certificate figure actually refers to. This article sets out those differences in third person, describing what the published literature and the chemical databases record.

For research use only — not for human or veterinary use. Nothing below is a claim about effects in people.

GHK: the free tripeptide

GHK is glycyl-L-histidyl-L-lysine, a three-residue peptide. PubChem records molecular formula C14H24N6O4 and an average molecular weight of 340.38 g/mol (CID 73587).

Its research history is unusually well documented. A 2015 review in BioMed Research International by Pickart, Vasquez-Soltero and Margolina records that the peptide was isolated in 1973 by Pickart as an activity associated with human albumin, and notes that GHK is present in human plasma, saliva and urine, with reported plasma concentrations declining with age. The sequence has since been studied extensively in cell-culture and animal models, and reviews describe it as having been investigated for effects on gene expression across a broad set of cellular pathways — work that remains preclinical.

As a research material the free peptide is an off-white lyophilised powder that dissolves to a colourless solution. TXLABS lists it as GHK (basic).

GHK-Cu: the copper(II) complex

GHK-Cu is not GHK with copper added as an ingredient; it is a defined coordination compound in which a copper(II) ion is held by the peptide. PubChem catalogues it under the name prezatide copper (also copper tripeptide-1), formula C14H23CuN6O4⁺, average molecular weight 402.92 g/mol (CID 71587328) — roughly 62 g/mol heavier than the free peptide, which is the copper.

How the copper is held

The coordination chemistry has been resolved crystallographically. Work published in Acta Crystallographica Section D in 2020 describes the copper(II) ion in a square-pyramidal coordination sphere, with the equatorial plane occupied by three nitrogen atoms: the glycine amino terminus, the amide nitrogen of the glycine–histidine peptide bond, and the histidine imidazole side chain. The remaining positions are occupied by labile ligands — water molecules or donor atoms from neighbouring molecules in the crystal.

The affinity is very high. The same work reports dissociation constants measured by isothermal titration calorimetry in the region of 10⁻¹⁴ M for GHK and a related peptide, and notes that binding is sensitive both to the peptide's protonation state and to the presence of competing chelators in the buffer.

Why the solution is blue

Copper(II) is a d⁹ transition-metal ion. In a ligand field, its partially filled d orbitals split in energy, and electronic transitions between those levels absorb light in the visible region. What the eye registers is the transmitted remainder — for GHK-Cu in aqueous solution, a distinct blue.

This matters practically because it is a frequent source of confusion. The colour is the bound copper, not an impurity, not degradation and not a contaminant. A GHK-Cu solution that is not blue is the anomalous case, not the reverse. The free tripeptide, having no metal centre, dissolves colourless; the complex dissolves blue. Colour is therefore a crude but real indicator of which material is in the vial, though it is no substitute for an analytical report. TXLABS lists the complex as GHK-Cu.

AHK and AHK-Cu

AHK is L-alanyl-L-histidyl-L-lysine — the same tripeptide architecture with alanine in place of glycine at the N-terminus. PubChem records C15H26N6O4, average molecular weight 354.41 g/mol (CID 7408502). The structural difference from GHK is a single methyl group on the N-terminal residue.

AHK-Cu is the corresponding copper complex; PubChem catalogues a copper AHK hydrochloride entry at CID 168431292, formula C15H24ClCuN6O4⁻, average molecular weight 451.39 g/mol. Because the histidine imidazole and the N-terminal amine that anchor copper in GHK are both retained, the coordination motif is closely analogous.

The honest statement about AHK-Cu is that the indexed literature on it is very much thinner than for GHK-Cu — a smaller number of studies, less mechanistic characterisation, and no comparable body of structural work. It is listed as AHK-Cu, and researchers comparing the two should not assume that findings reported for one transfer to the other.

Handling and analytical differences

Working with a metal complex is not the same as working with a free peptide, and several differences have practical consequences.

• Competing chelators change the picture. The crystallographic work notes explicitly that copper binding is influenced by other potential chelators present in the buffer. EDTA, citrate and some phosphate buffers compete for copper, so buffer selection is a genuine experimental variable rather than a background detail.
• pH affects coordination. Histidine protonation is pH-dependent, and the imidazole nitrogen is one of the three equatorial donors. Complex formation is correspondingly pH-sensitive, which is worth controlling deliberately rather than inheriting from whatever diluent is to hand.
• Chromatography behaves differently. A charged metal complex and a free peptide do not retain identically on a reversed-phase column, and the acidic ion-pairing conditions typical of peptide HPLC can affect the complex. Some laboratories therefore quantify the peptide portion rather than the intact complex.
• Label weight refers to the complex. When a vial is labelled 50 mg of GHK-Cu, that nominal mass is the complex, not the free peptide, and the two differ by roughly 18% on molecular weight alone. Comparing a GHK figure with a GHK-Cu figure without accounting for that is an arithmetic error waiting to happen.
• Copper redox and light. Ordinary peptide handling discipline applies, with the usual attention to light, moisture and freeze–thaw cycles; the metal centre is an additional reason not to leave solutions sitting at ambient temperature.
• Identity confirmation carries extra weight here. Because the free peptide and the complex share the same tripeptide backbone and differ chiefly by a coordinated metal ion, a report that establishes purity without establishing identity does not distinguish between them. Mass spectrometry does, and the analyte name on the certificate should state which of the two was measured.

The published TXLABS report for GHK-Cu illustrates the reporting convention: batch CS-gu50-0309, tested 18 March 2026, purity 99.780% with an assay of 60.04 mg against a 50 mg label weight. Purity and assay are separate figures answering separate questions, and both appear in the Certificate of Analysis library.

Summary

GHK is the free tripeptide at 340.38 g/mol; GHK-Cu is its square-pyramidal copper(II) complex at 402.92 g/mol, blue in solution because of the metal centre; AHK differs from GHK by one methyl group, and AHK-Cu is its copper complex with a much smaller supporting literature. Copper-containing and related research-grade materials are listed in the aesthetic and skin category.

For research use only. Products supplied by TXLABS are laboratory research chemicals intended for in-vitro and laboratory research by qualified professionals. They are not medicines, supplements, foods or cosmetics, and are not for human or veterinary use.