GHK-Cu and AHK-Cu compared

Two tripeptides, one methyl group apart

GHK-Cu and AHK-Cu are the two copper-binding tripeptides most often encountered in the research-peptide catalogue, and they are close enough in name and structure to be frequently confused. GHK is glycyl-L-histidyl-L-lysine; AHK is L-alanyl-L-histidyl-L-lysine. The only difference is the first residue, where alanine carries a methyl group that glycine lacks. Both are sold and studied as their copper(II) complexes, and both are blue.

Much of what circulates online treats them as interchangeable or attributes the findings of one to the other. They are not, and the literature for one is far larger than for the other. The full account of the better-studied compound is in the GHK-Cu monograph; the product pages are at GHK-Cu and AHK-Cu. Both are supplied by BuyUK Peptides as research chemicals for in-vitro laboratory use only, and nothing here is guidance on applying either compound to a person.

The copper chemistry

GHK was isolated from human plasma by Pickart and Thaler in 1973 [1], and in 1980 Pickart and colleagues proposed that its activity depended on binding copper and carrying it into cells [2]. Lau and Sarkar worked out the coordination chemistry in 1981: at physiological pH a single copper(II) ion is held by three nitrogen donors, the α-amino nitrogen of glycine, the deprotonated amide nitrogen of the glycine–histidine peptide bond and the imidazole nitrogen of histidine, with the lysine side chain outside the primary coordination sphere. The 1:1 complex is highly stable, with a binding constant comparable to that of the copper site on serum albumin [3].

AHK has all three donors in the same positions; alanine simply adds a methyl group on the α-carbon of the first residue. The same tridentate binding mode is therefore expected for AHK-Cu, and its blue colour, which arises from copper(II) coordinated by nitrogen, is consistent with it. We have not located a dedicated coordination study of AHK-Cu equivalent to Lau and Sarkar's work, so the binding constant, geometry and behaviour in competition with albumin are inferred from the parent compound rather than measured. That is the first substantive difference: for GHK-Cu the copper chemistry is established; for AHK-Cu it is assumed by analogy. A certificate for one should not be mistaken for a certificate for the other on the basis of colour alone.

Identifiers

For GHK-Cu the identifiers are well documented in the monograph. The free tripeptide is C14H24N6O4 (340.4 g/mol, CAS 49557-75-7); the copper complex is CAS 89030-95-5, commonly written as C14H24N6O4Cu, about 403.9 g/mol, with the INCI name Copper Tripeptide-1.

For AHK-Cu the position is less tidy. The free tripeptide, Ala-His-Lys-OH, has the molecular formula C15H26N6O4 and a formula weight of 354.4 g/mol, one CH2 unit more than GHK [4]. That is the only AHK identifier we have been able to verify against a supplier that publishes a formal name, formula and sequence. CAS registrations and formulae quoted for the copper complex vary between vendors and sometimes correspond to salt forms or chloride adducts, so we do not reproduce one here. A certificate for AHK-Cu should state the formula it reports against, and the mass spectrum should show the protonated free peptide at about 355. A mass of about 341 on a certificate labelled AHK-Cu means the laboratory has analysed GHK.

What the literature actually studied

GHK-Cu

GHK-Cu has five decades of literature, and although much of it carries Pickart's name and most of it is in vitro or in animals, it is a substantial record. The copper chemistry is established [2, 3]. Maquart and colleagues reported in 1988 that GHK-Cu stimulated collagen synthesis in cultured human skin fibroblasts at nanomolar concentrations [5], and Siméon and colleagues later reported increased matrix metalloproteinase-2 expression in the same system [6]. The reviews collect animal wound-healing studies in several species, small cosmetic studies in people, and gene-expression comparisons [7, 8]. The human evidence is small and mostly cosmetic, and there are no controlled clinical trials.

AHK-Cu

AHK-Cu's peer-reviewed literature is very thin by comparison, and it concerns one question: hair follicles.

The study that matters is Pyo and colleagues, in Archives of Pharmacal Research in 2007. In human hair follicles in organ culture and in cultured dermal papilla cells, a tripeptide-copper complex at 10⁻¹² to 10⁻⁹ M stimulated follicle elongation ex vivo and dermal papilla cell proliferation in vitro. At 10⁻⁹ M it reduced the number of apoptotic dermal papilla cells, though not to statistical significance, and raised the Bcl-2/Bax ratio while lowering cleaved caspase-3 and PARP [9].

The complex in that study was AHK-Cu, L-alanyl-L-histidyl-L-lysine-Cu²⁺, not GHK-Cu [9]. This matters because the paper is the single most-cited primary source for the claim that copper peptides promote hair growth, and it is routinely attributed to GHK-Cu by secondary sources, product listings and marketing copy. Under the GHK-Cu name, the hair data are largely animal work and patents rather than peer-reviewed ex vivo or human studies [7]. The hair-follicle evidence belongs to AHK-Cu; the collagen, matrix and wound evidence belongs to GHK-Cu. Neither record transfers to the other.

Beyond that, the verified AHK literature consists of a 2021 synthetic-chemistry paper on dimeric peptide derivatives in human follicle dermal papilla cells [4]. We have not found a fibroblast collagen study, an animal wound study, a gene-expression study or a human study for AHK-Cu. Claims about AHK-Cu and skin, wound healing or collagen are extrapolations from GHK-Cu, and should be labelled as such.

Cosmetic use and research use

GHK-Cu has been used in creams and serums since the 1990s under the INCI name Copper Tripeptide-1, regulated as an ingredient of a finished cosmetic product, and AHK-Cu appears in some hair-care formulations on the same footing. In both cases the regulatory assessment attaches to the finished cosmetic and the responsible person who places it on the market, not to the raw ingredient, and it does not transfer to a vial of lyophilised powder sold as a research chemical.

The material supplied here is a research chemical for in-vitro laboratory use under the Terms of Sale. It is not a cosmetic or a medicine, is not manufactured under a cosmetic safety assessment or a marketing authorisation, carries no claim about what it does on or in a person, and is not for human or veterinary administration, topical or otherwise. Our guide to the UK rules on research compounds explains why cosmetic-ingredient status belongs to the product, not the molecule.

Most GHK-Cu skin studies in people were cosmetic studies of formulated products, several published only as conference abstracts or book chapters [7, 8]; they say something about a cream and less about the compound at a defined concentration in a defined system.

Certificate considerations for copper complexes

A certificate for a copper peptide has to answer questions that do not arise for an ordinary peptide, and they apply to GHK-Cu and AHK-Cu alike. Our guide to reading a certificate of analysis covers the general items; the copper-specific ones are these.

Peptide-to-copper ratio. Both complexes are 1:1, and a certificate should report copper content, by ICP-OES, ICP-MS or atomic absorption, as a weight percentage or a molar ratio. For GHK-Cu written as C14H24N6O4Cu the theoretical copper content is about 15.7% by mass; for AHK-Cu, with one more CH2 unit, it is close to 15.2% on the same representation. A salt or hydrate will be lower. Copper well below theory means a significant fraction of free peptide; well above it means free copper salt, which is cytotoxic in culture. Because activity for both compounds is reported at nanomolar and picomolar concentrations [5, 9], often with a bell-shaped dose–response [5, 7], the copper figure is not a formality.

Colour. Copper(II) coordinated by peptide nitrogens absorbs in the visible range, so both complexes are blue to blue-violet and give blue solutions; the free tripeptides are white. Blue confirms the material is the complex rather than the free peptide; it does not distinguish GHK-Cu from AHK-Cu and says nothing about purity or stoichiometry. A solution that fades towards colourless, or throws a pale precipitate, has lost copper to a competing chelator, which is why EDTA, citrate and high phosphate are avoided in buffers for either compound.

Mass of the complex against mass of the free peptide. Reversed-phase HPLC is usually run at low pH with trifluoroacetic acid, which partly dissociates the complex on the column, so the purity figure measures the peptide rather than the intact complex. Electrospray mass spectrometry typically shows the free peptide, at about 341 for protonated GHK and about 355 for protonated AHK, and may also show the copper-bound species; copper's two stable isotopes, ⁶³Cu and ⁶⁵Cu at roughly 69:31, give any copper-bound peak a characteristic doublet two mass units apart. A certificate quoting only the free-peptide mass has confirmed the peptide; one that also shows the copper adduct has confirmed the complex. Either is acceptable, provided the certificate says which species it reports, against which formula, and reports copper content separately.

Water content. Both complexes are hygroscopic, so water by Karl Fischer titration or loss on drying matters more than for most peptides, and net content is what should be used when calculating molarity.

Choosing between them

The choice is set by the question. For fibroblast matrix metabolism, wound models, copper delivery or gene expression, GHK-Cu is the compound with the literature, the reference coordination chemistry and the reproducible in-vitro readouts [3, 5, 6, 7, 8]. For hair-follicle organ culture or dermal papilla cells, AHK-Cu is the compound that was actually tested [9]. Running both in the same assay, with a copper salt alone as a control, is the only way to find out whether the methyl group matters for the readout in hand.

References

1. Pickart L, Thaler MM. Tripeptide in human serum which prolongs survival of normal liver cells and stimulates growth in neoplastic liver. Nature New Biology. 1973;243(124):85–87. PMID 4349963.
2. Pickart L, Freedman JH, Loker WJ, et al. Growth-modulating plasma tripeptide may function by facilitating copper uptake into cells. Nature. 1980;288(5792):715–717. doi:10.1038/288715a0. PMID 7453802.
3. Lau SJ, Sarkar B. The interaction of copper(II) and glycyl-L-histidyl-L-lysine, a growth-modulating tripeptide from plasma. Biochemical Journal. 1981;199(3):649–656. doi:10.1042/bj1990649. PMID 7340824.
4. Cayman Chemical. AHK (trifluoroacetate salt), L-alanyl-L-histidyl-L-lysine; item 45938. Product datasheet listing formula C15H26N6O4 and formula weight 354.4, with references to Pyo et al. 2007 and Jeong J, Park K-Y, Kim J. Synthesis and biological evaluation of dimeric peptide derivatives as proliferation-stimulating agents in human follicle dermal papilla cells. Bulletin of the Korean Chemical Society. 2021;42(11):1435–1439. caymanchem.com/product/45938.
5. Maquart FX, Pickart L, Laurent M, Gillery P, Monboisse JC, Borel JP. Stimulation of collagen synthesis in fibroblast cultures by the tripeptide-copper complex glycyl-L-histidyl-L-lysine-Cu²⁺. FEBS Letters. 1988;238(2):343–346. doi:10.1016/0014-5793(88)80509-X. PMID 3169264.
6. Siméon A, Emonard H, Hornebeck W, Maquart FX. The tripeptide-copper complex glycyl-L-histidyl-L-lysine-Cu²⁺ stimulates matrix metalloproteinase-2 expression by fibroblast cultures. Life Sciences. 2000;67(18):2257–2265.
7. Pickart L, Vasquez-Soltero JM, Margolina A. GHK peptide as a natural modulator of multiple cellular pathways in skin regeneration. BioMed Research International. 2015;2015:648108. doi:10.1155/2015/648108. PMID 26236730.
8. Pickart L, Margolina A. Regenerative and protective actions of the GHK-Cu peptide in the light of the new gene data. International Journal of Molecular Sciences. 2018;19(7):1987. doi:10.3390/ijms19071987. PMID 29986520.
9. Pyo HK, Yoo HG, Won CH, et al. The effect of tripeptide-copper complex on human hair growth in vitro. Archives of Pharmacal Research. 2007;30(7):834–839. doi:10.1007/BF02978833.

This monograph summarises the published literature for laboratory researchers. It is not medical advice, and nothing on this site is for human or veterinary use.