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GHK-Cu (copper(II) glycyl-L-histidyl-L-lysine) is one of the more structurally interesting naturally occurring peptide-metal complexes. Unlike many research peptides that are purely organic molecules, GHK-Cu is an organometallic complex — a tripeptide that chelates a divalent copper ion as an integral part of its active structure. Understanding how this chelation works, and why the resulting geometry is chemically significant, is essential context for researchers working with this compound.

The Tripeptide Backbone: GHK

The peptide component of GHK-Cu consists of three amino acids in sequence: glycine (G), histidine (H), and lysine (K). This tripeptide — designated by the single-letter codes as GHK — was first isolated from human plasma by Loren Pickart in 1973 during research into age-related changes in plasma factors that influence liver cell function. Its identification as a copper-binding species came subsequently.

Copper(II) Coordination: The Chelation Chemistry

The defining chemical feature of GHK-Cu is the square planar coordination of a copper(II) (Cu2+) ion by four nitrogen-containing donor atoms from the GHK tripeptide. This type of coordination is typical of Cu2+ because the d9 electron configuration of copper(II) strongly favours square planar geometry via the Jahn-Teller effect, which distorts octahedral coordination toward a flattened, square planar arrangement.

The four donor atoms that coordinate the copper ion in GHK-Cu are:

This four-nitrogen square planar coordination is sometimes called the ATCUN motif (Amino Terminal Copper and Nickel binding motif), named after the structural pattern: a free alpha-amino group at the N-terminus, followed by a peptide bond nitrogen, followed by a histidine residue in the third position. The ATCUN motif is found in a number of endogenous copper-binding proteins and peptides, including serum albumin, which also binds copper through its N-terminal ATCUN sequence.

The two deprotonated amide nitrogens (N2 and N4) are particularly noteworthy from a chemical perspective. Under normal conditions, peptide bond amide protons (NH) are not ionised at physiological pH — they remain as NH groups. However, coordination to Cu2+ dramatically stabilises the deprotonated (amide anion) form by using the lone pair of the nitrogen to donate into the copper d-orbitals. The result is that binding to copper pulls the proton off the amide nitrogen, creating a highly stable chelate. This is called macrochelation or deprotonated amide coordination.

The Resulting Coordination Geometry

The four nitrogen donors (N1, N2, N3, N4) arrange themselves in a planar configuration around the Cu2+ centre, with bond distances in the range of 1.9–2.1 Å, typical for Cu2+-N bonds. The square planar geometry is nearly ideal, with donor atoms positioned at roughly 90° angles from each other relative to the copper centre.

This geometry places the lysine residue of the tripeptide extending away from the copper coordination plane — the lysine side chain (with its terminal epsilon-amino group) does not participate directly in copper coordination. This is significant: the positively charged lysine epsilon-amino group remains available for ionic interactions with negatively charged surfaces, such as the phospholipid head groups of cell membranes or the glycosaminoglycan chains of extracellular matrix components. This structural feature is proposed as one mechanism by which GHK-Cu anchors to tissue surfaces.

The Carboxylate: A Fifth Potential Ligand

In addition to the four nitrogen donors, the carboxylate group of the lysine C-terminus (-COO⁻) is positioned in proximity to the copper centre and is proposed to occupy an axial coordination position in the elongated octahedral geometry that many square planar Cu2+ complexes adopt in aqueous solution — where solvent molecules or additional ligands weakly occupy the axial positions above and below the square plane.

The participation of the C-terminal carboxylate as an axial ligand is pH-dependent and has been characterised by EPR (electron paramagnetic resonance) spectroscopy and potentiometric pH titration studies, which are the standard analytical methods for characterising coordination chemistry of metal-peptide complexes in solution.

Copper Oxidation State and Redox Activity

GHK-Cu as typically characterised contains Cu2+ (cupric copper). However, copper can also exist as Cu+ (cuprous). The interconversion between Cu2+ and Cu+ is redox-active — copper can accept and donate electrons. This redox activity is a double-edged property: copper enzymes (superoxide dismutase, lysyl oxidase, ceruloplasmin) exploit copper’s redox chemistry for catalysis, but free copper ions can also catalyse Fenton-type reactions that generate hydroxyl radicals and cause oxidative damage.

GHK-Cu, by chelating copper in a stable coordination complex, reduces the availability of free ionic copper for Fenton chemistry while delivering copper in a form that may be bioavailable for copper-dependent enzymes — particularly lysyl oxidase, which requires copper for its catalytic activity in collagen and elastin cross-linking. This is one proposed mechanism by which GHK-Cu influences connective tissue biology: as a copper delivery vehicle to lysyl oxidase and other copper-dependent enzymes at tissue remodelling sites.

Stability of the Complex

The stability constant of the GHK-Cu complex (log K approximately 16–17 in aqueous solution at physiological pH) is higher than that of many other peptide-copper complexes but lower than that of stronger chelating agents like EDTA. This intermediate affinity means that GHK-Cu is stable enough to circulate and resist premature copper release, but not so tightly bound that copper cannot be transferred to copper-acceptor proteins — a property that supports the copper chaperoning hypothesis.

In practical terms, reconstituted GHK-Cu solutions are stable at +4°C for up to 28 days when prepared with bacteriostatic water. The characteristic pale blue colour of the solution confirms the Cu2+ coordination state is intact. Loss of colour, precipitation, or change to a different hue would indicate complex degradation.

ℹ️ This article describes GHK-Cu chemistry for scientific research reference. GHK-Cu is a research compound supplied for laboratory and in-vitro research use only.

GHK-Cu is available in research-grade, lyophilized form from the Aminopept research catalogue. Supplied for laboratory and in-vitro research use only.

→ View GHK-Cu in the research catalogue