GHK-Cu is a copper-binding tripeptide — glycine, histidine and lysine (Gly-His-Lys) complexed with a copper(II) ion. It has the molecular formula C₁₄H₂₂CuN₆O₄, a molecular weight of 401.9 g/mol, and CAS number 89030-95-5. The free tripeptide without copper (GHK, CAS 49557-75-7) is a distinct compound: C₁₄H₂₄N₆O₄, 340.38 g/mol. Copper is held by three nitrogen donors — the glycine α-amino nitrogen, the deprotonated Gly–His amide nitrogen and the histidine imidazole — while the lysine side chain stays free. The resulting d-d absorption near 600 nm gives the complex its characteristic colour. First identified in human plasma in 1973, GHK-Cu is classified as a metallopeptide. It is supplied as a blue to blue-purple lyophilized powder, soluble in water, and should be kept refrigerated at 36‑46 °F (2‑8 °C) and protected from light. This product is strictly for research purposes only.