GHK-Cu

GHK-Cu is a copper(II) coordination complex of the tripeptide glycyl-L-histidyl-L-lysine, catalogued here in its one-to-one ligand-to-metal form: composition C₁₄H₂₃CuN₆O₄, average molecular mass 402.92 Da, monoisotopic mass 402.1077 Da. The free tripeptide has the composition C₁₄H₂₄N₆O₄ at 340.38 Da average and 340.1859 Da monoisotopic, with both termini unblocked. The 62.54 Da increment between the two is diagnostic and is not simply the mass of copper: it is one copper atom at 63.546 Da less the single proton lost when the amide nitrogen deprotonates on coordination. A mass reported as 340 Da alongside a copper-containing formula is therefore internally inconsistent, and the pair is a routine transcription error in this family.

Coordination is supplied by the tripeptide itself. The N-terminal amine, the deprotonated amide nitrogen of the first peptide bond and the imidazole nitrogen of the histidine side chain form the donor set characteristic of this ligand class, with the lysine side chain remaining free. Copper(II) in this donor environment produces a broad d-d absorption band in the visible region near 600 to 640 nm, and it is this band that gives complexes of this ligand class their colour. The tripeptide contains no tryptophan, tyrosine or phenylalanine, so the peptide component has no 280 nm chromophore and the visible band belongs entirely to the metal centre.

Two further species are routinely conflated with this one. A two-to-one complex of the same ligand and metal is catalogued at PubChem CID 133697840 with composition C₂₈H₄₈CuN₁₂O₈ and an average mass of 744.31 Da; catalogue entries in this family are not consistent about which stoichiometry they describe, and the 341.39 Da separation between the two makes them unambiguous by mass spectrometry. The free ligand is separated from the complex by the 62.54 Da increment above. In a metal-peptide complex of this type, copper content is determined by inductively coupled plasma mass spectrometry or flame atomic absorption and compared against the value calculated for the stated stoichiometry, while the natural copper isotope pattern, 63Cu to 65Cu at approximately 69 to 31, provides a confirmatory signature in the mass spectrum that a copper-free impurity cannot reproduce. Neither database names a protein target for the complex or for its free ligand.

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