Metal Complex (Copper–Peptide Complex)

A metal-peptide complex is a peptide holding a metal ion through coordinate bonds, in which nitrogen, oxygen or sulfur atoms in the chain donate electron pairs to the metal, histidine imidazoles and cysteine thiols most often. No peptide bond forms and no residue is added, and the association is reversible. That is why this site lists a metal centre among a compound's modifications rather than writing it into the residue string.

The example that matters is GHK-Cu. The tripeptide glycyl-histidyl-lysine was isolated from human plasma in the early 1970s and binds copper(II) with an affinity close to albumin's own copper site, so the two can pass the ion between them. Free GHK weighs about 340 daltons and is a white powder; the one-to-one complex weighs about 402, the copper adding 63.5 and displacing two protons, and is distinctly blue-violet.

The complex is the compound, not the peptide. GHK and GHK-Cu differ in charge, chromatography and biology, and most of the cell-culture work on collagen synthesis used the copper form, on the argument that the peptide delivers copper to enzymes such as lysyl oxidase, which is why cosmetic classification calls it a carrier peptide. Whether that is the mechanism in living skin has not been settled by controlled trials, and the compound has no drug approval anywhere.

The stoichiometry is where listings go wrong. Because the metal is held by coordination, a product's copper content depends on how it was prepared, from a clean one-to-one complex to free peptide with a trace of copper salt, and a blue colour proves copper is present but not in what ratio. A certificate reporting peptide purity by HPLC says nothing about the copper; only a copper assay addresses the complex itself.