Melanocortin & Copper Peptides: UK Research Peptides

Structural features

The two cyclic heptapeptides in this area share one ring. Written out, the core sequence is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]: a lactam (amide) bond between the side-chain carboxyl of aspartic acid and the side-chain amine of lysine closes a ring of six residues, and N-acetyl-norleucine sits outside the ring at the N-terminus. Norleucine is a non-proteinogenic isomer of leucine with an unbranched four-carbon side chain, and D-phenylalanine is the ring's one D-configured residue.

The two differ only at the C-terminus. One ends in a lysine amide (C50H69N15O9, 1024.18 g/mol) and the other in a free carboxylic acid (C50H68N14O10, 1025.18 g/mol). Exchanging NH2 for OH changes the formula by one nitrogen, one hydrogen and one oxygen, and the mass by about 1 Da, which is small enough that the pair has to be told apart deliberately rather than assumed.

GHK-Cu belongs to a different family. It is the copper(II) complex of the tripeptide glycyl-L-histidyl-L-lysine; the free tripeptide has the formula C14H24N6O4, and the complex listed here, C14H22CuN6O4, holds one copper ion per peptide. That formula has two hydrogen atoms fewer than the peptide and copper combined, consistent with a neutral complex of Cu(II) with the doubly deprotonated peptide. At three residues, GHK is the shortest peptide chain in the catalogue. In complexes of this kind the copper is typically bound by the N-terminal amine, a backbone amide nitrogen and a nitrogen of the histidine imidazole ring.

Identifiers at a glance

The table shows the two heptapeptides together, at 1024.18 and 1025.18 g/mol, and GHK-Cu at 401.91 g/mol for the complex. Residue counts cover the amino acids only, so the acetyl cap, the C-terminal group and the copper ion appear in the formula and the molecular weight but not in the count. The price column divides each vial's price by its content in milligrams, which places vials of different masses on one scale and makes the per-milligram figure the simplest way to read the area at a glance.

Structure notes within this area

Residue-level detail sits on each product page. Within this area, read the structure notes on MT2 and PT-141.

How this research area is defined

Two families share this area because both are short peptides whose identity depends on a feature beyond the linear sequence: a lactam ring in one case and a bound copper ion in the other. Neither feature can be read from a plain list of residues, so both families are written with extra notation, the cyclo[…] bracket for the ring and · Cu(II) for the metal. That shared need for a second check, cyclisation for one family and copper content for the other, is what defines the class.

The cyclic heptapeptides form the most closely related pair of compounds in the catalogue. They share the ring, the N-terminal acetyl-norleucine and the D-phenylalanine, and they differ only in the C-terminal group, an amide in one and a free acid in the other. GHK-Cu, at three residues, is the shortest chain in the store, and its formula, C14H22CuN6O4, is the only one in this area to include a metal. Its copper is held by nitrogen atoms of the glycine amine, the peptide backbone and the histidine imidazole.

Both families are compact enough for their full structures to be written on one line, which is how each product page gives them: the ring with its bracket and C-terminal group for the heptapeptides, and the tripeptide with its copper ion for GHK-Cu. The table on this page sets their identifiers next to one another for quick reference.

Analytical considerations

For the cyclic heptapeptides, forming the lactam bridge eliminates one molecule of water, so a cyclised molecule is 18 Da lighter than its open-chain precursor. That difference is a convenient mass-spectrometric check that cyclisation is complete. Tryptophan in the ring absorbs at 280 nm, giving a second ultraviolet window alongside the peptide-bond region around 214 nm, and the acid and amide forms are separated by HPLC retention as well as by their 1 Da difference in mass. The arginine in the ring stays positively charged across the usual chromatographic pH range, and an acidic ion-pairing agent such as trifluoroacetic acid in the mobile phase keeps the peaks sharp.

Metal complexes need analysis of both parts. Reversed-phase separations with acidified mobile phases can strip copper from the peptide, so a chromatogram may show the free tripeptide rather than the complex, and the copper content is usually established separately, for example by elemental analysis or atomic spectroscopy. Mass spectra of copper complexes carry a characteristic isotope pattern from the two stable isotopes, copper-63 and copper-65, in a ratio of roughly seven to three, which confirms that the metal is present in the ion being measured.

Further reading

• What Is GHK-Cu? Chemistry of the GHK Copper Peptide
• Peptide Sequence Notation: Reading an Amino Acid Sequence
• Mass Spectrometry and Peptide Identity
• How to Read a Peptide Certificate of Analysis
• Peptide chemistry glossary