AHK-Cu differs from its better-known relative by one atom's worth of side chain. The tripeptide is alanyl-histidyl-lysine instead of glycyl-histidyl-lysine, which means a methyl group where the other has a hydrogen. That is the entire structural difference, and it sits directly beside the part of the molecule that holds the copper.
Two sets of numbers, as with any metal complex
- AHK, the free tripeptide: C15H26N6O4, about 354 g/mol, CAS 126828-32-8, PubChem CID 7408502.
- The copper complex: CAS 682809-81-0, listed in PubChem as C15H24ClCuN6O4- at about 451 g/mol (CID 168431292).
The database entry for the complex carries a chloride and a negative charge. That is one way of writing it, not the only one: the formula of a coordination complex depends on the protonation state, the overall charge and the counterion chosen. Our note on copper peptides covers why the name does not pin the formula down.
Where the copper sits, and why residue one matters
Short sequences with a histidine near the amino terminus bind copper by bringing several donor atoms around the metal at once: the terminal amine, the imidazole of the histidine, and a backbone nitrogen that has to lose its proton to participate. (Reference: Harford & Sarkar, 1997) The geometry of this arrangement was worked out for the glycine version in 1982. (Reference: Freedman et al., 1982)
The first residue contributes the terminal amine, which is one of those donors. Replacing glycine with alanine adds a methyl group on the carbon next to it. The substitution does not remove a donor or add one; it changes what sits beside the metal, and comparing the two complexes is how the contribution of that position gets measured.
What a methyl group can and cannot do
It is worth being precise about the scale of the change, because a single residue swap invites two opposite errors. One is to assume the two compounds are interchangeable, since they differ so little. The other is to assume the difference must be large, since chemists bothered to make both.
A methyl group changes three things a chemist can reason about: it takes up space next to the terminal amine, it is slightly greasier than a hydrogen, and it removes the unusual flexibility that glycine gives a backbone. Glycine, with no side chain at all, can adopt conformations no other residue can. Replacing it therefore constrains the local geometry as well as adding bulk.
Whether those changes matter in a given system is a measurement, not a deduction. The structural difference is small and precisely defined; the consequences are whatever an experiment reports.
Why comparing near-identical sequences is standard practice
Changing one residue at a time is the basic move of structure-activity work. If two compounds differ at a single position and behave differently, the difference is attributable. If they behave the same, that position is not doing the work.
A pair like AHK and GHK is useful for exactly this reason, and it is also a trap for a supplier's paperwork: two compounds this similar are easy to mislabel and impossible to distinguish by eye.
The lysine at the end
Both tripeptides finish with lysine, whose side chain carries an amine that stays protonated at ordinary pH. That charge is not part of the copper site, and it does most of the work of keeping these compounds water-soluble.
It also has an analytical consequence. A positively charged residue interacts with the acidic additive used in reversed-phase methods, so retention depends on the additive as well as on the sequence. Two laboratories running the same sample with different acid concentrations can see the peak in different places, which is one more reason the method conditions belong beside the purity figure.
Telling them apart
A methyl group is fourteen daltons. Both free tripeptides and both copper complexes differ by that amount, which is comfortably within the resolution of any mass spectrometer used for this work. The identity measurement distinguishes them without difficulty.
The copper adds a second, independent check. Copper has two stable isotopes, copper-63 and copper-65, present at roughly 69 and 31 percent, so any species carrying one copper atom shows a pair of peaks two mass units apart in about that ratio. A spectrum of the free peptide shows nothing of the kind.
What a purity figure does not cover
A percentage from reversed-phase chromatography reports what share of the detected material was the target sequence. For a metal complex that leaves two questions open: how much copper is present, and whether it is bound at one copper per peptide.
Acidic conditions complicate it further. Reversed-phase peptide methods run at low pH with an acidic additive, and acid shifts a labile complex toward its dissociated form, so a clean peak can correspond to the free tripeptide. Copper content is worth reporting as a figure of its own, and the conditions of the analysis belong on the certificate alongside the number.
Making it
The peptide half is three coupling cycles of ordinary solid-phase synthesis with no difficult residues. Forming the complex is the step where a batch can go wrong while the peptide is perfectly good: the copper has to be introduced under controlled conditions so the ratio comes out right, and the pH has to sit where the intended species dominates. Under-supplied copper leaves free peptide in the product; over-supplied copper leaves metal the sequence is not holding.
Handling and verification
Material ships lyophilized, which is the stable form. Keep it dry, cool and out of direct light, and let a cold vial reach room temperature before opening so condensation does not form inside. Any Codex Research vial can be checked against its paperwork by batch number.
Research use only
All products sold by Codex Research, including AHK-Cu, are strictly for laboratory research and development. They are not for human or animal consumption and are not intended to diagnose, treat, cure, or prevent any disease.
The compound at Codex Research
AHK-Cu, 50 mg per vial · batch-verified, with its certificate of analysis.
Frequently asked questions
What does AHK-Cu stand for?
The copper complex of the tripeptide alanyl-histidyl-lysine, whose one-letter codes are A, H and K. The free tripeptide is C15H26N6O4 at about 354 g/mol, CAS 126828-32-8; the copper complex carries CAS 682809-81-0.
How is AHK-Cu different from GHK-Cu?
By one residue. The first position is alanine instead of glycine, which adds a methyl group next to the terminal amine that helps hold the copper. Nothing else in the sequence changes.
Does that single change affect copper binding?
It does not remove or add a donor atom. It changes what sits immediately beside the metal, and comparing the two complexes is how the contribution of that position is measured.
How are AHK-Cu and GHK-Cu told apart analytically?
By mass. A methyl group is fourteen daltons, a difference comfortably within the resolution of any mass spectrometer used for this work. The copper isotope pattern provides a second, independent check that the metal is present.
Why does PubChem list a chloride in the formula?
Because a coordination complex has to be written with some charge and some counterion, and that entry makes one particular choice. Other sources write the same compound differently, which is why the name alone does not pin down a formula.
Is a purity percentage enough for AHK-Cu?
No. It reports the share of detected material that was the target sequence and leaves open how much copper is present and whether it is bound one to one. Acidic chromatographic conditions can also dissociate the complex.
References
- Alanyl-histidyl-lysine compound summary. PubChem, National Library of Medicine (CID 7408502).
- AHK-Cu compound summary. PubChem, National Library of Medicine (CID 168431292).
- Harford, C., & Sarkar, B. (1997). Amino terminal Cu(II)- and Ni(II)-binding (ATCUN) motif of proteins and peptides. Accounts of Chemical Research, 30(3), 123-130.
- Freedman, J.H., Pickart, L., Weinstein, B., Mims, W.B., & Peisach, J. (1982). Structure of the glycyl-L-histidyl-L-lysine-copper(II) complex in solution. Biochemistry, 21(19), 4540-4544.