GHK-Cu colour: what blue, violet and green tell you
Why GHK-Cu is blue, what violet, green or colourless means, how storage protects the copper complex, and what a colour check cannot tell you.
GHK-Cu should be blue to blue-violet, both as a lyophilised solid and in solution. The colour comes from copper(II) held by the peptide, so it is the quickest check a researcher has that the copper complex is present and intact. A white or off-white solid points to free GHK or a different material. A green, brown or cloudy solution points to copper that has left the peptide, or to contamination. This note explains where the colour comes from, what shifts it, how storage affects it, and where a colour check stops being useful. All Alphex products are for laboratory and in-vitro research only.
Where the blue comes from
GHK (glycyl-histidyl-lysine) binds copper(II) through three nitrogen donors: the N-terminal amine, the deprotonated amide nitrogen between glycine and histidine, and the imidazole nitrogen of histidine. The fourth equatorial position on the copper stays open and can be taken by water or by another ligand. In this coordination environment the copper absorbs visible light in a broad band centred at about 605 to 606 nm, which is orange-red light. The eye sees the complementary colour, which is blue.
The same binding is very tight. Bossak-Ahmad and colleagues report a conditional dissociation constant for Cu(II)-GHK of about 0.24 pM at pH 7.4, and Ufnalska and colleagues give a conditional log K of 12.62 at the same pH. That affinity is why a well-made complex keeps its colour in neutral water for a long time, and also why anything that binds copper more strongly, or changes its oxidation state, changes the colour.
A colour reference for the bench
| What you see | Most likely meaning | What to do |
|---|---|---|
| Blue to blue-violet solid or film | Copper complex present, normal | Store as below and reconstitute as planned |
| Clear blue solution after reconstitution | Complex intact in solution | Record date and batch number, refrigerate in the dark |
| Slightly more violet than a previous lot | Normal variation in hydration and coordination | No action; check the batch certificate as usual |
| White or off-white solid | Free GHK or a different material | Do not use as GHK-Cu; contact the supplier with the batch code |
| Solution fades toward colourless | Copper stripped by a chelator, acid, or a reducing agent in the vehicle | Check the vehicle composition and pH |
| Green, brown or turbid solution | Copper released and precipitated, or contamination | Discard the stock |
What shifts the colour
- Ternary complexes. Because one copper position is open, other ligands in a buffer or medium can bind alongside GHK. Bossak-Ahmad and colleagues show that an external ligand blue-shifts the d-d band, by about 27 nm with excess GHK and about 46 nm with cis-urocanic acid. Imidazole-containing molecules, including histidine, form ternary complexes too. The solution can look a slightly different shade in a rich medium than in water, and that alone is not a fault.
- Chelators. EDTA and other strong chelators can pull copper away from the peptide. The blue fades as copper moves to a ligand with a weaker visible absorption. See GHK-Cu in assay buffers.
- Acid. The amide nitrogen only binds copper once it has lost its proton, so strongly acidic conditions loosen the complex and the colour weakens.
- Reducing agents. Thiols reduce Cu(II) to Cu(I), which has no d-d band in the visible region. Ufnalska and colleagues followed the reaction of Cu(II)-GHK with glutathione and saw the band shift from 606 nm to about 595 nm as intermediates formed. A vehicle containing a thiol reducing agent will bleach the colour over time.
- Light. The complex is light sensitive. See light-sensitive peptides.
Storage that keeps the colour
Storage for GHK-Cu follows the general rules for lyophilised peptides, with extra care over light and moisture.
- Lyophilised, sealed: -20 C for long-term storage, 2 to 8 C for short periods, in the original vial and in the dark.
- Before opening: let the vial reach room temperature, ideally in a desiccator, so water does not condense on the solid. Bachem's handling guidance makes the same point because peptides tend to be hygroscopic. See condensation and moisture uptake.
- In solution: 2 to 8 C in the dark, in a vehicle at about pH 6 to 7.5 without chelators. Aliquot and freeze at -20 C for longer storage. See freezing reconstituted peptides.
- Containers: glass or polypropylene, labelled with batch number, concentration and date.
A stock that has gone from blue to green or brown has changed chemically, and colour will not tell you by how much. Treat it as compromised rather than trying to correct for it.
What a colour check cannot tell you
Colour confirms that copper is bound to something that coordinates it like GHK. It does not measure purity. A blue solid can still contain deletion sequences, other peptides, or excess copper salt, and all of these would look much the same. Colour also cannot tell you the copper to peptide ratio. Free copper is active in many cell assays, so excess copper is a confounder that a colour check will not show.
Purity needs a chromatogram. Alphex GHK-Cu lot 0010 is reported at greater than 99.80% chromatographic purity by HPLC-UV/VIS (Vanguard Laboratory, ID V251215-25 001, reported 27 December 2025), and the certificate is published in the COA library. Type the code on your vial label there to open the certificate for your batch. How to read the result is covered in how to read a research peptide COA, and the difference between the free peptide and the complex is in GHK versus GHK-Cu.
A simple in-house check
Laboratories that want more than a visual check can record a UV-visible spectrum of a freshly reconstituted stock and note the position and absorbance of the band near 605 nm. Later readings of the same stock at the same concentration, or of a new lot made up the same way, can then be compared with that record. A falling absorbance at the band maximum, with no change in concentration, is an early sign of copper loss. This is a stability check only and does not replace the batch certificate. Record the spectrum alongside the batch number; see record keeping for research peptide purchases.
Frequently asked questions
Why is my GHK-Cu more violet than blue?
Hydration state, residual counter-ion and coordination geometry shift the shade slightly between lots. Blue through blue-violet is normal. Green or brown is not.
The solid is a thin film rather than a powder. Is that a problem?
No. Lyophilised peptides often dry as a film or a fragile cake, and the cake can break up in transit. See lyophilised cake appearance and defects.
Can I use colour to estimate concentration?
Only roughly, and only against a calibration of your own. The band is weak and shifts with the vehicle, so concentration should be set by mass and volume. See calculating concentration in mg per mL.
Where can I see the current lot?
On the GHK-Cu product page and in the COA library.
Sources and further reading
- Bossak-Ahmad K. et al. Ternary Cu(II) complex with GHK peptide and cis-urocanic acid as a potential physiologically functional copper chelate. Int J Mol Sci, 2020: coordination donors, the open fourth site, the 0.24 pM dissociation constant at pH 7.4 and the d-d band near 605 nm
- Ufnalska I. et al. Intermediate Cu(II)-thiolate species in the reduction of Cu(II)GHK by glutathione. Inorg Chem, 2021: conditional log K 12.62 and the band shift during reduction
- Pickart L., Margolina A. Regenerative and protective actions of the GHK-Cu peptide in the light of the new gene data. Int J Mol Sci, 2018: background on GHK as a copper(II) binding peptide
- Bachem. Handling and storage guidelines for peptides: hygroscopic solids, warming in a desiccator, storage temperature
Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 26 October 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.