Copper Peptide Lexicon

The vocabulary of a copper coordination complex.

A copper peptide is not a peptide with copper added — it is a defined coordination compound, and sourcing it well means speaking its language. This is the lexicon we work from: the chemistry of the bond, the colour it makes, the methods that measure it, and the sourcing terms on the lot report. Each entry is drawn from our Field Notes and kept strictly to the chemistry.

Four themes

Coordination Chemistry

What a copper peptide actually is — the metal, the ligands, the geometry, and why the complex (not the bare peptide) is the active.

GHK-CuCopper Tripeptide-1What is GHK-Cu?: GHK-Cu is the 1:1 coordination complex of the tripeptide glycyl-L-histidyl-L-lysine (Gly-His-Lys) with a copper(II) ion. Its INCI name is Copper Tripeptide-1 and the complex CAS is 89030-95-5; the copper is part of the active molecule, not an additive, which is why the lot is characterised as a copper complex rather than as a peptide that happens to list copper.
AHK-CuCopper Tripeptide-3What is AHK-Cu and how does it differ from GHK-Cu?: AHK-Cu is the copper(II) complex of alanyl-L-histidyl-L-lysine — the same coordination motif as GHK-Cu with alanine in place of glycine at the N-terminus. That single methyl group at the α-carbon slightly distorts the geometry around the copper, giving AHK-Cu a marginally lower formation constant and a d-d band shifted a few nanometres from GHK-Cu. The two are characterised against their own separate references and are not interchangeable in a formula.
Copper coordinationWhat does copper coordination mean in a copper peptide?: Coordination is the bonding arrangement in which donor atoms from the peptide ligand attach directly to a central copper(II) ion, holding it in a defined geometric pocket. In GHK-Cu the copper is held by nitrogen and oxygen donors from the peptide rather than sitting loose alongside it — and because the bioactivity tracks the intact coordinated complex, the coordination state is the thing a copper-peptide spec is really describing.
ChelationWhat is chelation and why does it matter for copper peptides?: Chelation is coordination in which a single ligand grips a metal ion through two or more donor atoms at once, forming a ring that holds the metal far more tightly than isolated bonds would. GHK chelates copper(II) through several donor atoms in the same molecule, which is what makes the complex stable; conversely a competing chelator in a formula (notably EDTA) can pull the copper away from the peptide over shelf life, which is the most common way a copper-peptide product loses its active.
Cu²⁺ : peptide molar ratioWhat Cu²⁺ : peptide molar ratio should a copper peptide have?: The Cu²⁺ : peptide molar ratio is how many copper ions are present per peptide molecule in the finished powder. For a 1:1 complex like GHK-Cu or AHK-Cu the theoretical value is 1.00, and well-coordinated commercial lots cluster between 0.95 and 1.05. Below 0.95 part of the powder is uncoordinated peptide (not the active); above 1.05 the excess is free copper, a pro-oxidant in finished formulations.
Apo-peptide (free GHK)What is the difference between GHK and GHK-Cu?: The apo-peptide is the metal-free ligand — free GHK is the bare glycyl-histidyl-lysine tripeptide (a white powder, molecular weight about 340.4), as opposed to GHK-Cu, the blue copper complex (about 401.9). They are distinct actives, not two grades of one: the activity attributed to GHK-Cu is a property of the intact complex, so free GHK is not a cheaper substitute, and a lot labelled GHK-Cu that is white or shows no copper is apo-peptide regardless of the name on the bag.
Blue chromophoreWhy is GHK-Cu blue?: The blue of a copper peptide is not added pigment — it is the chromophore created when copper(II) sits in its square-planar peptide pocket, the colour arising from the d-d electronic transition of the bound metal. The free apo-peptide is white; the appearance of the blue is direct visual evidence that the copper is coordinated, which is why a pale, patchy, or green-shifted cake is treated as a coordination problem worth a hold-and-review.
Square-planar geometryWhat is the coordination geometry of GHK-Cu?: Square-planar geometry describes a copper(II) centre with four donor atoms arranged in a plane around it — the arrangement GHK-Cu adopts at its working pH. That geometry is what produces the characteristic d-d band and the blue colour; a distorted, split, or shifted band on the UV-Vis trace is read as the copper sitting in a different or mixed coordination environment rather than the clean square-planar field.
Coordination numberWhat is the coordination number of copper in GHK-Cu?: The coordination number is the count of donor atoms bonded directly to the central metal ion. In the square-planar GHK-Cu complex the copper(II) has a coordination number of four — three nitrogen donors (the histidine imidazole, the N-terminal amine, and a deprotonated backbone amide) plus a carboxylate oxygen completing the fourth position.
Ligand / donor atomWhat is a ligand and a donor atom in a copper peptide?: A ligand is the species that binds a metal ion; the donor atoms are the specific atoms within it that supply the electron pairs forming each bond. In GHK-Cu the peptide is the ligand, and its donor atoms are the nitrogens of the histidine imidazole, the N-terminal amine, and a backbone amide, together with a carboxylate oxygen — the set of donors that defines the complex's geometry and stability.
Histidine imidazole coordinationHow does histidine anchor copper in GHK-Cu?: The imidazole ring on the histidine side chain supplies a nitrogen donor that acts as the strongest single anchor for the copper(II) in GHK-Cu and AHK-Cu. Because this imidazole nitrogen is central to the binding, the histidine residue is the reason the His-containing tripeptides coordinate copper so tightly — and its protonation at low pH is part of why the complex dissociates outside the working window.
Coordination integrityWhat does coordination integrity mean for a copper-peptide lot?: Coordination integrity is the degree to which the copper in a lot is actually held in its proper peptide pocket rather than sitting free or in a mixed environment. It is read together from several signals — a clean d-d band at the expected wavelength, a Cu²⁺ : peptide ratio near 1.00, a high bound-copper fraction, and a colour that matches the master reference — because no single number proves the complex is intact on its own.
Formation constantWhat is a formation constant for a copper peptide?: The formation constant is the equilibrium measure of how strongly a ligand holds a metal ion — a larger value means a more stable complex. GHK-Cu has a high formation constant, which is why its copper stays coordinated through normal handling; AHK-Cu's is slightly lower (a small log-K difference) owing to the geometric strain from its extra methyl group, which makes it marginally more sensitive to pH drift and reductive interference in a formula.

Colour & Spectroscopy

Why a clean lot is blue, what the d-d band reads on a UV-Vis trace, and how colour is anchored to a master reference as a numeric quality signal.

d-d transition band~622 nmWhat is the d-d band of a copper peptide?: The d-d band is the broad, weak absorbance in the visible range that arises from electronic transitions between copper(II) d-orbitals split by the surrounding ligand field; for GHK-Cu in its 1:1 complex it sits near 600–625 nm and is the origin of the blue colour. Its position and shape report on the copper's coordination environment, so the d-d band is the workhorse signal for confirming the complex is intact at lot release.
UV-Vis spectroscopyHow is UV-Vis used to characterise a copper peptide?: UV-Vis spectroscopy measures how a solution absorbs ultraviolet and visible light across a wavelength scan, and for a copper peptide it shows both a strong charge-transfer band in the UV (where the peptide backbone absorbs) and the d-d band in the visible. Read against a master reference, the d-d band maximum (λmax), its absorbance, and its symmetry tell you whether the copper is in the right coordination geometry — a shift, a flattened trace, or a split peak warrants a cross-check against the copper number.
λmaxAbsorbance maximumWhat is λmax and what does it tell you about a copper peptide?: λmax is the wavelength at which a band reaches its peak absorbance. For a copper peptide the d-d band's λmax is read against the master reference under matched conditions — a clean GHK-Cu lot sits near 600–625 nm, and a λmax shifted more than a few nanometres signals a different coordination environment (often a pH or composition change) and triggers a re-run.
CIELAB colour spaceWhat is the CIELAB colour space?: CIELAB is the perceptually uniform colour space defined by the CIE, locating any colour by three coordinates: L* (lightness), a* (green–red), and b* (blue–yellow). A copper-peptide lyophilate sits in a defined region — moderate L*, slightly negative a*, and strongly negative b* as the blue dominates — which makes the colour a measurable, reproducible quantity rather than a remembered impression.
ΔE colour differenceWhat is ΔE and how is it used on a copper-peptide COA?: ΔE is the distance between two colours in CIELAB space — the difference between a sample and a reference — most often reported as ΔE2000 (CIEDE2000), the current perceptually-weighted standard. A copper-peptide lot is measured against a master reference under defined illumination and observer geometry; a small ΔE means the lot matches the expected blue, and a value beyond the release threshold prompts a parallel UV-Vis and copper investigation rather than auto-rejection.
Master swatch / colour anchoringWhat is a colour master swatch for copper peptides?: A master swatch is the characterised reference colour every lot is measured against — at Cupratec a Pantone-anchored CIELAB master, recharacterised annually so the absolute colour stays fixed. Colour anchoring means reporting each lot's ΔE against that named master under stated conditions (D65 illumination, 10° observer), so lot-to-lot visual continuity is a defensible number rather than a side-by-side eyeball comparison.

Quantification & QC

The methods that count copper and peptide separately, separate bound copper from free, and decide whether a lot leaves the bench.

Atomic absorption spectroscopyAASWhat does atomic absorption (AAS) measure on a copper-peptide lot?: AAS atomises the sample and measures absorbance at copper's characteristic atomic line (324.8 nm), giving the total copper content regardless of how that copper is bound. It is robust, low-cost, and well-suited to routine total-copper quantification on release samples — but on its own it cannot tell bound copper from free copper, which is why it is paired with a speciation method.
ICP-OES / ICP-MSInductively coupled plasmaWhat does ICP measure for a copper peptide?: Inductively coupled plasma methods ionise the sample in a high-temperature argon plasma and quantify the resulting emission (ICP-OES) or ions (ICP-MS), measuring total copper plus essentially every other trace metal at the same time. The typical use on a copper-peptide programme is the heavy-metal screen (Pb, As, Hg, Cd and others) needed for a finished-product regulatory notification; like AAS, ICP reads total copper and does not by itself report whether the copper is bound.
HPLC-DAD copper quantificationDiode-array detectionWhy is HPLC-DAD used to measure bound copper?: HPLC with diode-array detection separates the peptide-copper complex from free copper, free peptide, and coordination variants, then quantifies the complex peak by its absorbance — so it measures peptide-bound copper specifically rather than total copper. The diode array also records the d-d band per peak, so HPLC-DAD reports both how much copper is actually coordinated and whether that coordination looks right.
Bound-copper fractionWhat is the bound-copper fraction and what value is clean?: The bound-copper fraction is the share of total copper that is actually coordinated to the peptide — computed by comparing bound copper (HPLC-DAD) against total copper (AAS or ICP). A clean copper-peptide lot sits at or above about 0.95; materially lower means part of the copper in the vial is uncoordinated free copper, so the active fraction is less than the total copper suggests.
Free copperWhat is free copper and why is it a concern?: Free copper is uncoordinated Cu²⁺ — copper present in the lot or formula that is not held by the peptide. It is the fraction worth knowing about because, unlike the bound complex, free copper is a pro-oxidant: it can accelerate oxidation of unsaturated lipids and destabilise oxidation-sensitive co-actives such as retinoids in a finished product.
Net peptide contentWhat is net peptide content on a copper-peptide COA?: Net peptide content is the actual mass of peptide per gram of dry product, after subtracting water, counter-ions, and impurities, and is quantified by HPLC-UV (often against an internal standard). For a copper peptide it is one half of the specification: combined with the copper number it yields the Cu²⁺ : peptide molar ratio, the figure that says whether the copper and peptide are present in the right proportion.
Copper assay (COA)What should a copper-peptide certificate of analysis include?: A copper-peptide certificate of analysis (COA) has to answer more than a generic peptide COA, because the active is a coordination complex. Beyond peptide identity and HPLC purity, look for total copper (AAS or ICP), the bound-copper fraction (HPLC-DAD), the Cu²⁺ : peptide molar ratio, the UV-Vis d-d band near 622 nm, and the colour ΔE against a master reference; the absence of those copper line items means the document is describing a peptide rather than a copper complex.
UV / d-d absorbance ratioWhat is the UV to d-d absorbance ratio used for?: The ratio of the UV charge-transfer band to the visible d-d band is read as a complex-purity indicator — it tracks the balance between peptide and coordinated copper in the lot. Held within a stated tolerance across lots, it is one of the orthogonal release signals; a drift usually corresponds to a shift in the peptide-to-copper balance that the HPLC and copper numbers then corroborate.

Sourcing & Handling

The procurement and stability vocabulary on a copper-peptide lot report and quote sheet — assay, stability window, MOQ, lead time, cold chain.

LyophilisationFreeze-dryingWhy are copper peptides supplied as a lyophilised powder?: Lyophilisation removes water by sublimation under vacuum, leaving a stable solid cake. A copper peptide is far more stable as a dry lyophilate than in solution — the dissociation and oxidation pathways that act on the complex in water slow dramatically in the solid state — which is why GHK-Cu and AHK-Cu ship as a blue powder for reconstitution at the point of formulation.
Solution stability vs pHWhat pH range keeps a copper peptide stable in solution?: The square-planar copper coordination depends on a specific protonation state of the peptide ligands, so the complex is only stable in a narrow pH band — broadly pH 4.5–7.4 in aqueous solution, with the optimal window for finished formulas around pH 5.0–6.5. Below roughly pH 4.5 the backbone amide donor protonates and the copper begins to dissociate (the blue fades or shifts green); above about pH 7.4 hydroxide coordination and precipitation become a risk.
Counter-ionAcetate saltWhat counter-ion form do copper peptides ship in?: Because the lysine residue carries a positive charge, the dry powder is a salt paired with an associated anion — the counter-ion. Cosmetic copper peptides are typically supplied ion-exchanged to the acetate form, which is the cleaner counter-ion to formulate around; the counter-ion form is stated on the lot report alongside the copper and stability data.
INCI nameWhat is the INCI name of a copper peptide?: The INCI name is the standardised ingredient name used to declare a cosmetic ingredient on a product label. GHK-Cu's INCI name is Copper Tripeptide-1; AHK-Cu is Copper Tripeptide-3. The INCI identity is what makes these copper peptides cosmetic-grade actives intended for topical finished products, and the ingredient-disclosure obligations apply per molecule, not collectively.
Net active vs gross massIs the labelled mass of a copper peptide all active?: Gross mass includes water and the counter-ion alongside the active complex, so the net active is always less than the weight in the vial. For a copper peptide the meaningful active is the coordinated complex specifically — which is why the lot report carries net peptide content, total copper, and the bound fraction rather than a single weight, and why dosing a formula off gross mass alone under-delivers the active.
MOQMinimum order quantityWhat is a typical MOQ for copper peptides?: MOQ is the smallest quantity a supplier will produce or sell for a given active. Cupratec works at atelier scale — gram-scale pilot lots through to multi-kilogram commercial campaigns — so the minimum is set per programme rather than as a fixed catalogue number, with a 25 g sample available before a first commercial lot is committed.
Lead timeWhat lead times apply to copper-peptide lots?: Lead time is the interval between order confirmation and shipment. For copper peptides it covers synthesis, ion-exchange to the acetate counter-ion, lyophilisation, and the full copper-coordination release packet — the per-lot characterisation (Cu²⁺ : peptide ratio, UV-Vis, stability, colour ΔE) is part of the timeline, because a lot is not released until that data is reviewed.
Cold chainDo copper peptides need cold-chain shipping?: Cold chain is temperature-controlled handling through transit and storage. A lyophilised copper peptide is stable at ambient temperature for the duration of typical international shipping when packaged appropriately, with cooler conditions used for longer-term storage; cold-chain handling (gel packs or similar) is reserved for temperature-sensitive shipments or where a buyer specifically requires temperature-logged transport.

Every entry here is the short form of something written out at length in the Field Notes — the coordination chemistry, the d-d band, the Cu²⁺ : peptide ratio, the colour workflow, each with the reasoning a formulator can act on.

Read the Field Notes

From the Atelier

Sample a Cupratec lot.

First response under 12 hours. We send a 25 g sample of the active you specify, the lot's UV-Vis spectrum, the Cu²⁺ : peptide ratio measurement, and a feasibility note for the formulation application you have in mind.

The vocabulary of a copper coordination complex.