GHRP-2 vs GHRP-6: Two Substitutions on One Hexapeptide Backbone Compared

GHRP-6 and GHRP-2 are hexapeptides at the same receptor that differ at exactly two positions. GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2; GHRP-2 is D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2. Positions three through six — Ala-Trp-D-Phe-Lys with a C-terminal amide — are identical. Only the first two residues were changed, and the whole of the difference between a first-generation and a second-generation growth hormone-releasing peptide is contained in those two swaps. In a GHRP-2 vs GHRP-6 comparison the question is which of those two positions your study cares about: the histidine that was removed, or the tryptophan that was replaced with something that cannot oxidise.

Both are supplied as lyophilized powder in 5 and 10 mg vials at ≥ 99% by HPLC with lot-matched certificates, both are GHS-R1a agonists, and both appear in co-lyophilized blends with a GHRH analogue. All of it describes laboratory reference materials for in-vitro and preclinical work.

GHRP-2 vs GHRP-6 at a glance

GHRP-6 is C46H56N12O6, 873.01 g/mol, CAS 87616-84-0, also catalogued as growth hormone releasing hexapeptide or SKF-110679. GHRP-2 is C45H55N9O6, 817.97 g/mol, CAS 158861-67-7, also called pralmorelin, KP-102 or GPA-748. Both carry D-amino acids at two positions for peptidase resistance and both end in a C-terminal amide. Neither contains a cysteine, so there is no disulfide to scramble and no reducing-agent incompatibility in either.

The two substitutions, read off the formulas

The molecular formulas are consistent with the sequences in a way that is worth checking by hand, because it is a useful sanity test on any certificate for either peptide. GHRP-6 has twelve nitrogens: six in the backbone, two in the histidine imidazole, one in each of the two tryptophan indoles, one on the lysine side chain and one in the terminal amide. GHRP-2 has nine, and the three that disappeared are exactly the two imidazole nitrogens and one indole nitrogen — because histidine became D-alanine, which has no side-chain nitrogen, and one tryptophan became D-2-naphthylalanine, whose naphthalene ring has none either.

The carbon count confirms the same two swaps from the other direction. Histidine to alanine loses three carbons; tryptophan to 2-naphthylalanine gains two. Net change, one carbon: C46 to C45. And the mass difference of 55.04 g/mol is, to within rounding, the sum of the two residue substitutions — minus 66.06 for histidine to alanine, plus 11.03 for tryptophan to 2-naphthylalanine. Two independent checks on the same pair of changes.

Position 1 — removing the histidine

Histidine is the most chemically interesting residue in GHRP-6 and the one GHRP-2 does without. Its imidazole has a side-chain pKa near 6, which means it is the only group in either peptide whose charge state changes across the physiological pH range: GHRP-6 titrates between pH 5 and pH 8 while GHRP-2 does not. Imidazole is also a strong metal-coordinating group, so GHRP-6 binds divalent transition metals in a way GHRP-2 cannot — relevant if a buffer carries trace copper or zinc, or if the peptide is ever co-formulated with a copper complex.

Hexarelin shows what else can be done at the front of this scaffold. It is GHRP-6 with a methyl group added to the position-2 tryptophan: C47H58N12O6 against C46H56N12O6, a single CH2, and 887.04 g/mol against 873.01 — a difference of 14.03, which is a methylene to the second decimal place. The histidine stays, and hexarelin additionally carries documented activity at the CD36 scavenger receptor studied in cardiac tissue models. The three peptides together are a near-ideal structure–activity series on one hexapeptide backbone.

Position 2 — replacing one of two tryptophans

This is the substitution with the clearest handling consequence. Tryptophan is the most photolabile and most oxidation-prone aromatic residue in peptides: the indole oxidises to N-formylkynurenine and kynurenine, and it does so under ambient light alone. GHRP-6 carries two tryptophans and therefore twice the indole burden. GHRP-2 carries one, because the other has been replaced by a naphthalene ring, which is aromatic and hydrophobic like indole but has no oxidisable nitrogen and no photochemistry of consequence.

That difference shows up in the storage lines on the two specifications. Both are stored lyophilized at −20 °C protected from light and moisture, but the reconstituted entry for GHRP-6 specifies protection from light while GHRP-2's asks only that the solution be used within the study window. The practical reading is that an amber vial and a dark fridge matter more for GHRP-6 stock than for GHRP-2 stock, and that a slow loss of assay value across a long GHRP-6 experiment should be checked against a fresh chromatogram before it is interpreted as biology. Both peptides also retain a tryptophan at position four, so neither is light-indifferent.

Same receptor, different reported profiles

Both are agonists at growth hormone secretagogue receptor 1a, the ghrelin receptor, and both are standard tools in secretagogue pharmacology. The research areas listed against each diverge in a way that reflects where the published work actually sits. GHRP-2's specification names GHS-R1a pharmacology, secretagogue potency ranking and neuroendocrine signalling — it is the compound used when the question is how potent a secretagogue is relative to others. GHRP-6's names GHS-R1a pharmacology, appetite and feeding models, gastric motility and cytoprotection — the orexigenic and gastrointestinal literature is largely built on GHRP-6, which is unsurprising given that the receptor's endogenous ligand is ghrelin.

One shared property is worth stating because it is a recurring confounder rather than a feature. Peptides of this family are reported to raise prolactin and cortisol alongside growth hormone in human secretagogue studies, which the selective pentapeptide ipamorelin does not do to the same degree. The catalogue treats that as a study endpoint rather than a side note — the Ipamorelin + GHRP-2 blend lists "cortisol and prolactin cross-reactivity" among its research areas. Any experiment using either hexapeptide as a GH-axis probe should measure those two hormones rather than assume a clean readout. The selectivity contrast itself is covered in the ipamorelin versus GHRP-2 comparison.

Normalising the comparison

Potency ranking is a molar question and these two vials are sold by mass. A 5 mg vial of GHRP-6 contains 5.73 micromoles; a 5 mg vial of GHRP-2 contains 6.11 — 6.7% more molecules for the same mass, because GHRP-2 is the lighter molecule. At 10 mg the figures are 11.45 and 12.23 micromoles. A mass-matched head-to-head therefore gives GHRP-2 a 6.7% molar advantage before any pharmacology is involved, which is small but not negligible when the measured difference between two secretagogues is itself modest. Converting both arms to molarity first is the fix, and the arithmetic is the same single division set out in the reconstitution maths guide.

The same correction matters more inside the blends. CJC-1295 (No DAC) + GHRP-2 and CJC-1295 (No DAC) + GHRP-6 are both 10 mg vials at a 1:1 mass split, but the GHRH analogue is 3,367.93 g/mol. In the GHRP-2 version the molar ratio is 1:4.1 in the hexapeptide's favour; in the GHRP-6 version it is 1:3.9. A 1:1 label describes neither. Ipamorelin + GHRP-2, by contrast, pairs two molecules of similar size — 711.85 and 817.97 — so its 1:1 mass split is 1.15:1 molar, close enough that the label is a usable approximation. Why co-formulated vials fix these ratios permanently, and when a multi-vial kit is the better architecture, is set out in the peptide stacks guide.

Which to choose for which research question

Choose GHRP-6 for feeding, motility and the histidine

If the endpoint is food intake in a rodent model, gastric emptying or motility, or cytoprotection in a tissue preparation, GHRP-6 is the molecule with the matching literature and the one most of that work was done with. It is also the correct choice for any study in which the position-1 histidine is the variable — pH-dependent charge behaviour, metal coordination, or a structure–activity panel run against hexarelin, which differs from it by a single methyl group.

Choose GHRP-2 for potency ranking and for long runs

If the study is a secretagogue potency comparison, GHRP-2 is the conventional second-generation reference. It is also the more robust material for anything extended: one fewer oxidisable indole, no titratable side chain, and no metal-coordinating imidazole means fewer ways for the stock to become something else between the first time point and the last. Where the design holds a single reconstituted stock over weeks, that is the argument that decides it.

Run both when the variable is the scaffold

The pair is unusually clean for structure–activity work: identical C-terminal tetrapeptide, identical receptor, two defined substitutions, and a third member of the series — hexarelin — sitting one methyl group away from GHRP-6. Running all three at matched molarity isolates the contribution of the first two positions about as well as any commercially available set allows, and it is the design that makes the 6.7% mass-to-molar correction worth doing properly.

Purity, identity and COA checks

Ask for the chromatogram, not only the percentage. On GHRP-6 specifically, look for oxidation satellites: a +16 Da species on the mass spectrum indicates indole oxidation, and with two tryptophans a +32 Da species is possible. On both, confirm identity by mass against 873.01 and 817.97 rather than trusting the label, since the two differ by 55 g/mol and are easy to tell apart on any instrument — which also means a mislabelled vial is cheap to catch. Confirm the C-terminal amide: a hydrolysed amide gives the free acid at +1 Da, a shift small enough to overlook and large enough to matter, since the amide is part of the pharmacophore. And check water content, because a lyophilized cake that has absorbed moisture reports more peptide on its label than the vial contains. The sections are explained in the certificate-of-analysis guide.

Regulatory framing

GHRP-2 and GHRP-6 are supplied as research chemicals for in-vitro and preclinical laboratory work only. Neither is approved as a medicine in the United States or the European Union, and both are prohibited in sport. GHRP-2 has been studied in human clinical trials under the name pralmorelin and used as a diagnostic agent for growth-hormone-deficiency testing in Japan; that history attaches to a specific regulated pharmaceutical article and does not transfer to research-grade powder supplied anywhere else. Nothing on this page describes human use, and no dosing or protocol guidance is given or implied.