GHRP-2 vs GHRP-6: Sequence, Mass and How to Tell Them Apart

Supplier catalogues tend to list GHRP-2 vs GHRP-6 as if they were two strengths of the same thing. They are not. Both are synthetic, amidated hexapeptides that act at the same receptor in published binding work, but they differ at two of six positions, carry different numbers of D-amino acids, and sit about 55 daltons apart. For a lab that buys secretagogue peptides in quantity, knowing exactly where those differences come from makes identity checks faster and certificate problems easier to spot.

GHRP-2 vs GHRP-6: the sequences

• GHRP-6: His-D-Trp-Ala-Trp-D-Phe-Lys-NH2
• GHRP-2 (pralmorelin): D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2

Residues 3 to 6 are shared: Ala-Trp-D-Phe-Lys, finished with a C-terminal amide. The first two positions are where the compounds part ways. GHRP-6 opens with L-histidine and D-tryptophan. GHRP-2 opens with D-alanine and D-2-naphthylalanine (D-2-Nal), a non-proteinogenic residue that carries a naphthalene ring in place of tryptophan’s indole.

That gives GHRP-2 three D-residues and GHRP-6 two. Neither peptide is a fragment of a natural protein. The family grew out of structure-activity work on enkephalin analogues, and both compounds are described in the receptor literature as agonists at the growth hormone secretagogue receptor, GHSR-1a.

Where the 55 daltons come from

The gap is easy to reconstruct from residue masses, and doing it once is a good way to trust the number:

• Position 1: histidine (137.06) replaced by alanine (71.04), a loss of 66.02 Da.
• Position 2: tryptophan (186.08) replaced by 2-naphthylalanine (197.08), a gain of 11.00 Da.
• Net: 55.02 Da lighter for GHRP-2.

The formulas tell the same story. GHRP-2 has three fewer nitrogens because it has lost histidine’s imidazole ring and tryptophan’s indole nitrogen. Any mass spectrometer used for peptide work separates the two species comfortably, so a dominant ion near m/z 818 points to GHRP-2 and one near m/z 873 points to GHRP-6, with no overlap between them.

The rest of the family on one table

For labs holding several secretagogues at once, it helps to keep the neighbours in view. Masses are average values for the free base.

Ipamorelin is a pentapeptide, and it shares the D-2-Nal residue with GHRP-2. Its receptor selectivity is covered in our article on ipamorelin and GHSR-1a.

How the two behave on HPLC

Retention

A naphthalene ring is more hydrophobic than an indole, and GHRP-2 also lacks the charged histidine. Under the same reversed-phase gradient, GHRP-2 therefore usually elutes later than GHRP-6. The order is repeatable on a given column and method, but absolute retention times do not transfer between columns with different bonded phases. Retention time is identity evidence only when compared with a reference run on the same system.

Peak shape

Both peptides have a free lysine side chain, and GHRP-6 adds an imidazole. Without an ion-pairing agent, both tail on silica-based C18 packings. TFA at 0.05 to 0.1% is the usual remedy, which is why most certificates for this family show acidic mobile phases.

UV spectra

GHRP-6 has two indole rings and absorbs strongly near 280 nm. GHRP-2 has one indole and one naphthalene, and the naphthyl group changes the shape of the absorbance band between roughly 270 and 290 nm. A detector that records full spectra across each peak can use that shape as a second, independent identity check alongside retention and mass.

What mass spectrometry cannot tell you

A D-residue and its L-counterpart have identical composition. Swap any of the three D-residues in GHRP-2, or the two in GHRP-6, for the wrong enantiomer and the result is a diastereomer with exactly the same mass. MS will not see it.

Two approaches address stereochemistry:

1. A chromatographic method that resolves the diastereomers, confirmed against a reference standard.
2. Chiral amino acid analysis: hydrolyse the peptide, derivatise the amino acids with a chiral reagent such as Marfey’s reagent, and compare the D and L ratios with what the sequence requires.

Routine certificates rarely include either. That is normal for research-grade material, but it means a certificate confirms composition and purity, not configuration at every position.

Impurities you are likely to meet

Oxidation. Tryptophan picks up oxygen readily, giving a +16 Da species that usually elutes earlier. GHRP-6 has two tryptophans, so it can form two different mono-oxidised products that may or may not resolve. GHRP-2 has one tryptophan, and its naphthalene ring is much less prone to the same reaction.

Deletion sequences. A failed coupling during synthesis leaves a peptide missing one residue. The mass loss identifies the gap directly: about 71 Da for alanine, 128 Da for lysine, 147 Da for D-phenylalanine, 186 Da for tryptophan, 137 Da for histidine in GHRP-6, and 197 Da for 2-Nal in GHRP-2. These appear as separate, usually earlier, peaks with a clear mass assignment.

Names to watch on paperwork

GHRP-2 also appears as pralmorelin and KP-102. GHRP-6 is sometimes written as growth hormone releasing hexapeptide, or just “GHRP” without a number. That bare abbreviation could refer to any member of the family, since the digit is what distinguishes them. When a document uses an older code or an unnumbered name, the sequence and mass on the same page are what settle it.

For a volume order of either compound, specify the full sequence with D-residues marked, the expected mass, HPLC purity at a stated wavelength and a single lot for all vials. Bulk Peptides does not list GHRP-2 or GHRP-6; our secretagogue range includes ipamorelin and related compounds, with mix-and-match volume pricing across every vial in the cart.

GHRP-2, GHRP-6 and the other compounds named here are covered purely as laboratory research materials. This article makes no reference to use in people or animals.