GHRP-2 and GHRP-6: Sequences, Masses and Analytical Separation
GHRP-2 and GHRP-6 are often listed side by side, and the listings rarely explain what separates them. Both are synthetic hexapeptides, both are C-terminally amidated, both contain D-configured residues, and they differ by about 55 daltons. That difference, and where it comes from, is the whole of the analytical story.
The two sequences
GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2. GHRP-2, also written as pralmorelin, is D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2. Positions three through six are shared. The first two positions differ: GHRP-6 opens with histidine and D-tryptophan, GHRP-2 with D-alanine and D-3-(2-naphthyl)alanine.
Neither sequence is a fragment of anything endogenous. Both were constructed, and the D-residues and the terminal amide are deliberate synthetic features rather than natural ones.
Masses and registry numbers
- GHRP-6 — C46H56N12O6, monoisotopic 872.44 Da, average 873.0 Da, CAS 87616-84-0.
- GHRP-2 — C45H55N9O6, monoisotopic 817.43 Da, average 818.0 Da, CAS 158861-67-7.
Both are usually supplied as acetate or trifluoroacetate salts, so a weighed quantity contains less peptide than the figure on the label unless net peptide content is stated separately. The masses above are free-base masses and are what a mass spectrum reports.
Why 2-naphthylalanine matters analytically
The unnatural residue in GHRP-2 is not there for bulk alone. A naphthalene ring is substantially more hydrophobic than an indole, which changes retention on a reversed-phase column, and its ultraviolet absorbance profile differs from tryptophan’s.
GHRP-6 carries two tryptophan residues and absorbs strongly near 280 nm. GHRP-2 carries one tryptophan and one naphthylalanine, and the naphthyl group contributes a characteristic shoulder in the 270 to 290 nm region. A diode-array detector recording full spectra across a peak can therefore distinguish the two by spectral shape as well as by retention time.
What the 55 dalton gap does and does not settle
A 55 dalton separation is comfortably resolved by any mass spectrometer used for peptide work, including low-resolution instruments. On that basis alone, a spectrum showing a protonated molecule at m/z 873.4 is not GHRP-2 and one at m/z 818.4 is not GHRP-6.
What mass does not settle is stereochemistry. Both sequences contain D-residues, and a D-to-L substitution at any position produces a diastereomer of identical elemental composition and identical mass. Mass spectrometry is blind to that substitution. Only a separation that resolves diastereomers, or a chiral analysis of the hydrolysate, addresses it.
Behaviour on a reversed-phase column
Both peptides are basic, carrying a free lysine side chain and, in GHRP-6, an imidazole. Without an ion-pairing additive they tail badly on silica-based C18 packings because the protonated amines interact with residual surface silanols. Trifluoroacetic acid at 0.05 to 0.1 percent is the usual remedy, and it is why most peptide chromatograms are run under acidic conditions.
The hydrophobicity difference between the two means GHRP-2 generally elutes later under the same gradient. That ordering is reproducible within a method but is not transferable between columns of different bonding chemistry, which is why retention time is only identity evidence against a standard run on the same system.
Impurities the sequences invite
Tryptophan is the residue most prone to oxidation during synthesis, purification and storage, and both peptides contain at least one. An oxidised tryptophan adds 16 daltons and typically elutes earlier. In GHRP-6, with two tryptophans, a mono-oxidised species can arise at either position and the two products may or may not separate.
The other common family is deletion sequences, where a coupling step failed and one residue is missing. A missing alanine removes 71 daltons, a missing lysine 128, a missing tryptophan 186. These appear as distinct earlier-eluting peaks with masses that identify the omission directly.
Naming confusions worth knowing
GHRP-2 appears in the literature as pralmorelin and as KP-102. GHRP-6 appears as SKF-110771 and as growth hormone releasing hexapeptide, sometimes abbreviated GHRP without a number, which is ambiguous because the number is the only thing distinguishing the family members. When a certificate names a compound by an older code, the sequence and the mass on the same document are what resolve it.
The single check that confirms identity
An accurate mass measurement against the calculated free-base value, read alongside a chromatogram from the same lot, separates these two from each other and from the rest of the hexapeptide family. It does not address stereochemistry, and it does not establish how much peptide a vial contains. Those are separate measurements, reported separately.
Where this sits in the wider family
The hexapeptide secretagogues sit alongside the growth hormone releasing hormone analogs, which work through a different receptor and are built on a different structural template. The distinction between the two classes is covered in GHRH analogs versus growth hormone secretagogues, and the receptor selectivity question specifically in ipamorelin and GHSR-1a selectivity.
