GHRP-2 and GHRP-6 Are Not Two Sizes of One Thing
Catalog listings put GHRP-2 and GHRP-6 next to one another without saying much about what actually divides them. The shared features are easy to enumerate: both are synthetic hexapeptides, both carry a C-terminal amide, both include D-configured residues. What sets them apart is roughly 55 daltons, and tracing where those 55 daltons come from accounts for nearly everything an analyst needs to know.
Where the sequences diverge
GHRP-6 reads His-D-Trp-Ala-Trp-D-Phe-Lys-NH2. GHRP-2, which also travels under the name pralmorelin, reads D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2. From position three onward the two are identical. Only the opening pair differs: histidine and D-tryptophan in GHRP-6, D-alanine and D-3-(2-naphthyl)alanine in GHRP-2.
Neither one derives from an endogenous protein. Both were designed from scratch, and the D-configured residues along with the terminal amide are intentional synthetic choices rather than anything found in nature.
Formulas, masses, CAS numbers
- GHRP-6 is C46H56N12O6: monoisotopic 872.44 Da, average 873.0 Da, CAS 87616-84-0.
- GHRP-2 is C45H55N9O6: monoisotopic 817.43 Da, average 818.0 Da, CAS 158861-67-7.
Those are free-base values, which is what a mass spectrum returns. Supply is generally as the acetate or trifluoroacetate salt, so unless net peptide content appears as its own figure, a weighed amount holds less peptide than the label number implies.
The naphthyl ring has analytical consequences
The unnatural residue in GHRP-2 does more than add bulk. Naphthalene is considerably more hydrophobic than indole, which shifts reversed-phase retention, and its ultraviolet absorbance profile is not tryptophan’s.
Two tryptophans in GHRP-6 give it strong absorbance near 280 nm. GHRP-2, holding one tryptophan and one naphthylalanine, shows a distinctive shoulder from the naphthyl group somewhere between 270 and 290 nm. A diode-array detector collecting full spectra across the peak can therefore tell the two apart on spectral shape, independently of where they elute.
What 55 daltons proves, and what it leaves open
Any mass spectrometer used for peptide work, low-resolution units included, separates a 55 dalton gap without effort. A protonated molecule sitting at m/z 873.4 is therefore not GHRP-2, and one at m/z 818.4 is not GHRP-6.
Stereochemistry is the blind spot. D-residues occur in both sequences, and swapping a D for an L at any position yields a diastereomer with the same elemental composition and the same mass. Mass spectrometry cannot see it. Addressing that requires either a separation capable of resolving diastereomers or chiral analysis performed on the hydrolysate.
Running them on a C18 column
Basic character is common to both, thanks to a free lysine side chain and, in GHRP-6, an imidazole ring. Protonated amines interact with leftover surface silanols on silica-based C18 packings, producing severe tailing when no ion-pairing additive is present. Trifluoroacetic acid at 0.05 to 0.1 percent is the standard fix, and it explains why acidic conditions predominate in peptide chromatography.
Given the hydrophobicity gap, GHRP-2 will normally elute later than GHRP-6 on the same gradient. That ordering repeats reliably within a single method but does not carry over to columns with different bonding chemistry, which is why retention time only serves as identity evidence when a standard has been run on the same system.
Impurity classes these sequences invite
Oxidation targets tryptophan more readily than any other residue, whether during synthesis, purification or storage, and each of these peptides contains at least one. Oxidized tryptophan adds 16 daltons and generally elutes earlier than the parent. GHRP-6, carrying two, can pick up a single oxygen at either site, and the resulting pair of products may or may not separate chromatographically.
Deletion sequences form the other recurring family, arising when a coupling step fails and leaves a residue out. Losing an alanine costs 71 daltons, a lysine 128, a tryptophan 186. Each shows up as its own earlier-eluting peak, and the mass names the omitted residue outright.
Alternative names in circulation
GHRP-2 turns up in the literature as pralmorelin and as KP-102. GHRP-6 appears as SKF-110771 and as growth hormone releasing hexapeptide, occasionally shortened to GHRP with no number attached, which is unhelpful given that the number is the only thing distinguishing one family member from another. Where a certificate uses one of the older codes, the sequence and mass printed on the same page are what settle which compound is meant.
The one measurement that does the identifying
An accurate mass compared against the calculated free-base value, read together with a chromatogram from that same lot, will distinguish these two from each other and from the remaining hexapeptides. It says nothing about stereochemistry and nothing about how much peptide the vial actually holds. Both of those are separate determinations, reported on their own terms.
The neighboring class
These hexapeptide secretagogues sit beside the growth hormone releasing hormone analogs, a group built on a different structural template and acting at a different receptor. That class distinction is examined in the comparison of GHRH analogs with growth hormone secretagogues, while receptor selectivity in particular is taken up in the article on ipamorelin and GHSR-1a selectivity.