For research use only. The two compounds discussed here are research-grade peptides studied in laboratory settings; nothing below describes human or animal use. GHRP-6 and GHRP-2 are easy to mix up, because they share most of their structure — yet they carry different names, different molecular formulas, and different catalog numbers. This article walks the GHRP-6 vs GHRP-2 chemical structure difference one position at a time. Think of it as a spot-the-difference puzzle in which only two of six building blocks actually change.
The common ground: two hexapeptides cut from the same cloth
Start with the shared ground. Both molecules are hexapeptides — short chains of exactly six amino-acid residues — and both belong to the family of synthetic growth hormone secretagogues derived from met-enkephalin. That common lineage is why the two are constantly mentioned in the same breath.
They also point at the same molecular target. Each one acts as an agonist of the growth hormone secretagogue receptor (GHSR), the receptor whose natural endogenous ligand turned out to be ghrelin. So both were built as synthetic ghrelin mimetics at that receptor — even though they were designed before ghrelin itself was characterized. If you're researching this corner of peptide chemistry, it helps to see them as siblings alongside Ipamorelin, another ghrelin-receptor peptide: same receptor neighborhood, different residue choices.
One detail rounds out the picture. Despite the met-enkephalin ancestry, neither peptide carries opioid activity. The scaffold was deliberately reshaped to keep the growth-hormone-releasing behavior seen in cell and animal models while shedding the opioid character of the parent molecule. The family resemblance is real — but it's the resemblance of two purpose-built research tools, not two accidental look-alikes.
Reading the two sequences side by side
The short answer: the two sequences are identical for the back four residues and differ only at the first two. Here they are, in the standard peptide shorthand:
- GHRP-6: His–D-Trp–Ala–Trp–D-Phe–Lys–NH2 (GHRP-6 reference)
- GHRP-2: D-Ala–D-2-Nal–Ala–Trp–D-Phe–Lys–NH2 (pralmorelin / GHRP-2 reference)
Two notation points, since this is where readers new to peptide chemistry get tripped up. The "D-" prefix marks a mirror-image version of the amino acid — more on why that matters below. The trailing "NH2" is an amide cap on the tail end of the chain, a finishing touch shared by both molecules. If the way a residue chain is written out is unfamiliar, the same convention is unpacked in our explainer on how a peptide's residue sequence is written out.
Line the two up and the pattern jumps out. Positions three through six — Ala–Trp–D-Phe–Lys–NH2 — are letter-for-letter the same in both peptides. Everything that sets GHRP-6 apart from GHRP-2 is packed into positions one and two. That's an unusually tidy contrast: two molecules with their own names, formulas, and regulatory histories that nonetheless agree on two-thirds of their sequence.
One target, two keys: the position-1 and position-2 swap
The short answer: GHRP-6 opens with histidine then D-tryptophan; GHRP-2 opens with D-alanine then a bulky two-ring residue called D-2-naphthylalanine. That's the entire molecular divergence.