Hexarelin and GHRP-6 Differ by a Single Methyl Group
Of all the synthetic hexapeptides in its class, hexarelin is the one most likely to be misidentified, and the culprit is a single methyl group. Fourteen daltons separate it from GHRP-6. On a mass spectrum that gap is obvious. Almost everywhere else it is easy to overlook entirely.
One position out of six
Written out, hexarelin is His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH2. GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2. Line them up and every position matches except the second, where hexarelin carries a methyl group at the two-position of the indole ring.
What sits there is a synthetically modified version of an otherwise standard residue rather than a genuinely different amino acid. That subtlety goes a long way toward explaining why the two compounds get run together so frequently, both in catalog listings and in older publications.
Composition and registry numbers
Hexarelin: C47H58N12O6, monoisotopic 886.46 Da, average 887.0 Da, CAS 140703-51-1. GHRP-6: C46H56N12O6, monoisotopic 872.44 Da. The whole compositional difference amounts to one CH2 unit, 14.016 Da.
The same caveat that applies across this family applies here: a spectrum reads the free base. Acetate or trifluoroacetate counter-ions add to what the balance registers but never show up in the molecular ion.
Separating them by retention
Hanging a methyl group off an aromatic ring nudges hydrophobicity upward. On a C18 column under a shallow acetonitrile gradient, hexarelin therefore comes off marginally later than GHRP-6. The margin is narrow, though, and it scales with gradient slope. Run the gradient steeply enough and the two peaks compress into one another; when an overlapped pair is integrated as a single peak, the area percentage attributed to whichever compound the method targeted is inflated accordingly.
The practical takeaway is that resolving close analogs calls for a shallow gradient, whereas a survey of gross impurities does not. Roughly one percent organic per minute or less will separate species that a five percent per minute ramp leaves merged.
Separating them by mass
Mass resolves the ambiguity cleanly. Singly protonated hexarelin lands near m/z 887.5 against m/z 873.5 for GHRP-6, a gap that any quadrupole handles. The doubly charged pair, near m/z 444.2 and 437.2, is just as clearly split.
One trap deserves flagging. Fourteen daltons is also exactly the distance between any methylated species and its unmethylated parent, and stray methylation is a recognized synthesis artifact. Within a mixture, then, a peak fourteen daltons above an expected mass need not be the methylated analog at all; it could be a methylated impurity of the parent compound. Sorting that out depends on retention time and the surrounding chromatographic context.
What the modification was for
In tryptophan-containing peptides, the two-position of the indole ring is a point of chemical and metabolic vulnerability, and substituting there shifts the electronic environment of the ring. Published characterization work on this family reported that the modification changes stability and receptor interaction compared with the unmethylated hexapeptide. That is an observation about structure taken from the literature, and nothing more than that.
Oxidation is still on the table
Position four holds an unmodified tryptophan, so hexarelin remains oxidation-prone at that site. Oxidized material shows up sixteen daltons heavier than the parent, near 902.5 for the protonated ion, and normally elutes earlier, since the extra oxygen raises polarity.
Methylation at position two makes that residue less reactive without making it inert. Certificates reporting a plus-sixteen impurity rarely specify which of the two tryptophans took the oxygen, and telling them apart calls for fragmentation rather than intact mass on its own.
Two wavelengths, two different numbers
With two indole rings plus a phenylalanine, hexarelin absorbs strongly around 280 nm, while 214 nm continues to report on the peptide bond itself. Area percentages taken at the two wavelengths can diverge, sometimes by a lot, because impurities without an aromatic residue register fully at 214 nm and not at all at 280 nm. A purity number means nothing until the wavelength behind it is known.
What actually pins down a lot
Three items do the work: an accurate mass of 886.46 Da for the neutral species, a chromatogram from a gradient shallow enough to resolve the fourteen dalton neighbor, and a stated detection wavelength. Together they separate hexarelin from the compound it is most often mistaken for. Neither measurement says anything about stereochemistry at the three D-positions, which stays unaddressed.
The wider structural class
Hexarelin sits within the same structural class as the other synthetic hexapeptides, and that class is separate from the growth hormone releasing hormone analogs even though the two are regularly conflated. The comparison is drawn out in this piece on GHRH analogs and growth hormone secretagogues, with a specific case examined in the article treating sermorelin as a GHRH 1-29 reference analog.