Hexarelin: One Methyl Group From GHRP-6
Hexarelin is the hardest compound in its family to confirm, and the reason is a single methyl group. It differs from GHRP-6 by fourteen daltons, which is trivial to see on a mass spectrum and easy to miss everywhere else.
The sequence and where the methyl sits
Hexarelin is His-D-2-methyl-Trp-Ala-Trp-D-Phe-Lys-NH2. Set that against GHRP-6, which is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, and the sequences are identical apart from position two, where hexarelin carries a methyl group on the two-position of the indole ring.
That substitution is a synthetic modification of a residue that is otherwise standard. It is not a different amino acid in the ordinary sense, which is part of why the two compounds are so often conflated in listings and in older literature.
Formula, mass and registry
Hexarelin is C47H58N12O6, monoisotopic 886.46 Da, average 887.0 Da, CAS 140703-51-1. GHRP-6 is C46H56N12O6 at 872.44 Da monoisotopic. One CH2 unit, 14.016 Da, is the entire difference in composition.
As with the rest of the family, the free-base mass is what a spectrum reports. The salt counter-ion, usually acetate or trifluoroacetate, contributes to weighed mass but not to the molecular ion.
Fourteen daltons on a chromatogram
Adding a methyl group to an aromatic ring increases hydrophobicity slightly. Under a typical shallow acetonitrile gradient on a C18 column, hexarelin elutes a little after GHRP-6, but the gap is small and depends on the gradient slope. A steep gradient can compress the two peaks until they overlap, and an overlapping pair reported as a single peak inflates the area percentage of whichever compound the method was calibrated for.
This is a practical argument for a shallow gradient when the analysis is intended to resolve closely related analogs rather than simply to quantify gross impurities. A gradient of one percent organic per minute or less separates species that a five percent per minute ramp will not.
Fourteen daltons on a mass spectrum
By mass the two are unambiguous. A singly protonated hexarelin appears near m/z 887.5 and GHRP-6 near m/z 873.5, a separation any quadrupole resolves. The doubly charged ions at roughly m/z 444.2 and 437.2 are equally distinct.
One caution applies. A fourteen dalton difference is also the difference between a methylated species and its unmethylated parent anywhere else in a spectrum, and methylation is a known synthesis artefact. In a mixture, a peak fourteen daltons above an expected mass is not automatically the methylated analog; it may be a methylated impurity of the parent. Retention time and the chromatographic context are what disambiguate this.
Why the methyl group was introduced
The two-position of the indole ring is a site of metabolic and chemical vulnerability in tryptophan-containing peptides. Substituting it changes the electronic environment of the ring. In the published characterisation work on this family, the modification was reported to alter stability and receptor interaction relative to the unmethylated hexapeptide. That is a structural observation drawn from the literature, not a claim about any application.
Oxidation behaviour
Hexarelin still contains an unmodified tryptophan at position four, so it remains susceptible to oxidation at that site. An oxidised species appears sixteen daltons above the parent, at roughly 902.5 for the protonated ion, and typically elutes earlier because the added oxygen increases polarity.
The methylated residue at position two is less reactive but not inert. A certificate that reports a plus-sixteen impurity does not usually say which tryptophan carried it, and distinguishing the two requires fragmentation rather than intact mass alone.
Detection wavelength
Two indole rings and a phenylalanine give hexarelin strong absorbance near 280 nm, and 214 nm remains the wavelength that responds to the peptide bond itself. Area percentages differ between the two wavelengths, sometimes substantially, because impurities lacking an aromatic residue are invisible at 280 nm and fully visible at 214 nm. A purity figure is only interpretable alongside the wavelength it was measured at.
What confirms a hexarelin lot
An accurate mass at 886.46 Da for the neutral species, a chromatogram run on a gradient shallow enough to resolve the fourteen dalton neighbour, and a stated detection wavelength. Those three together distinguish hexarelin from the compound it is most often confused with. Stereochemistry at the three D-positions remains outside what either measurement addresses.
Where this sits in the wider family
Hexarelin belongs to the same structural class as the other synthetic hexapeptides, and that class is distinct from the growth hormone releasing hormone analogs despite frequent conflation. The two are compared in GHRH analogs versus growth hormone secretagogues, with a worked example in sermorelin as a GHRH 1-29 reference analog.
