Short Peptide Bioregulators: The Name Is Not the Sequence
Trade names such as epitalon, pinealon, vesugen, thymogen and vilon look like they describe something. They do not. Each of these laboratory materials is only two to four residues long, and nothing in the word itself encodes which residues those are. For this family, the written sequence is the identity; the name is decoration.
Six labels, six sequences, six masses
Laid out together, the group is small enough to tabulate in full:
- Vilon – Lys-Glu; two residues; monoisotopic 275.15 Da.
- Thymogen – Glu-Trp; two residues; monoisotopic 333.13 Da.
- Epitalon – Ala-Glu-Asp-Gly; four residues; monoisotopic 390.14 Da.
- Vesugen – Lys-Glu-Asp; three residues; monoisotopic 390.18 Da.
- Pinealon – Glu-Asp-Arg; three residues; monoisotopic 418.18 Da.
- Cortagen – Ala-Glu-Asp-Pro; four residues; monoisotopic 430.17 Da.
Two entries in that list are a problem. Epitalon and vesugen carry average masses of 390.35 and 390.39 – the same figure once rounded – and their monoisotopic values sit only 0.036 daltons apart. Put either one in front of a low-resolution mass spectrometer and the instrument cannot say which it is looking at.
The arithmetic behind the collision
Epitalon is AEDG, vesugen is KED. Trading an alanine and a glycine for a single lysine leaves the composition almost untouched: 71.037 for alanine plus 57.021 for glycine comes to 128.058, while lysine alone is 128.095. That 0.036 dalton gap is the textbook near-isobaric case, where a CH4 is exchanged for an O and the nominal mass barely moves.
Two routes separate them. High-resolution measurement is one: 0.036 daltons at mass 390 works out to roughly 90 parts per million, well within reach. Chromatography is the other, since a lysine-bearing tripeptide and a glycine-terminated tetrapeptide do not behave alike on column. Retention time measured against a reference standard resolves the question. A nominal mass never will.
Why none of them stick to a C18 column
Every sequence here is small, polar and carries multiple charges. Each one holds at least one glutamate or aspartate, most hold a lysine or an arginine, and not one contains a meaningful hydrophobic residue. Run on a conventional C18 packing, they come off in or beside the void volume.
Laboratories fall back on the standard toolkit for very polar analytes: aqueous-compatible reversed-phase packings at low organic content, ion-pairing chromatography, or hydrophilic interaction chromatography. Because the choice among those is a matter of local preference, purity numbers quoted for these compounds travel between sources far less reliably than purity numbers for longer peptides.
Detection is weak, and thymogen is the outlier
Only one sequence in the group contains tryptophan, and that is thymogen, Glu-Trp. Its single tryptophan produces genuine absorbance at 280 nm, which makes thymogen the one member whose 280 nm chromatogram carries information.
Everything else is watched at 214 nm, where response tracks the number of peptide bonds present. A dipeptide has exactly one. The resulting signal is faint, which is why these materials are frequently analyzed at higher concentrations than a longer peptide would need, and why comparing area percentages between a short peptide and a larger impurity is an especially poor guide.
Counter-ions weigh more than you expect
These compounds reach the bench as free acids, as amides, and as an assortment of salts. On a short sequence those distinctions scale up: an acetate counter-ion attached to a 275 dalton dipeptide accounts for a far greater share of the weighed mass than the identical counter-ion on a 3000 dalton peptide.
That is the concrete reason net peptide content deserves more attention at this end of the size range. Weigh out a nominal quantity of a dipeptide acetate salt and the actual peptide delivered falls noticeably short of the figure printed on the vial – a real gap, not a rounding artifact.
Reading the label for what it is
The names come out of one particular research program and are applied consistently inside that literature, so they are not random. They are simply not systematic. Nothing about the word pinealon points to Glu-Asp-Arg, and a reader without the mapping in hand has no way to reconstruct it.
The workable convention is straightforward: treat the trade name as a label and the three-letter sequence as the identifier. Documentation carrying both, with a mass that agrees with the sequence, leaves no ambiguity. Documentation carrying a trade name and nothing else has not yet said what the vial holds.
The short version of adequate documentation
A sequence spelled out in three-letter code, a mass consistent with it, and – in the specific case of epitalon and vesugen – either a high-resolution measurement or a retention comparison against a reference standard, since nominal mass alone cannot separate that pair.
Related reading
Of this group, epitalon attracts the most standalone discussion, and the published record is surveyed in our review of the epitalon tetrapeptide literature. For the wider issue of names that fail to encode structure, see peptide nomenclature, analogs, fragments and salts.
