Short Peptide Bioregulators: The Name Is Not the Sequence
A family of very short peptides circulates under trade names that carry no structural information at all. Epitalon, pinealon, vesugen, thymogen, vilon and their relatives are two to four residues each, and the name tells you nothing about which residues. The sequence is the only thing that identifies them.
The names and the sequences
- 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.
Note epitalon and vesugen. Their average masses are 390.35 and 390.39, which round to the same figure, and their monoisotopic masses differ by 0.036 daltons. A low-resolution mass spectrometer cannot tell them apart.
Why that near-collision happens
Epitalon is AEDG and vesugen is KED. Replacing an alanine plus a glycine with a lysine gives almost the same total composition: alanine at 71.037 plus glycine at 57.021 is 128.058, against lysine at 128.095. The difference is 0.036 daltons, and it is the classic near-isobaric pair: swapping a CH4 for an O changes composition without meaningfully changing nominal mass.
Distinguishing them requires either high-resolution measurement, where 0.036 daltons at mass 390 is roughly 90 parts per million and comfortably resolved, or chromatography, where a lysine-containing tripeptide and a glycine-terminated tetrapeptide behave differently. Retention time against a reference standard settles it; a nominal mass does not.
Everything short is hard to retain
All of these are small, polar and multiply charged. Every one contains at least one glutamate or aspartate, most contain a lysine or arginine, and none contains a substantial hydrophobic residue. On a conventional C18 column they elute in or near the void volume.
The methods that work are the ones used for any very polar analyte: aqueous-compatible reversed-phase packings run at low organic content, ion-pairing chromatography, or hydrophilic interaction chromatography. Different laboratories choose differently, which means purity figures for these compounds are less comparable across sources than figures for larger peptides.
Detection, with one exception
None of these sequences contains tryptophan except thymogen, which is Glu-Trp. That single tryptophan gives thymogen real absorbance at 280 nm, and it is the only member of the group for which a 280 nm chromatogram means anything.
For the rest, detection is at 214 nm, where the response scales with the number of peptide bonds. A dipeptide has one. That is a weak signal, and it is why these compounds are often analysed at higher concentrations than a longer peptide would require, and why area percentage comparisons between a short peptide and a larger impurity are particularly unreliable.
Terminal state and salt form
Short peptides are supplied as free acids, as amides, and as various salts, and the differences are proportionally larger than they would be on a long sequence. An acetate counter-ion on a 275 dalton dipeptide is a much larger fraction of the weighed mass than the same counter-ion on a 3000 dalton peptide.
This is the practical reason net peptide content matters more for short sequences than for long ones. Weighing out a stated quantity of a dipeptide acetate salt delivers noticeably less peptide than the number on the vial, and the gap is large enough to matter rather than being a rounding consideration.
What the trade names do and do not tell you
These names originate in a specific research programme and are used consistently within that literature, so they are not arbitrary. But they are not systematic: nothing in the word pinealon encodes Glu-Asp-Arg, and a reader who does not already know the mapping cannot derive it.
The reliable practice is to treat the trade name as a label and the three-letter sequence as the identifier. A document giving both, with a mass consistent with the sequence, is unambiguous. A document giving only a trade name has not stated what is in the vial.
What confirms a lot
The sequence written out in three-letter code, a mass matching it, and — for epitalon and vesugen specifically — either a high-resolution measurement or a retention comparison against a reference standard, because those two cannot be separated on nominal mass alone.
Related reading
Epitalon is the member of this group most often discussed on its own terms, and the published work on it is reviewed in the epitalon tetrapeptide evidence review. The general problem of a name that does not encode a structure is covered in peptide nomenclature, analogs, fragments and salts.
