Buying · 5 min read
Why Two Suppliers Sell 'TB-500' at Molecular Weights Five Times Apart
One name, two molecules, a fivefold difference in mass, and two separate bodies of literature. This is the clearest example of why you check the molecular weight before you check the price.
The name does not identify a molecule
Thymosin beta-4 is a defined 43-amino-acid peptide with a molecular weight of approximately 4963 g/mol. It is the principal G-actin sequestering protein in mammalian cells, and it has a substantial published literature spanning wound models, corneal research, and cardiac studies.
Its actin-binding activity localizes largely to a short internal motif — LKKTETQ, residues 17 through 23. Some suppliers synthesize that fragment alone and sell it as TB-500. Its molecular weight is near 889 g/mol.
Both are sold under the same product name. Neither supplier is necessarily lying. But they are shipping molecules that differ by more than a factor of five in mass, and the research supporting one does not automatically transfer to the other.
The literature you are citing was almost certainly performed on full-length thymosin beta-4. Confirm that is what is in the vial.
Why it matters for a protocol
Mass-based comparisons break immediately. A milligram of the fragment contains roughly five and a half times as many molecules as a milligram of full-length Tβ4. Any protocol specified in mass rather than molar terms will be off by that factor.
Pharmacokinetics differ. A 43-residue peptide and a 7-residue fragment have different clearance behaviour, different stability, and different distribution.
The evidence base differs. The actin-sequestering mechanism and the bulk of the published work concern full-length Tβ4. Claims specific to the isolated fragment rest on a considerably thinner literature.
How to tell which you have
Read the molecular weight on the certificate of analysis. Near 4963 g/mol is full-length thymosin beta-4. Near 889 g/mol is the fragment. There is no ambiguity once you look.
Check the CAS number. Full-length Tβ4 is 77591-33-4. A fragment product may carry a different identifier or none at all.
If the product page states neither molecular weight nor CAS number, that omission is the answer to your question.
Common questions
Is TB-500 the same as thymosin beta-4?
Sometimes. TB-500 is a product name, not a chemical identity. Some suppliers ship full-length thymosin beta-4 at approximately 4963 g/mol; others ship a short fragment based on the LKKTETQ actin-binding motif at approximately 889 g/mol. Check the molecular weight on the certificate of analysis.
Which form does the published research use?
The great majority of published work — the actin-sequestering mechanism, the corneal and wound-model studies — concerns full-length thymosin beta-4. Research specific to the isolated LKKTETQ fragment is a much smaller literature.
All products are intended strictly for laboratory research use. Not for human or veterinary use. Not for diagnostic or therapeutic use. Not a drug, food, or cosmetic.
