TB-400 vs TB-500: The Full Molecule, the Fragment, and a Name Used for Both
They are not two brands of the same thing. One is a 43-amino-acid protein; the other is a seven-amino-acid piece of it — and the name "TB-500" has, at different times, been sold as each.
The Name Came From a Product, Not a Molecule
Product names alone are not a sufficient chemical description. This site uses TB-400 and TB-500 as catalogue labels; that naming convention is not independent evidence of the contents of a sample.
The precise descriptions used here are full-length thymosin beta-4 and its N-terminally acetylated 17–23 fragment, Ac-LKKTETQ. Check the stated sequence, modification and analytical evidence rather than resolving identity from a commercial name alone.
The earlier account of who first used the TB-500 name and how it changed rested on an industry interview that was not independently verified. We have withdrawn that history and the interview citation. No biological claim should rely on it.
The Molecular Arithmetic
The two molecules differ in size by roughly a factor of five and a half, and that is a statement about mass — nothing more.
Full-length thymosin beta-4: 43 amino acids, approximately 4,963 Da (~4.96 kDa), N-terminally acetylated.
Fragment 17–23: 7 amino acids, approximately 889 Da.
The working: a milligram is a fixed mass, so the number of molecules it contains rises as the mass of each molecule falls. 4,963 ÷ 889 = 5.58. So one milligram of the fragment contains roughly 5.6 times as many molecules as one milligram of the full protein. This is a calculation from the stated approximate molecular masses, not an experimental result.
What Each One Is Studied For
The full-length molecule contains the LKKTETQ sequence at residues 17–23 — the actin-binding region. The fragment is essentially that motif synthesised on its own, and the published work on it concerns actin binding and cell migration.
The full protein carries that motif and also the regions either side of it. The literature on the whole molecule is substantially larger, and includes the foundational cell-migration work of Malinda and colleagues, cardiac-injury models, corneal repair, and the actin-sequestering role reviewed by Goldstein, Hannappel and Kleinman. Its published research history is covered in the Thymosin Beta-4 guide.
The asymmetry worth knowing: most published research describes the full protein, while much of what is sold and discussed is the fragment. Papers about one are frequently cited as evidence for the other. That is a reason to read the methods section of any study closely, and to check which molecule it actually used.
Neither molecule holds regulatory approval. Both are research-grade compounds, supplied for laboratory research only, and are not approved by the TGA for human or veterinary use.
What We Sell
Catalogue naming and analytical identity are separate. Refer to the current certificate register and each report’s own notes for the documents attached to a product. This comparison does not resolve an inconsistency within a laboratory report or authenticate vial contents.
Frequently Asked Questions
Compiled by the Southern Pacific Labs research team. Last updated September 2026.
Published References
- Goldstein AL, Hannappel E, Kleinman HK. “Thymosin beta4: actin-sequestering protein moonlights to repair injured tissues.” Trends in molecular medicine, 2005. PubMed
- Malinda KM, Sidhu GS, Mani H, et al. “Thymosin beta4 accelerates wound healing.” The Journal of investigative dermatology, 1999. PubMed
- Hannappel E. “beta-Thymosins.” Annals of the New York Academy of Sciences, 2007. PubMed
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