Thymosin beta-4 is a 43-residue, roughly 4,960 Da intracellular protein. TB-500, as supplied in research catalogues, is a short acetylated fragment with a molecular weight of 889.02 Da. They are related by sequence, they are not interchangeable, and most of the literature people cite for "TB-500" was generated with the protein. The five-fold difference in molecular weight is also the fastest way to tell which one is actually in a vial.
The two molecules, side by side
| Thymosin beta-4 (Tβ4) | TB-500 (as commonly supplied) | |
|---|---|---|
| Length | 43 residues | 7 residues |
| Molecular weight | ≈ 4,960 Da | 889.02 Da |
| Sequence | Full β-thymosin sequence, N-terminally acetylated in vivo | Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln |
| CAS | Distinct from the fragment | 885340-08-9 |
| Native role | Principal G-actin sequestering protein in mammalian cells | Corresponds to the actin-binding motif region |
| Typical production | Recombinant or long-chain synthesis | Standard solid-phase synthesis |
The heptapeptide corresponds to the LKKTETQ motif, the region of Tβ4 most associated with actin binding. That motif is necessary for the interaction, but a seven-residue peptide cannot reproduce the binding surface that a 43-residue chain wraps along an actin monomer. Calling the fragment an actin-sequestering agent is, at minimum, an over-extension of what the shorter molecule can do structurally. The concept is defined at fragment.
Where the published biology actually came from
The well-known thymosin beta-4 findings — cardiac work reported in a high-profile 2004 study, corneal and dermal wound-repair models, hair follicle work, endothelial migration assays — were performed with full-length Tβ4, usually recombinant. The clinical development programmes that reached human trials, including ophthalmic and dermal formulations, likewise used the full-length protein. That is a substantial and legitimate literature, and none of it is a study of the heptapeptide.
The fragment has its own smaller literature, generally focused on the motif's contribution to cell migration and angiogenic signalling in vitro. It is not nothing, but it is a different and much thinner evidence base. When a summary cites "TB-500 research" and describes cardiac or corneal findings, it has silently substituted one molecule for another.
This matters for experimental design in an obvious way: if your hypothesis derives from full-length Tβ4 biology, the fragment is not the reagent that tests it. Structural background is in what is TB-500.
Why beta-thymosins are hard to shrink
The reason the fragment cannot stand in for the protein is structural rather than semantic. Beta-thymosins are intrinsically disordered in free solution and fold into an extended conformation only on binding actin, draping an N-terminal helix, a central region containing the LKKTETQ motif, and a C-terminal segment across separate surfaces of the monomer. The binding energy is distributed along that whole length. Excise seven residues and you retain a recognition element without the scaffold that positions it, which is why isolated motif peptides in the beta-thymosin literature show far weaker interactions than the parent protein.
This is a general lesson about peptide fragments rather than a quirk of this one. A fragment reproduces a sequence, not necessarily a function, and the burden of evidence sits with anyone claiming otherwise. The same caution applies across the catalogue wherever a short peptide is named after a larger parent — the fragment deserves its own literature before inheriting the parent's.
The naming problem in research supply
"TB-500" is a catalogue name, not a chemical identity, and it has been applied to both molecules by different suppliers. Some sell the heptapeptide; some sell full-length Tβ4 under the same label. Neither is wrong per se, but a protocol that does not specify which one is not reproducible.
Resolving it takes one look at the certificate of analysis. A mass spectrum showing a species near 889 Da — or a doubly charged ion near 445 — is the heptapeptide. A spectrum showing roughly 4,960 Da, typically as a charge-state envelope across several m/z values, is the full protein. There is no ambiguity between those two numbers. The method for reading it is in mass spectrometry and peptide identity, with the general reading guide in how to read a peptide COA and the unit itself at molecular weight (Da).
The N-terminal acetyl group is worth noting too. Native Tβ4 is N-terminally acetylated in cells, and the research heptapeptide carries the same modification — which is why the catalogue name includes it and why the observed mass runs 42 Da above the free-amine sequence. Defined at acetylation.
Practical handling of the fragment
At 889.02 Da the heptapeptide is small, highly polar and readily water-soluble — an easier molecule to handle than most of the catalogue. Sequence-based risk assessment is short: no methionine, no cysteine, no tryptophan, so oxidation, disulfide scrambling and photodegradation are all low concerns. The threonine and glutamine residues are the ones to think about in prolonged aqueous storage.
Molar arithmetic is straightforward: a 5 mg vial corresponds to about 5.62 µmol of peptide before net-content correction, which is a much larger molar quantity than the same mass of a 4–5 kDa peptide. When comparing a fragment against a full-length protein in any assay, working in molar rather than mass terms is essential — 5 mg of the heptapeptide contains roughly five and a half times the molar quantity of 5 mg of Tβ4.
TB-500 research vials are supplied at 5, 10 and 20 mg, with an oral research format in TB-500 capsules and a fixed-ratio combination in the BPC-157 + TB-500 blend. The head-to-head with the other widely studied repair peptide is in BPC-157 vs TB-500, and the category sits in tissue repair peptides.
What to write in a methods section
Name the molecule by sequence and mass, not by catalogue nickname. "Ac-LKKTETQ, observed [M+H]+ 890.0, lot XXXX" is unambiguous; "TB-500" is not. If the work depends on full-length thymosin beta-4 biology, source and state the full-length protein. If it depends on the motif specifically, state the fragment. The distinction costs one line and prevents an entire class of misinterpretation downstream.