Thymosin beta-4 versus TB-500: protein and fragment
A 43-residue protein and a 7-residue fragment of it, routinely confused. The relationship, the practical differences, and how to tell which you have.
Thymosin beta-4 and TB-500 are routinely treated as the same thing, on supplier websites and in informal discussion, and they are not. One is a 43-residue protein found in almost every mammalian cell; the other is a synthetic seven-residue fragment of it. Confusing them leads to wrong molar calculations, misread literature and, on occasion, the wrong compound in the vial. This note sets out the relationship precisely and explains how to tell which you have. All Alphex products are for laboratory and in-vitro research only.
Thymosin beta-4
Tβ4 is a 43-amino-acid, N-terminally acetylated protein (about 4,963 g/mol) and the most abundant member of the beta-thymosin family. Its principal known role is sequestering monomeric G-actin, buffering the cell's actin pool; it is present at high concentration in platelets, leukocytes and wound fluid. It has been produced recombinantly and synthetically for research, and has a substantial literature in cell biology and in preclinical models of dermal, corneal and cardiac tissue, plus early-stage clinical investigation. Full-length Tβ4 is not in the Alphex catalogue.
TB-500
TB-500 is the research name for the synthetic peptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ, 889.0 g/mol, CAS 885340-08-9), corresponding to residues 17-23 of Tβ4. That heptapeptide is the actin-binding motif of the parent protein, which is why it was chosen for study: the question was whether the motif alone retains any of the protein's activity. It is supplied by Alphex as a laboratory reagent; see the TB-500 guide.
The differences, side by side
- Size: 43 residues versus 7. Molecular weight 4,963 versus 889 g/mol, a factor of 5.6.
- Structure: Tβ4 is an intrinsically disordered protein that folds on binding actin; the fragment is a short linear peptide.
- Literature: the protein's literature is large; the fragment's is small and mostly in vitro. Findings do not transfer automatically in either direction.
- Synthesis: the fragment is a routine solid-phase synthesis; the full protein is a long, expensive synthesis or a recombinant product, which is one reason genuine full-length Tβ4 is much costlier per mg.
- Handling: both are water-soluble; the fragment is the more stable of the two in solution.
How to tell which you have
Mass spectrometry. An observed mass of about 889 is TB-500; about 4,963 is Tβ4. Nothing else on a label or COA is as definitive. A product sold as "TB-500" whose certificate shows a mass near 5,000, or as "thymosin beta-4" with a mass near 900, is mislabelled. Retention time on HPLC also differs substantially. See mass spectrometry explained. The two are trivial to tell apart by mass: 889.0 for the fragment against roughly 4,963 for the full protein.
Reading the literature carefully
When a source discusses "TB-500" and cites work on Tβ4, check the methods of the cited study: which molecule was used, at what molar concentration, in what model. Many secondary sources cite protein studies as if they were fragment studies. For a researcher designing work with the fragment, the relevant primary literature is the fragment's own, plus the actin-binding biochemistry that explains why the motif was chosen. See BPC-157 versus TB-500 for a similar disentangling.
Frequently asked questions
Does the fragment do what the protein does?
The literature does not establish that. It retains the actin-binding motif; whether that translates to the parent's behaviour in any given model is exactly what the small fragment literature has been examining, with mixed results.
Why is TB-500 so much cheaper than Tβ4?
Seven residues versus forty-three. Synthesis cost scales steeply with length. See how peptide pricing works.
Does Alphex sell full-length Tβ4?
No; see why the catalogue is small.
Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 4 August 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.