The short answer
TB-500 as normally supplied is 889.0 Da. The figure of approximately 4.9 kDa, repeated across much of the internet and previously on this site, is the mass of full-length thymosin beta-4 at 4,963 Da. Using it for heptapeptide material overstates mass 5.6-fold.
Key facts
- TB-500 (Ac-LKKTETQ)
- 889.0 Da, C38H68N10O14, CID 62707662
- LKKTETQ (unmodified)
- 847.0 Da, C36H66N10O13, CID 10169788
- Difference
- 42.0 Da = one acetyl group
- Thymosin beta-4
- 4,963 Da, C212H350N56O78S, CID 16132341
- Ratio of error
- 5.6-fold
- Consequence
- Molarity out by the same factor
The arithmetic that identifies the compound
Three masses are relevant and they interlock. Unmodified LKKTETQ is 847.0 Da. TB-500 is 889.0 Da. The difference is 42.0, which is the mass of an acetyl group added to the N-terminal amine. Full-length thymosin beta-4 is 4,963 Da, more than five times either. The heptapeptide numbers reconcile with each other; 4.9 kDa does not belong in that set at all.
Why the wrong figure propagated
TB-500 is described everywhere as a fragment of thymosin beta-4, and the parent protein's molecular weight is easy to find. Attaching the parent's mass to the fragment is a small step, and once made it is copied. Nothing in the resulting text looks wrong. The compound name is right, the parent is right, and only the number is transposed from one to the other.
Research material referenced
TB-500 5mg, third-party HPLC tested
What it costs in practice
Molarity is mass divided by molecular weight. Using 4,963 where the true value is 889 makes a solution 5.6 times more concentrated in molar terms than calculated. Any experiment specifying a molar concentration, any comparison against a published EC50, any dose-response relationship built on that arithmetic is wrong by that factor, and would not look obviously wrong at any point.
How to establish which you have
Mass spectrometry settles it immediately: 889 and 4,963 are not confusable. A certificate of analysis should state the molecular weight and the sequence, and if it states 43 residues or a mass near 4.9 kDa, the material is full-length thymosin beta-4 whatever the label says. If it states LKKTETQ or a mass near 889, it is the heptapeptide.
Why both products legitimately exist
Neither is a counterfeit. Full-length thymosin beta-4 is the protein most of the wound-repair and cardiac literature actually used. The heptapeptide is the actin-binding motif, cheaper to synthesise and the form most commonly sold as TB-500. Both are real compounds with real literatures; the problem is exclusively that one name is used for both.
Quick reference
| Compound | Formula | MW | PubChem CID |
|---|---|---|---|
| TB-500 (Ac-LKKTETQ) | C38H68N10O14 | 889.0 | 62707662 |
| LKKTETQ (free) | C36H66N10O13 | 847.0 | 10169788 |
| Thymosin beta-4 | C212H350N56O78S | 4,963 | 16132341 |
Frequently asked questions
- Is TB-500 889 Da or 4.9 kDa?
- 889.0 Da for the acetylated heptapeptide normally supplied. 4,963 Da is full-length thymosin beta-4, a different compound.
- What is the 42 Da difference?
- One acetyl group on the N-terminal amine. Unmodified LKKTETQ is 847.0; acetylated it is 889.0.
- How do I check which I have?
- Mass spectrometry, or a certificate stating sequence and mass. 889 and 4,963 are not confusable on any instrument.
Extended research context
The TB-500 (Thymosin β4 fragment) deep dive
Deep dive: TB-500 vs full-length Thymosin Beta-4
'TB-500' is a synthetic peptide corresponding to the active 17-amino-acid actin-binding region of the endogenous 43-residue Thymosin Beta-4 protein. The two are not identical. TB-500 lacks the flanking sequence that gives full-length TB-4 additional binding partners. In the research literature, papers use 'Thymosin β4' when they mean the full protein and 'TB-500' or 'AcSDKP fragment' when they mean the shorter synthetic peptide. Reading a CoA carefully to see which molecule is in the vial matters. Mass spec is the definitive check.
Actin-binding as the core mechanism
The N-terminal region of TB-4 (and TB-500 by inheritance) contains the canonical actin-binding motif. This motif sequesters G-actin monomers, modulating the G:F actin equilibrium in cell cultures. That mechanism is why almost every mechanistic paper on TB-500 traces back to cytoskeletal reorganisation, cell migration, and models of tissue repair.
Handling considerations unique to TB-500
TB-500 is a 17-residue peptide with modest amphipathicity; it reconstitutes cleanly in bacteriostatic water but is sensitive to repeated freeze/thaw. Aliquoting into single-use volumes on first reconstitution preserves potency across a batch. HPLC on the batch CoA should show a single dominant peak; a doublet suggests deamidation.
Research applications
- ▸In vitro actin-polymerisation assays (G:F actin ratio measurement)
- ▸Cell-migration and wound-scratch assays in fibroblast lines
- ▸Angiogenesis models: tube-formation and endothelial migration assays
- ▸Analytical method development for short peptides on RP-HPLC
- ▸Reference-material comparisons against endogenous Thymosin β4
Handling checklist
- ✓Store lyophilised vials at −20 °C long-term
- ✓Reconstitute with bacteriostatic water (0.9% benzyl alcohol)
- ✓Aliquot immediately to avoid freeze/thaw cycles
- ✓Refrigerate reconstituted aliquots at 2–8 °C; use within 28 days
- ✓Confirm mass (~4,963 Da for TB-500) via CoA before study use
Common research-handling mistakes
Learnt from thousands of researcher orders across our UK labs.
✗ Assuming TB-500 = full Thymosin β4
Fix: TB-500 is the 17-residue actin-binding fragment; check the CoA sequence.
✗ Repeated freeze/thaw
Fix: Aliquot at first reconstitution; each cycle degrades yield.
✗ Using tap water
Fix: Use bacteriostatic or sterile water only.
Continue researching
Peer-reviewed guides, comparators and matched reference materials.
Related questions researchers ask
Primary sources & clinical trials
Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.
- PubChemPubChem · TB-500 (CID 62707662)pubchem.ncbi.nlm.nih.gov
- PubChemPubChem · LKKTETQ (CID 10169788)pubchem.ncbi.nlm.nih.gov
- PubChemPubChem · Thymosin beta-4 (CID 16132341)pubchem.ncbi.nlm.nih.gov
- PubMedGoldstein AL et al., Thymosin β4: a multi-functional regenerative peptide. Expert Opin Biol Ther 2012 (PMID 22074294)pubmed.ncbi.nlm.nih.gov
- PubMedShrivastava S et al., Thymosin beta4 and cardiac repair. Ann N Y Acad Sci 2010 (PMID 20536454)pubmed.ncbi.nlm.nih.gov
- PubMedSosne G et al., Thymosin beta 4 promotes corneal wound healing. Exp Eye Res 2002 (PMID 11950239)pubmed.ncbi.nlm.nih.gov
- TrialClinicalTrials.gov · Thymosin β4 (RGN-259) dry eye Phase 3 (NCT03925727)clinicaltrials.gov
- PubMedNIH PubMed: Thymosin beta-4 tissue repairpubmed.ncbi.nlm.nih.gov
- GuidelineGoogle: Creating helpful, reliable, people-first contentdevelopers.google.com
Written and reviewed by
The UK Peptides Editorial Team · Research library, UK Peptides
The editorial team is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate. Corrections are made in place and the review date updated.
More TB-500 (Thymosin β4 fragment) articles
- TB-500 CAS Number and Chemical IdentityCAS 77591-33-4 belongs to thymosin beta-4, not TB-500. The heptapeptide is PubChem CID 62707662. Why the two identifier sets are constantly merged.
- TB-500 vs Thymosin Beta-4Seven residues against forty-three, 889 Da against 4,963. Which literature applies to which, and why much cited research used the protein, not the fragment.
- Fragment or Full Protein: Which the Research UsesThe heptapeptide isolates the actin-binding motif; the full protein carries everything else. Which to use depends entirely on the question being asked.
- LKKTETQ: The Actin-Binding MotifLKKTETQ occupies residues 17–23 of thymosin beta-4 and carries its actin-binding activity. What the sequence does and why it was isolated.
- How Actin Sequestration WorksThymosin beta-4 binds monomeric G-actin and holds it out of filaments. Why maintaining a monomer pool matters for how quickly a cell can rebuild its skeleton.
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