What is TB-500's molecular weight?

889.0 Da for the acetylated heptapeptide normally supplied. The widely quoted 4.9 kDa is full-length thymosin beta-4, a different molecule.

Is TB-500 the same as thymosin beta-4?

No. TB-500 is a seven-residue fragment of its actin-binding motif with an acetylated N-terminus. The parent protein is 43 residues.

Is CAS 77591-33-4 TB-500's CAS number?

No. That is thymosin beta-4's. TB-500 as the heptapeptide is a distinct compound, PubChem CID 62707662.

What Is TB-500? Ac-LKKTETQ, Not Thymosin Beta-4

The short answer

TB-500 (also written TB500 or TB 500) is a synthetic peptide based on the actin-binding region of thymosin beta-4. As normally supplied it is the N-acetylated heptapeptide Ac-LKKTETQ at 889.0 Da, not the 43-residue 4,963 Da parent protein, a distinction frequently lost, and a 5.6-fold difference in mass.

Key facts

TB-500 (as supplied): Ac-LKKTETQ, 889.0 Da
Formula: C38H68N10O14
PubChem CID: 62707662
Parent protein: Thymosin beta-4, 43 residues, 4,963 Da
Tβ4 CAS: 77591-33-4 (the PARENT, not TB-500)
Motif: LKKTETQ, residues 17–23 of Tβ4
Status: Investigational; WADA-prohibited in sport

What TB-500 actually is

Thymosin beta-4 is a 43-residue protein whose actin-binding activity is concentrated in a short internal motif, LKKTETQ, at residues 17 to 23. TB-500 is that motif synthesised as a standalone peptide with an acetylated N-terminus. As supplied it is seven residues and 889.0 Da, which is a different molecule from the 4,963 Da protein it was derived from.

The mass confusion, and how to settle it

TB-500's molecular weight is widely quoted as approximately 4.9 kDa. That figure belongs to full-length thymosin beta-4. The arithmetic settles which is which: unmodified LKKTETQ is 847.0 Da, TB-500 is 889.0 Da, and the 42.0 Da difference is exactly one acetyl group. Everything reconciles at the heptapeptide. Nothing reconciles at 4.9 kDa unless the material is the whole protein.

Research material referenced

TB-500 5mg, third-party HPLC tested

Buy TB-500 · £23.99

The complication nobody states

What is sold as TB-500 is not consistent. Some material is the acetylated heptapeptide; some is full-length thymosin beta-4 sold under the TB-500 name. They differ by more than 4 kDa, have different CAS numbers, and are supported by different literatures. The actin-sequestration work applies to the motif, but much of the wound-repair and cardiac work used the full protein. Establishing which one a vial contains is not a formality.

Where the parent protein came from

Thymosin beta-4 was sequenced by Low and colleagues and published in the Proceedings of the National Academy of Sciences in February 1981, from bovine thymus, described at the time as a thymic hormone. Its actin-binding role was characterised later, and it is now understood primarily as the principal G-actin sequestering protein in many cell types rather than as a thymic hormone.

What the literature reports

Published work describes actin sequestration, effects on cell migration, angiogenesis in model systems, corneal wound healing in animal models and cardiac repair models. The evidence is overwhelmingly in vitro and animal, and a substantial share used full-length thymosin beta-4 rather than the heptapeptide.

Regulatory and sporting status

TB-500 holds no marketing authorisation from the MHRA, EMA or FDA. It is prohibited in sport under the World Anti-Doping Agency list, which is relevant to any research context involving competitive athletes. It is supplied here for laboratory research only.

Frequently asked questions

What is TB-500's molecular weight?: 889.0 Da for the acetylated heptapeptide normally supplied. The widely quoted 4.9 kDa is full-length thymosin beta-4, a different molecule.
Is TB-500 the same as thymosin beta-4?: No. TB-500 is a seven-residue fragment of its actin-binding motif with an acetylated N-terminus. The parent protein is 43 residues.
Is CAS 77591-33-4 TB-500's CAS number?: No. That is thymosin beta-4's. TB-500 as the heptapeptide is a distinct compound, PubChem CID 62707662.

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.

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.

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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.

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What Is TB-500? The Heptapeptide and Its Parent Protein