What Is TB-500? Identity, Thymosin β4 and Lot Verification

BioLabs Research

By BioLabs Research · Last reviewed: October 28, 2026

TB-500 is a catalog name for a short synthetic peptide related to thymosin beta-4 (Tβ4), a naturally occurring 43-residue peptide [1]. When analytical chemists examined a product sold as TB-500, they identified it as Ac-LKKTETQ: the N-acetylated fragment matching residues 17–23 of Tβ4 [4]. BioLabs Research lists TB-500 as this fragment, supplied for laboratory research only.

Storefronts often use "TB-500" and "thymosin beta-4" as if they were the same thing. Analytically, they are not. This page sets out the difference, where the fragment sits in the full sequence, how mass spectrometry tells the two apart, and what a lot certificate for the TB-500 listing should show.

TB-500 peptide identity at a glance

Values are from PubChem [5] and UniProt [1]. "TB-500" is used loosely across the market, so confirm the sequence and mass on the MS report for your lot.

Where the seven residues come from

UniProt lists the human Tβ4 precursor as 44 residues [1]. The initiator methionine is removed, leaving the 43-residue mature chain that starts SDKPDMAEIEKFDKSK. Counting from that serine, positions 17 to 23 read LKKTETQ. The catalog fragment is that excerpt with an acetyl group added to the leucine. It keeps seven residues and drops the other thirty-six: it does not contain the N-terminal SDKP stretch, the methionine at position 6, or anything past glutamine 23.

That has a practical consequence. Any identity field that belongs to the full-length chain, such as its formula, its mass or its registry number, cannot describe the fragment, and the reverse is also true.

Where the actin-binding motif sits

Tβ4 binds single actin molecules (G-actin) and keeps them from assembling into filaments, a process called actin sequestration [1]. In 1991 it was shown to be the same molecule as Fx, a previously described actin-sequestering peptide [2]. Mutational work later mapped part of its actin-binding site to a short motif at residues 17–22 (LKKTET), with lysine 18 among the residues that contact actin [3]. The TB-500 fragment, residues 17–23, spans this motif.

Research context

UniProt summarizes the documented role of Tβ4 as organizing the cytoskeleton [1], and Tβ4 has been a standard subject in cytoskeleton research since the 1991 identification. At a high level, the laboratory literature uses Tβ4 and related sequences to study actin monomer sequestration and filament assembly in purified-protein assays, cytoskeletal organization in cultured cells, and structure–activity questions about which part of the sequence carries the actin-binding motif [3]. These are findings about molecular mechanism. They are not evidence of safety or efficacy in people, and BioLabs Research makes no such claims.

The fragment's own record is analytical. Esposito and colleagues synthesized the acetylated 17–23 fragment as a reference material and characterized it [4]. We cite that work only because it documents what the "TB-500" name refers to chemically. The consequence for a buyer is simple: the name alone does not define the material, and the certificate has to.

Mass is the deciding field

The two masses differ more than fivefold, so a mass spectrometry result settles which molecule is in the vial. A certificate for TB-500 should report an observed mass close to 889 Da. A result close to 4963 Da describes full-length Tβ4, whatever the carton says.

Small differences matter too. These offsets are our own calculations from standard average residue masses:

• Missing acetyl group. Non-acetylated LKKTETQ is about 42 Da lighter, near 847 Da.
• C-terminal amide instead of free acid. An amidated version would read about 1 Da lower than the free acid listed in PubChem.
• Truncated sequence. Losing a single residue removes roughly 100–130 Da, depending on which one.

Salt form is a separate question. Counter-ions such as acetate or trifluoroacetate change the weight of the powder in the vial, but they are not part of the peptide ion the spectrometer measures, so they explain a weighing difference, not a mass mismatch. With two lysines the fragment is basic and usually carries counter-ions, which is one reason HPLC area percent is not net peptide content.

Reading the spectrum, not just one peak

Electrospray mass spectrometry often reports ions carrying several charges. The fragment's singly protonated ion sits near m/z 890 and its doubly protonated ion near m/z 445.5. Full-length Tβ4 carrying five or six protons appears near m/z 994 and m/z 828, in the same window. A single peak "around 900" therefore proves nothing on its own. A useful certificate states the deconvoluted neutral mass, or shows the charge-state series it was calculated from. (The m/z values are our own calculations from the average masses above.)

Where catalog names collide

• "TB-500", "TB500" and "thymosin beta-4 fragment" are used for the heptapeptide, but some sellers also use "TB-500" loosely for full-length Tβ4. A listing that says "thymosin beta-4 peptide" without a sequence leaves both readings open.
• Registry numbers. Public records list full-length Tβ4 under CAS 77591-33-4 [5]. A certificate that prints that number against a seven-residue fragment is the wrong paper. Registry numbers for the fragment differ between public sources, so we do not print one here. Sequence and observed mass decide identity.
• Thymosin alpha-1 is a third, unrelated peptide of 28 residues (UniProt P06454, residues 2–29 of prothymosin alpha) [6]. See Thymosin alpha-1 is not TB-500.
• BPC-157 is a separate 15-residue sequence with its own CAS; see What is BPC-157?. A two-component blend certificate must identify each component separately; see BPC-157 / TB-500 blend: two sequences, two masses.

How to verify a TB-500 lot

A lot's certificate of analysis (COA) should show:

• Lot number matching the vial, and the test date.
• Intact mass by MS, compared with about 889 Da for Ac-LKKTETQ, as a neutral mass or charge-state series.
• Acetylation check: a peak about 42 Da lower points to non-acetylated fragment.
• HPLC chromatogram with method ID, column and detection wavelength. Area percent is not net peptide content; see HPLC area percent vs net peptide content.
• Testing laboratory named on the certificate. If it cites an accreditation such as ISO/IEC 17025, it can be checked in the accreditation body's public directory.
• Registry number, if printed, belonging to the fragment and not to full-length Tβ4.

Lab COA PDFs are published in our COA library. For a field-by-field walkthrough, see how to read a peptide COA.

Receiving checklist and questions for a seller

Before a vial goes into stock, require three fields to agree: the sequence on the label and certificate (Ac-LKKTETQ), the expected mass printed on the certificate (about 889 Da), and the observed mass reported by the testing lab. Also confirm the vial lot appears on the certificate and the vial is sealed, dry and intact. If any field disagrees, record the lot as "identity unresolved" and ask for a corrected certificate.

If a listing leaves identity open, four questions usually settle it: Is the material the seven-residue fragment or the 43-residue protein? What expected mass does the certificate compare against, and is the N-terminus acetylated? Is the C-terminus a free acid or an amide? Which registry number, if any, is printed, and which public record does it belong to?

Storage and safety data

TB-500 ships as a sealed, lyophilized solid. Its safety data sheet lists storage at 2–8 °C or below, in the original sealed vial, protected from light and moisture. Limit humidity and repeated warming, log receipt date, storage location and lot number against the certificate, and follow institutional chemical-hygiene and disposal rules. Download the SDS from the SDS library.

Frequently asked questions

View TB-500 research material →

References

1. UniProt Consortium. UniProtKB P62328, Thymosin beta-4 (TYB4_HUMAN). https://www.uniprot.org/uniprotkb/P62328/entry
2. Safer D, Elzinga M, Nachmias VT. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. J Biol Chem. 1991;266(7):4029–4032. PubMed 1999398
3. Van Troys M, Dewitte D, Goethals M, Carlier MF, Vandekerckhove J, Ampe C. The actin binding site of thymosin beta 4 mapped by mutational analysis. EMBO J. 1996;15(2):201–210. PubMed 8617195
4. Esposito S, Deventer K, Goeman J, Van der Eycken J, Van Eenoo P. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500 […]. Drug Test Anal. 2012;4(9):733–738. doi:10.1002/dta.1402. PubMed 22962027
5. National Center for Biotechnology Information. PubChem Compound Summaries for CID 62707662 (TB-500, Ac-LKKTETQ), CID 45382195 (thymosin beta-4) and CID 16132341 (thymosin beta-4 record listing CAS 77591-33-4).
6. UniProt Consortium. UniProtKB P06454, Prothymosin alpha (PTMA_HUMAN); thymosin alpha-1, residues 2–29. https://www.uniprot.org/uniprotkb/P06454/entry