TB-500 vs Thymosin β4: Which Sequence Is on the Vial

BioLabs Research

By BioLabs Research · Last reviewed: October 10, 2026

Two different molecules travel under the shop name "TB-500". Full-length thymosin β4 (Tβ4) is a 43-residue, N-terminally acetylated peptide of about 4963 Da. The material that analytical chemists identified in a product sold as TB-500 is much smaller: Ac-LKKTETQ, the N-acetylated fragment matching residues 17–23 of Tβ4, about 889 Da (Esposito et al., 2012). The TB-500 catalog listing is this fragment. This page compares the two side by side so a receiving lab can tell from the paperwork which one it holds.

Side by side

Formula and weight values are from PubChem; the sequence position is from UniProt.

Where the seven residues come from

UniProt lists the human Tβ4 precursor as 44 residues. 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 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.

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.

Reading the spectrum, not just one peak

Electrospray mass spectrometry often reports ions carrying several charges, and that can confuse a quick read. 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

Naming is the root of most mix-ups:

• "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. 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). See Thymosin alpha-1 is not TB-500.

The three-field check at receiving

Before a vial goes into stock, compare three fields and require all of them to agree:

1. the sequence on the label and the certificate: Ac-LKKTETQ for this listing;
2. the expected mass printed on the certificate: about 889 Da;
3. the observed mass reported by the testing lab, as a neutral mass or a charge-state series.

If any one disagrees, record the lot as "identity unresolved" and ask for a corrected certificate before it is booked into inventory.

Questions to send before ordering

If a seller's listing or certificate leaves the identity open, four questions usually settle it:

• Is the material the seven-residue Ac-LKKTETQ 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?

A seller who cannot answer these from the lot certificate has not yet documented which molecule is in the vial.

Reading the certificate

Our guide on how to read lot documents explains where the sequence, expected mass, observed mass and method fields sit on a certificate. Complete examples for catalog listings are in the COA library. For what TB-500 is and how it is studied, see What is TB-500?. For lot documentation and storage of this listing, see TB-500 and thymosin β4: fragment vs full-length identity.

Next step: Request the lot certificate for the TB-500 listing before you order.

Related reading

• What is TB-500?
• TB-500 lot documentation
• Lot docs are not a purity percentage

References

1. UniProtKB P62328 (TYB4_HUMAN), Thymosin beta-4. https://www.uniprot.org/uniprotkb/P62328/entry · UniProtKB P06454 (PTMA_HUMAN), Prothymosin alpha; peptide thymosin alpha-1, residues 2–29. https://www.uniprot.org/uniprotkb/P06454/entry
2. Esposito S, Deventer K, Goeman J, et al. Drug Test Anal. 2012;4(9):733–738. https://pubmed.ncbi.nlm.nih.gov/22962027/
3. PubChem Compound Summaries: CID 62707662 (Ac-LKKTETQ, C38H68N10O14, MW 889.0); CID 45382195 (thymosin beta-4, C212H350N56O78S, MW 4963); CID 16132341 (thymosin beta-4 record listing CAS 77591-33-4). https://pubchem.ncbi.nlm.nih.gov/compound/62707662 · https://pubchem.ncbi.nlm.nih.gov/compound/45382195 · https://pubchem.ncbi.nlm.nih.gov/compound/16132341

For research use only. Not for human or veterinary use.