TB-500 (thymosin beta-4): what the name does not tell you

TB-500 is a trade name, not a chemical identity. It is used both for full-length thymosin beta-4, a 43-residue protein of about 4963 Da, and for a short synthetic fragment of about 889 Da. The two differ by more than a factor of five in mass. Before buying anything sold as TB-500, the number to ask for is the mass.

Nomenclature: TB-500, thymosin beta-4, Ac-SDKP and the fragment

Thymosin beta-4 is a naturally occurring protein of 43 amino acid residues. Its public record gives the formula C₂₁₂H₃₅₀N₅₆O₇₈S and a mass of about 4963 Da: PubChem CID 45382195. The compound most often traded under the name TB-500 is not that protein but a short synthetic fragment, formula C₃₈H₆₈N₁₀O₁₄, mass about 889 Da: PubChem CID 62707662. Both are sold under the same three characters and the same product photograph.

A related fragment, the N-terminal tetrapeptide Ac-SDKP, appears in the literature under its own name and is a third thing again. None of these names resolves the ambiguity on its own. The mass does, immediately and without argument, which is why a listing that gives a name and no mass has not described a product.

Peptorium makes no claim about what any of these molecules does. The published record on thymosin beta-4 and its fragments can be searched on PubMed, and it is not summarised here.

Why the mass is the whole question here

On a certificate of analysis, purity is a ratio - the area of the main peak as a share of all peak area separated by HPLC - while the measured mass, set beside the mass calculated from the sequence, is the identity check. For most compounds that ordering is a piece of good practice. For TB-500 it is the entire transaction. A batch can be reported at any purity you like and still be the other molecule, at a fifth of the mass, sold at the price of the larger one.

Ask which sequence the vial contains, ask for the theoretical mass the supplier is working to, and ask for the measured mass beside it. If those three answers do not line up, nothing else on the certificate is worth reading.

Regulatory status in the UK and the European Union

Neither thymosin beta-4 nor the fragment holds a marketing authorisation in the United Kingdom or the European Union. Nothing under the name TB-500 appears in the EMA medicines database, and in the UK regulation 46 of the Human Medicines Regulations 2012 prohibits selling, supplying or offering an unauthorised medicinal product, while regulation 2 defines a medicinal product partly by how it is presented. A substance with no authorisation can be turned into an unauthorised medicine by the copy written beside it. Competition rules are separate again and are set by sporting bodies, on their own schedule; anyone subject to them reads the list their federation publishes.

"Research use only" is a purpose, not a permission

The phrase states that the seller offers the material for laboratory work rather than for use in people. It is not a licence, not a legal category in the UK or the EU, and not a statement that anything has been assessed. It is also not a quality claim, and where it substitutes for saying who made the material and what was measured, it is doing the opposite of what it appears to do.

Purity, peptide content and what the vial actually holds

Purity by HPLC is the share of the separated peak area that belongs to the target. Peptide content is the share of the vial's mass that is peptide at all, the remainder being residual water and the counter-ions left from synthesis, usually acetate or trifluoroacetate. The two answer different questions and neither can be calculated from the other, so a certificate states both. Water is determined separately by Karl Fischer titration. The guide on purity and peptide content works through the difference, and the guide to reading a certificate goes through the document line by line.

What Peptorium publishes about this material

This page publishes identity: which molecule the name refers to, the sequence, the molecular formula and the calculated mass that follows from it. Each of those can be checked against a public register rather than taken on trust. Purity, peptide content and water describe one batch rather than a compound. Those figures belong on a certificate of analysis issued by the laboratory that tested that batch, and they travel with the material itself.

What to ask before buying TB-500

• Which molecule is it - the 43-residue protein or the short fragment? Ask for the sequence, not the trade name.
• What theoretical mass is the supplier working to, and what measured mass does the certificate report beside it?
• Is there a batch number on the label, and a certificate for that batch rather than one reused across the product?
• Are impurity peaks listed individually, with a chromatogram?
• Is peptide content given separately from purity, with water by Karl Fischer?
• Are the method and the test date named, and can the result be checked with the laboratory that produced it?

What is not known

• What a given vial contains cannot be known without analysing it. A trade name that covers two molecules makes that more true here, not less.
• The published work concerns defined material characterised in a laboratory; it says nothing about material bought elsewhere under the same name.
• Long-term stability of a lyophilisate outside the stated storage conditions has no reliable general answer. Storage guidance is convention.

Frequently asked questions

A lyophilisate is dissolved in bacteriostatic water, and the volume of solvent chosen fixes the concentration; the peptide calculator converts between vial content, solvent volume and concentration, and recommends no quantity for any purpose. Material described on this site is for laboratory and in vitro research use only, and the research declaration sets out what that means.