What Is the Difference Between TB-500 and Full-Length Thymosin Beta-4?

The label "TB-500" is used in the research materials market for at least two chemically distinct substances. They differ in length, in molecular weight by roughly a factor of five, and in what the published literature actually describes. A buyer comparing prices across suppliers is frequently comparing two different molecules without knowing it. This article sets out what each one is, how the confusion propagates, and how to resolve it from a specification sheet alone.

What Thymosin β4 is

Thymosin β4 is a naturally occurring 43-amino-acid peptide, N-terminally acetylated, with a molecular weight of approximately 4963 Da. Its sequence begins SDKPDMAEIE and it is the subject of most of the published work that the name "TB-500" is used to invoke [1]. Within the molecule, the region responsible for its actin-binding behavior is a short segment at positions 17 to 23: LKKTETQ.

What the fragment is

That seven-residue segment, acetylated at the N-terminus (Ac-LKKTETQ), is itself synthesized and sold, and it is this fragment that most commercial listings labeled "TB-500" actually contain. Its molecular weight is approximately 889 Da. It is a fragment of the parent molecule, around one sixth of it by mass, not the parent molecule itself. Published research on the full-length protein does not automatically describe the fragment, and the reverse is equally true.

How the confusion propagates

The naming problem is inherited rather than invented. Peptide manufacturers quote whichever molecule their synthesis line produces; resellers copy specification text from suppliers; content sites copy from resellers. By the time a listing reaches a buyer, a sheet may read "TB-500 (Thymosin β4 Acetate)", a phrase that is internally contradictory, because it names the fragment and then defines it as the parent. Sheets of this kind are common, and not always deliberate. The result is a market in which two substances with a fivefold weight difference trade under one name.

The two side by side

Thymosin β4, full length: 43 amino acids; sequence beginning SDKPDMAEIEKFDKSKLKKTETQ; molecular weight approximately 4963 Da; CAS 77591-33-4. It is the parent molecule.

TB-500, heptapeptide fragment: 7 amino acids; sequence Ac-LKKTETQ; molecular weight approximately 889 Da; CAS 885340-08-9. It corresponds to positions 17 to 23 of the parent.

Telling them apart from the paperwork

Two figures resolve the question without ambiguity. The first is molecular weight: 889 and 4963 are not close, and no analytical method returns both for one substance. A mass spectrometry identity result on the certificate of analysis states which molecule was actually in the vial. The second is the sequence itself: a supplier who can state the sequence and produce a mass spectrometry result consistent with it has answered the question. A supplier who can produce neither has not.

Price is a weaker but useful third signal. Synthesizing a 43-residue peptide costs considerably more than synthesizing seven residues. A full-length product offered at fragment prices raises the molecular-weight question before anything else.

What FLASK publishes

FLASK lists the sequence, molecular formula, molecular weight and CAS number for every compound it offers. Where a name is ambiguous in the market, the specification is not. Materials listed as TB-500 on this site are the heptapeptide fragment, stated as such. FLASK does not currently list full-length Thymosin β4; if it is ever added, it will appear under its own name, with its own sequence and analytical records.

Frequently asked questions

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

No. TB-500 as sold is usually the seven-residue fragment Ac-LKKTETQ at about 889 Da; thymosin beta-4 is the 43-residue parent at about 4963 Da.

Which molecule does FLASK sell under the name TB-500?

The heptapeptide fragment, stated as such. FLASK does not currently list full-length thymosin beta-4.

Can price alone tell the two apart?

It is a weaker signal. Full-length material offered at fragment prices raises the molecular-weight question, and the certificate of analysis resolves it.