What TB-500 is
TB-500 is a name that, critically, refers to two chemically distinct molecules. Most sources blur the two; this page keeps them separate, because their molecular weights differ more than fivefold and conflating them produces meaningless reference data.
- Strict / registered TB-500 is a synthetic, N-acetylated heptapeptide with the sequence Ac-LKKTETQ. This corresponds to residues 17–23 of thymosin beta-4 — the conserved actin-binding motif of the parent protein. It carries CAS Registry Number 885340-08-9 and PubChem CID 62707662.
- Commonly-supplied vendor "TB-500" is instead the full 43-amino-acid thymosin beta-4 (Tb4; International Nonproprietary Name timbetasin), CAS Registry Number 77591-33-4, catalogued under UniProt P62328 (gene TMSB4X).
Both forms belong to the same class: synthetic, actin-binding, G-actin-sequestering peptides of the beta-thymosin family. Both are N-terminally acetylated. The distinction is one of size and sequence, not chemical family.
Neither form is approved for human therapeutic use by any regulator. Both are investigational, research-use-only compounds. This page is a molecular and scientific reference intended for researchers. It is not medical advice, and it makes no therapeutic, efficacy or dosing claims. For a broader research-context orientation, see the TB-500 research guide.
Structure and amino acid sequence
The two molecules sold as TB-500 differ fundamentally in length and sequence.
The strict heptapeptide is seven residues long, with the one-letter sequence:
Ac-LKKTETQ — the N-acetylated actin-binding motif corresponding to thymosin beta-4 residues 17–23.
The full thymosin beta-4 is 43 amino acids in its mature, N-acetylated form, with the one-letter sequence:
Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES — the complete 43-residue beta-thymosin (INN timbetasin).
Reading the full sequence, the substring LKKTETQ appears internally (residues 17–23) — this is precisely the motif isolated as the strict heptapeptide. In other words, the heptapeptide is the conserved actin-binding fragment carved out of the larger protein. Both sequences begin with an N-terminal acetyl group, which improves resistance to aminopeptidase degradation by blocking the free alpha-amino terminus.
For a research-context article that uses TB-500 alongside another peptide, see the BPC-157 and TB-500 blend research guide.
Physicochemical properties
The table below gives the confirmed physicochemical and identifier data for both molecules side by side, so the two identities are never confused. The headline figure in each case is the average molecular weight; monoisotopic masses are not asserted here.
| Property | Heptapeptide (strict TB-500) | Full thymosin beta-4 (vendor TB-500) |
|---|---|---|
| Identity | Ac-LKKTETQ (Tb4 residues 17–23) | Full 43-aa thymosin beta-4 (Tb4) |
| INN | — | Timbetasin |
| CAS Registry Number | 885340-08-9 | 77591-33-4 |
| Molecular formula | C38H68N10O14 | C212H350N56O78S |
| Average molecular weight | ~889 g/mol | ~4963 g/mol |
| Peptide length | 7 amino acids | 43 amino acids (mature) |
| N-terminus | Acetylated | Acetylated |
| Class | Synthetic actin-binding beta-thymosin peptide | Synthetic actin-binding beta-thymosin peptide |
| PubChem CID | 62707662 | — |
| UNII | QHK6Z47GTG | — |
| UniProt | — | P62328 (gene TMSB4X) |
This reference does not assert solubility or storage specifications as authoritative. For laboratory handling, reconstitution and storage of research-grade peptides, see the peptide reconstitution and storage guide.
Stability and identity notes
Both molecules carry an N-terminal acetyl group. Acetylation caps the free alpha-amino terminus, which improves resistance to aminopeptidase degradation — aminopeptidases cleave residues from an exposed N-terminus, and a blocked terminus is a poorer substrate. This is a structural property shared by the heptapeptide and the full protein, and it is one reason both are described as N-acetylated in their respective registry entries.
The most consequential identity note is the one this page is built around: the two molecules are not interchangeable. A reference value valid for the heptapeptide (average MW ~889 g/mol, formula C38H68N10O14) is wrong for the full protein (average MW ~4963 g/mol, formula C212H350N56O78S), and vice versa. Any molar calculation, characterisation or analytical method must first establish which molecule is in hand. Independent verification by mass spectrometry or sequence analysis is the only way to confirm identity; the label name "TB-500" alone does not resolve it.
This page does not assert free-base versus salt-form masses, monoisotopic masses, or storage and solubility specifications as authoritative values. The molecular formulae and average molecular weights given are for the peptide forms as described in the cited registry sources.
Mechanism context
The biological role behind the TB-500 name is anchored in thymosin beta-4, the full 43-residue protein. Thymosin beta-4 is the principal intracellular G-actin (monomeric actin) sequestering peptide — it binds free monomeric actin and holds it in a sequestered pool, helping regulate the available concentration of polymerisation-competent actin inside cells. This is a biological mechanism, described here without any therapeutic, efficacy or clinical claim.
Within that protein, the heptapeptide motif LKKTETQ (residues 17–23) is the conserved actin-binding motif — the short sequence most directly associated with the actin-binding interaction. The strict TB-500 heptapeptide is, in effect, this motif isolated and N-acetylated. Understanding the parent protein therefore explains why the heptapeptide carries the TB-500 designation at all: it is the active-site fragment of a G-actin-sequestering beta-thymosin.
No claim is made here about what either molecule does in an organism, in a disease context, or at any dose. The mechanism described is the established intracellular actin-sequestering role of thymosin beta-4 as documented in the cited sources.
Sources