Is TB-500 an N-terminal fragment?

No. It is internal, from residues 17 to 23. Both its termini are artefacts of synthesis rather than features of the parent.

Why does acetylation matter?

It caps a free amine that does not exist in the protein, restoring closer electrostatics and slowing aminopeptidase degradation.

Can TB-500 be quantified at 280 nm?

No. There is no tryptophan, tyrosine or phenylalanine, so it has no usable absorbance there.

TB-500 Structure and Origin

The short answer

TB-500 is Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, seven residues taken from positions 17 to 23 of thymosin beta-4 with an acetylated N-terminus. At 889.0 Da it is small, highly polar, and carries no cysteine, methionine or aromatic residue.

Key facts

Sequence: Ac-LKKTETQ
Residues: 7
Molecular weight: 889.0 Da
Formula: C38H68N10O14
Origin: Residues 17–23 of thymosin beta-4
N-terminus: Acetylated (+42.0 Da)
Cys / Met / aromatic: None of any

An internal fragment, not a terminal one

TB-500 comes from the middle of its parent, which is unusual among research peptides. Most fragments in circulation are N-terminal or C-terminal pieces. Taking an internal segment means both ends are artificial: the natural sequence continues in both directions in the parent, and the fragment's termini exist only because synthesis stopped there.

Why the N-terminus is acetylated

An internal residue has no free alpha-amino group. It is engaged in a peptide bond with the residue before it. Synthesising the fragment alone creates a free amine that does not exist in the protein, carrying a positive charge that is an artefact of the excision. Acetylation caps it, restoring something closer to the parent's electrostatics and incidentally slowing aminopeptidase attack.

Research material referenced

TB-500 5mg, third-party HPLC tested

Buy TB-500 · £23.99

The composition

Two lysines, one glutamate, two threonines, one leucine, one glutamine. Strongly polar throughout with no hydrophobic core, and net positively charged at physiological pH. There is no cysteine, no methionine and no aromatic residue, which removes disulfide chemistry, methionine oxidation, and the possibility of quantification by absorbance at 280 nm respectively.

What the structure implies

A short, highly charged, aromatic-free peptide will be very water-soluble and conformationally flexible. It will adsorb readily to surfaces, which matters for small quantities. And it offers no convenient spectroscopic handle, so concentration must be established by other means.

How the parent differs

Thymosin beta-4 at 43 residues contains methionine and other residues absent from the fragment, and behaves quite differently in handling as well as biology. Guidance written for one does not automatically apply to the other, which is a recurring theme in this category and a direct consequence of two products sharing a name.

Frequently asked questions

Is TB-500 an N-terminal fragment?: No. It is internal, from residues 17 to 23. Both its termini are artefacts of synthesis rather than features of the parent.
Why does acetylation matter?: It caps a free amine that does not exist in the protein, restoring closer electrostatics and slowing aminopeptidase degradation.
Can TB-500 be quantified at 280 nm?: No. There is no tryptophan, tyrosine or phenylalanine, so it has no usable absorbance there.

Extended research context

The TB-500 (Thymosin β4 fragment) deep dive

Deep dive: TB-500 vs full-length Thymosin Beta-4

'TB-500' is a synthetic peptide corresponding to the active 17-amino-acid actin-binding region of the endogenous 43-residue Thymosin Beta-4 protein. The two are not identical. TB-500 lacks the flanking sequence that gives full-length TB-4 additional binding partners. In the research literature, papers use 'Thymosin β4' when they mean the full protein and 'TB-500' or 'AcSDKP fragment' when they mean the shorter synthetic peptide. Reading a CoA carefully to see which molecule is in the vial matters. Mass spec is the definitive check.

Actin-binding as the core mechanism

The N-terminal region of TB-4 (and TB-500 by inheritance) contains the canonical actin-binding motif. This motif sequesters G-actin monomers, modulating the G:F actin equilibrium in cell cultures. That mechanism is why almost every mechanistic paper on TB-500 traces back to cytoskeletal reorganisation, cell migration, and models of tissue repair.

Handling considerations unique to TB-500

TB-500 is a 17-residue peptide with modest amphipathicity; it reconstitutes cleanly in bacteriostatic water but is sensitive to repeated freeze/thaw. Aliquoting into single-use volumes on first reconstitution preserves potency across a batch. HPLC on the batch CoA should show a single dominant peak; a doublet suggests deamidation.

Research applications

▸In vitro actin-polymerisation assays (G:F actin ratio measurement)
▸Cell-migration and wound-scratch assays in fibroblast lines
▸Angiogenesis models: tube-formation and endothelial migration assays
▸Analytical method development for short peptides on RP-HPLC
▸Reference-material comparisons against endogenous Thymosin β4

Handling checklist

✓Store lyophilised vials at −20 °C long-term
✓Reconstitute with bacteriostatic water (0.9% benzyl alcohol)
✓Aliquot immediately to avoid freeze/thaw cycles
✓Refrigerate reconstituted aliquots at 2–8 °C; use within 28 days
✓Confirm mass (~4,963 Da for TB-500) via CoA before study use

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Assuming TB-500 = full Thymosin β4

Fix: TB-500 is the 17-residue actin-binding fragment; check the CoA sequence.

✗ Repeated freeze/thaw

Fix: Aliquot at first reconstitution; each cycle degrades yield.

✗ Using tap water

Fix: Use bacteriostatic or sterile water only.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

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Written and reviewed by

The UK Peptides Editorial Team · Research library, UK Peptides

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TB-500: Structure and Where It Comes From