TB-500 vs thymosin beta-4: 889 Da fragment or 4963 Da protein?

The name TB-500 was coined commercially and is often described loosely as synthetic thymosin beta-4. When doping-control laboratories analysed products sold as TB-500, they found the short acetylated fragment Ac-LKKTETQ rather than the full 43-residue protein (Esposito et al., 2012; Ho et al., 2012). That heptapeptide contains the actin-binding motif LKKTETQ that earlier work had mapped within thymosin beta-4 (Sosne, Qiu and Goldstein, 2010).

Product pages and spreadsheets then copied identity numbers from whichever source came first. The result is common: a listing for the 889 Da fragment printed with the 4963 Da molecular weight and CAS 77591-33-4 of the full protein. We found this on several vendor pages during a market review on 2026-10-29, and our own catalog carried the full-length values until the same date. It has been corrected to the fragment values that match our lot report.

Ignore the product name and read the mass spectrum. A 7-residue peptide of 889 Da ionizes mainly as a singly and a doubly charged ion, so the spectrum shows peaks near m/z 889-890 and 445, often with a sodium adduct near 911. A 4963 Da protein produces a series of higher charge states (for example near m/z 1242, 994 and 828) and nothing at 445 that could be its molecular ion.

As a worked example, the published Vitality Chems TB-500 10 mg lot report (lot 3457, Freedom Diagnostics, reported 2026-07-31) lists observed ions at m/z 445.1 ([M+2H]2+) and 889.3 / 890.3 ([M+H]+), with 911.2 consistent with a sodium adduct. Those values match the 889 Da fragment, not full-length thymosin beta-4.

1. Peaks near 445 and 889 point to the TB-500 fragment.
2. A charge-state series near 828, 994 and 1242 points to full-length thymosin beta-4.
3. If the certificate lists only a purity figure and no mass data, identity is not documented either way.

Molar concentrations, expected m/z values and literature comparisons all depend on using the right molecular weight. Using 4963 g/mol for an 889 g/mol peptide understates the molar amount by a factor of about 5.6. Record the sequence, formula and CAS that match the mass data on the lot certificate, not the ones on a marketing page.

UHPLC-high-resolution MS methods for TB-500 and its metabolites have been published (Rahaman et al., 2024) and are a useful reference for the expected ions and retention behaviour of the fragment.