TB-500

A fragment, not the parent protein

TB-500 is routinely described as “synthetic thymosin beta-4.” It is not. Thymosin beta-4 is a 43-residue protein of roughly 4,960 g/mol. TB-500 as supplied by essentially every research vendor is the seven-residue acetylated fragment corresponding to residues 17-23, at 889.02 g/mol — about a fifth of the parent’s mass.

The distinction is worth checking on any certificate. A product sold as TB-500 whose stated mass sits near 4,960 is full-length Tβ4; one at 889 is the fragment. Vendor pages that quote the parent’s residue count alongside the fragment’s formula are internally inconsistent, and that inconsistency is a useful signal about how the copy was produced.

Handling notes

N-terminal acetylation adds 42 daltons and blocks aminopeptidase attack. Two lysines give the peptide net positive character despite the glutamate, so counterion load is meaningful and adsorption to negatively charged plastic is worth considering at dilute working concentrations. The C-terminal glutamine is a slow deamidation site — a reason not to store solutions alkaline, though far less pressing than an asparagine would be.