For laboratory research use only; not for human consumption. This article covers the chemistry, analysis and handling of the material sold under the name TB-500.
What the name refers to
"TB-500" is a trade-style label, not a systematic chemical name. Two products with the same label can differ in sequence, terminal modification or length. The material discussed here is the N-terminally acetylated fragment corresponding to residues 17-23 of thymosin beta-4, a 43-residue protein. The heptapeptide sequence is Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, written Ac-LKKTETQ. Synthesis and characterisation of this acetylated fragment are described in the literature (PMID: 22962027).
Because the label is ambiguous, a certificate should state the sequence and the measured mass, not only the name. A reader can then check the reported mass against the value calculated from the stated sequence.
Formula, mass and expected ions
The molecular formula is C38H68N10O14 and the monoisotopic mass is 888.49 Da. As a cross-check, the residue masses of Leu, Lys, Lys, Thr, Glu, Thr and Gln sum to about 828.47 Da. Adding water (18.01) for the free C-terminus and an acetyl group (42.01) gives about 888.49 Da.
In positive-mode electrospray ionisation, protonation happens at basic sites. This peptide has two lysine side chains, and the acetyl group blocks the N-terminal amine, so there is no free alpha-amine to protonate. The doubly charged ion [M+2H]2+ at m/z 445.25 is therefore prominent, and it is often the base peak. The singly charged [M+H]+ appears at m/z 889.50 and is usually weaker. A sodium adduct [M+Na]+ near m/z 911.48 may also appear, and a triply charged ion near m/z 297.17 is expected to be minor. A report that lists only the 889.50 ion has left out the ion most likely to dominate the spectrum.
How identity is confirmed on a certificate
Three measurements are commonly combined, and each answers a different question.
HPLC with UV detection compares the retention time of the sample with that of a reference standard run under the same conditions. A matching retention time is consistent with identity, but it does not prove it, because different peptides can co-elute. The chromatogram also shows how many other components absorb at the detection wavelength.
Mass spectrometry of the molecular ion tests whether the measured m/z matches the calculated value for the stated formula. The certificate should say which ion was observed, such as 445.25 for 2+, and which instrument type was used. High-resolution data can distinguish small mass differences that a low-resolution instrument cannot.
Tandem MS fragments the selected precursor and checks the product ions against the sequence. Calculated singly charged fragments for Ac-LKKTETQ include b2 at about m/z 284.20 and y1 at about m/z 147.08. The acetyl group shifts the whole b-series up by 42.01 Da relative to a free N-terminus. LC-MS method work on this peptide has also been published (PMID: 23084823).
A certificate reports what one laboratory measured on one sample. It should name the testing laboratory and carry a Report ID or Laboratory ID that ties it to the vial. Some samples may not yet have a report.
Stability: solid state and solution
As a lyophilised solid, a short peptide like this is most exposed to moisture, since water uptake accelerates hydrolysis and deamidation. Vials should stay sealed and be brought to room temperature before opening, so that condensation does not form on the cold powder.
In solution, the main concern for this sequence is deamidation of the glutamine side chain. Gln is converted to Glu, and the mass increases by 0.984 Da. Here Gln is the C-terminal residue, and the reaction is generally slower than for asparagine. It is faster at higher pH and temperature and with longer storage in solution.
Deamidation is easy to miss in mass spectra. At the 2+ charge state the shift is about 0.49 in m/z, which is close to the 0.5 spacing between isotope peaks of the intact peptide. The product therefore overlaps the M+1 isotope peak unless resolution is high or the two species are separated chromatographically. A retention-time shift on HPLC, together with high-resolution MS, is the more reliable way to see it.
Bench handling and storage
Store the lyophilised material at or below -20 °C, sealed and protected from light and moisture. Weigh it quickly, in a low-humidity environment where possible, because the powder is hygroscopic.
When a stock solution is needed for an assay, dissolve the peptide in a buffer or solvent compatible with the assay, and use low-binding tubes because short cationic peptides can adsorb to plastic and glass. Divide the stock into single-use aliquots, freeze them, and avoid repeated freeze-thaw cycles. Record the preparation date, solvent, pH and nominal concentration. Where the result depends on exact concentration, confirm it by an independent method such as amino acid analysis, because the weighed powder also contains counter-ions and residual water.
If a stock has been stored for some time, re-running LC-MS and comparing the retention time and the m/z 445.25 ion with the original report will show whether the material has changed. This material is supplied for laboratory research use only; not for human consumption.
For research use only. This guide covers bench handling of laboratory research materials. It does not describe an application, benefit, dose, route or outcome, and nothing in it is guidance for use in a person or an animal. Nothing sold by PepXtide has been evaluated by the FDA.
Where this applies on the site
The certificate library
Every published certificate, scanned as the laboratory issued it.
Quality control
The method, the laboratory, and the real spread of results.
TB-500
Named in this guide. Matrix & Cytoskeletal.
BPC-157
Has a published certificate to read against this guide.
GHK-Cu
Has a published certificate to read against this guide.
GLP-2 (Tirzepatide) 20mg
Has a published certificate to read against this guide.
Tesamorelin
Has a published certificate to read against this guide.
Thymosin beta-4
Named in this guide. Sequence and properties in the library.