Peptide reference · Acetylated heptapeptide fragment of Thymosin Beta-4
TB-500 COA check: molecular weight, formula and what to look for
Use these reference values to check the identity result on a TB-500 Certificate of Analysis. The theoretical average molecular weight of TB-500 is 889.01 Da and the monoisotopic mass is 888.492 Da.
Check a TB-500 COA nowTB-500 reference data
| Name | TB-500 |
|---|---|
| Also known as | Thymosin Beta-4 fragment (17-23), Ac-LKKTETQ, TB500 |
| Type | Acetylated heptapeptide fragment of Thymosin Beta-4 |
| Molecular formula | C38H68N10O14 |
| Average molecular weight | 889.01 g/mol (Da) |
| Monoisotopic mass | 888.4916 Da |
| Residues | 7 |
| Sequence | Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln |
| Reference source | PubChem CID 62707662 (formula and mass cross-checked) |
Also accepted: Full-length Thymosin Beta-4 (sometimes sold as "TB-500"): C212H350N56O78S, 4,963.44 Da average / 4,960.486 Da monoisotopic. See Thymosin Beta-4.
Expected mass spectrometry ions
A TB-500 COA may list one of these m/z values instead of the neutral mass. That is still consistent with TB-500. How charge states work.
| Ion | m/z (monoisotopic) | m/z (average) |
|---|---|---|
| [M+H]+ | 889.50 | 890.02 |
| [M+2H]2+ | 445.25 | 445.51 |
| [M+3H]3+ | 297.17 | 297.34 |
What else can show up on a TB-500 COA
Masses calculated from TB-500's formula and sequence for the adducts, degradation products and synthesis impurities a lab report can show. A match tells you what the extra peak probably is.
| Signal | Value | What it means |
|---|---|---|
| Na adduct, deconvoluted (M + 21.98) | 910.47 Da | Sodium adduct: an MS artifact, not a separate impurity. The [M+Na]+ ion itself is at M + 22.99 |
| K adduct, deconvoluted (M + 37.96) | 926.45 Da | Potassium adduct: an MS artifact. The [M+K]+ ion itself is at M + 38.96 |
| TFA adduct (M + 113.99) | 1,002.48 Da | Trifluoroacetic acid from purification seen as an adduct; residual TFA salt is real vial content |
| M + 0.98 (deamidation) | 889.48 Da | Asn/Gln deamidation; overlaps the natural isotope peak, needs high-resolution MS |
| M − 42 (missing acetyl) | 846.48 Da | N-terminal acetyl group missing |
Deletion sequences (5): one residue missing
| Signal | Value | What it means |
|---|---|---|
| des-Leu/Ile (M − 113.08) | 775.41 Da | Deletion sequence missing one Leu/Ile |
| des-Lys (M − 128.09) | 760.40 Da | Deletion sequence missing one Lys |
| des-Thr (M − 101.05) | 787.44 Da | Deletion sequence missing one Thr |
| des-Glu (M − 129.04) | 759.45 Da | Deletion sequence missing one Glu |
| des-Gln (M − 128.06) | 760.43 Da | Deletion sequence missing one Gln |
Adduct rows are deconvoluted masses (the shift seen against [M+H]+); the singly charged [M+Na]+ ion itself sits at M + 22.99. Deletion masses alone can't say which copy of a repeated residue is missing.
Short peptides like TB-500 (7 residues) are easy to make cleanly, so 98% HPLC purity should be the floor, not the goal. The full checklist is in how to read a peptide COA.
TB-500-specific things to watch
The name "TB-500" is used inconsistently. Strictly, TB-500 refers to the acetylated 17-23 fragment of Thymosin Beta-4 (Ac-LKKTETQ, about 889 Da). Many vendors, however, label full-length 43-residue Thymosin Beta-4 (about 4963 Da) as "TB-500". These are completely different molecules.
Read the COA mass carefully: a result near 889 Da means the fragment; a result near 4963 Da (or multiply-charged ions around 1241, 993 or 828 m/z) means full-length Thymosin Beta-4. Neither is automatically wrong, but the product label and the COA must agree.
This checker accepts both and tells you which one the reported mass matches, so you can compare it with what the product page says you are buying.
TB-500 (Ac-LKKTETQ) carries an N-terminal acetyl group. A main peak near 846.5 Da means the acetyl is missing: the cheaper, non-acetylated fragment, or a synthesis error.
Its two lysines carry the charges (the acetyl cap removes the N-terminal amine), so a strong [M+2H]2+ ion near 445.3 m/z is normal. Deamidation of the C-terminal glutamine (+0.98 Da) is possible but slow, and only high-resolution MS can separate it from the natural isotope peak.
Regulatory note, as of November 2, 2026. On October 28, 2026, HHS issued an interim enforcement discretion policy for 503A pharmacy compounding of six peptides: BPC-157, Epitalon, KPV, MOTS-c, Semax and TB-500. It covers single-ingredient preparations made by state-licensed pharmacies and physicians under listed conditions; blends are not covered, and it does not approve these peptides as drugs. For reading a COA, the useful detail is that the policy names specific forms (here, TB-500 acetate), so a COA that states the salt form tells you more than one that does not.
Compounds with similar mass
If a COA's mass is off, compare it with these neighbors. A match to one of them can point to a mislabeled product.
Sources
- PubChem CID 62707662: TB-500 formula and reference mass.
- CIAAW: Standard atomic weights: Used to calculate average molecular weights.
- NIST CODATA: proton mass: Used to convert m/z charge states to neutral mass.
- PeptideCOACheck methodology: How the checker's thresholds work.
- HHS will permit compounding of six peptides ahead of formal rulemaking (RAPS, Oct 29, 2026): Summary of the October 28, 2026 HHS enforcement discretion policy.
- Federal Register public inspection, FR Doc 2026-22192: The policy document itself.
Last updated . Written and maintained under our editorial policy. Found an error? See corrections.
Frequently asked questions
My TB-500 COA shows about 4963.5 Da. Is it fake?
Not necessarily. That is full-length Thymosin Beta-4, a different molecule. Check which one the product page says you are buying.
What does a TB-500 result near 846 Da mean?
The peptide is missing its N-terminal acetyl group (about 42 Da).
Why does a TB-500 spectrum show 445 m/z?
That is the doubly charged ion ([M+2H]2+) of the 888.5 Da fragment.
For laboratory research use only. This page describes analytical chemistry data. It is not medical advice and makes no claim about effects in humans or animals.