The short answer: CAS 77591-33-4 identifies thymosin beta-4, so verify the sequence and purity, not just the formula
TB-500 is listed under CAS number 77591-33-4, which refers to thymosin beta-4 (Tβ4), the parent protein — but the “TB-500” fragment sold for research is a shorter synthetic peptide, which is exactly why molecular formulas vary between suppliers. The practical lesson for researchers is that the CAS number confirms the family, while the sequence, the molecular weight, and the purity data are what actually tell you what is in the vial. This article explains how to read all four like a researcher — strictly for laboratory reference, not for human or animal use.
What TB-500 actually is: a fragment, not the full protein
Thymosin beta-4 is a 43-amino-acid protein, but the research peptide sold as “TB-500” is a shorter synthetic fragment of it — most commonly the N-terminal region or the actin-binding 17–23 sequence (the LKKTETQ motif). Because different manufacturers produce slightly different fragments, the molecular formula you see on a COA can legitimately differ from one supplier to the next. That variability is normal; it is not by itself a red flag.
The CAS number: what it confirms and what it does not
The CAS registry number 77591-33-4 is a unique identifier for thymosin beta-4, and it confirms that the peptide belongs to the Tβ4 family. What it does not confirm is the exact fragment length, the purity, or the batch quality. CAS numbers are easy for a reseller to copy onto a listing, so treat the CAS as a starting check, never as proof of quality.
Molecular formula vs. molecular weight: which one to check
The molecular weight is the more useful number, because it is what a mass-spectrometry report directly measures. A supplier that lists a molecular weight but no MS spectrum is giving you a claim without evidence. The molecular formula (for example, a string like C212H350N56O78S for the full 43-mer) is derived from the sequence, so the key is that the formula, the stated sequence, and the measured molecular weight all agree with each other on the COA.
Here is the practical checklist: confirm the sequence is stated, confirm the molecular weight matches that sequence, and confirm the COA shows an MS spectrum consistent with that weight. If any one of the three is missing or mismatched, treat the listing with caution.
Why purity matters more than the formula
Between a perfectly stated molecular formula and a real HPLC purity report, the purity report is worth far more. A formula is a text string anyone can copy; a purity report with a chromatogram and a lot-matched mass spectrum is evidence. For research, the working standard is a purity of 98% or higher backed by HPLC, with the MS spectrum confirming the molecular weight. The formula only matters to the extent that it matches the sequence and the measured mass.
How to verify the formula and structure match your COA
When you receive a vial, cross-check the paper against the product. Confirm the lot number on the vial matches the COA, confirm the sequence on the COA matches the fragment you ordered, and confirm the MS spectrum peaks at the molecular weight that sequence predicts. A COA where the lot number, sequence, and mass data all line up is a strong signal; a COA that is missing any one of them is a reason to ask questions before relying on the material.
- Lot number on vial = lot number on COA.
- Sequence on COA = the fragment you actually ordered.
- MS spectrum peak = the molecular weight the sequence predicts.
- HPLC purity = 98% or higher, with a chromatogram.
FAQ: TB-500 molecular formula
What is the CAS number for TB-500?
TB-500 is commonly listed under CAS 77591-33-4, which identifies thymosin beta-4. The CAS confirms the peptide family, not the exact fragment or purity.
Why do different suppliers list different molecular formulas?
Because “TB-500” refers to a synthetic fragment of thymosin beta-4, and manufacturers produce slightly different fragments (for example, the 17–23 LKKTETQ motif versus the N-terminal region). Different fragments have different formulas, which is normal.
Is the molecular formula or the molecular weight more important?
The molecular weight is more useful because mass spectrometry measures it directly. Confirm the formula, sequence, and measured molecular weight all agree on the COA.
What purity should TB-500 be for research?
The working standard is 98% or higher, verified by HPLC with a lot-matched mass spectrum, not just a text claim.
Important: All products are sold strictly for laboratory and research purposes only — not for human or veterinary use. No medical or dosing guidance is provided.