Is Thymosin Alpha-1 the Same as TB-500 or Thymulin? Sequences, CAS Records and Salt Form

No: thymosin alpha-1 is not the same compound as TB-500 or thymulin. Thymosin alpha-1 (INN thymalfasin) is an N-acetylated 28-residue peptide, formula C129H215N33O55, 3108.26 Da, CAS 62304-98-7; TB-500 as listed by Pepta Labs is N-acetylated 43-residue thymosin beta-4 (INN timbetasin), formula C212H350N56O78S, 4963.44 Da, CAS 77591-33-4; and thymulin is a 9-residue peptide with a pyroglutamate N-terminus, formula C33H54N12O15, 858.86 Da, CAS 78922-62-0 on the listing. The three share part of a name, not a sequence, a formula or a registry record. This page sets the three identities side by side, then covers thymosin alpha-1 by identifier, its salt form (the "thymalfasin acetate" question) and the calculated masses that tell the three apart on a mass-spectrometry report. Every value is the one shown on the Pepta Labs listing (checked 2026-09-24) unless it is labelled calculated.

Is thymosin alpha-1 the same as thymosin beta-4 (TB-500) or thymulin?

No. Thymosin alpha-1 is an N-acetylated 28-residue peptide, C129H215N33O55, 3108.26 Da, CAS 62304-98-7, and TB-500 as listed by Pepta Labs is N-acetylated 43-residue thymosin beta-4, C212H350N56O78S, 4963.44 Da, CAS 77591-33-4, so the two are different compounds. Thymosin alpha-1 is not thymulin either: thymulin is the 9-residue peptide pE-AKSQGGSN-OH, C33H54N12O15, 858.86 Da, CAS 78922-62-0 on the Pepta Labs listing. The three are separate peptides with different lengths, sequences, formulas, masses and CAS Registry Numbers. A certificate or listing for one of them cannot stand in for either of the others, because each identifier below points to a different molecule.

Purity figures are the values shown on each listing on 2026-09-24 for the sourced production run. Monoisotopic masses are calculated from the formulas above with standard isotope masses; they are not copied from an analytical report. The thymulin formula C33H54N12O15 is that of the metal-free nonapeptide, which is what the listing describes. Registries hold more than one record for thymulin, so its alternative CAS and PubChem entries are set out on the registry discrepancies page rather than in this table.

Do the three sequences overlap?

Barely. Thymosin alpha-1 and thymosin beta-4 share their first two residues, Ac-Ser-Asp, and no run of more than two consecutive residues anywhere else. Thymulin shares no two consecutive residues with thymosin alpha-1 at all. The composition differs just as clearly: thymosin alpha-1 has no methionine, proline, glycine or glutamine, while thymosin beta-4 carries one methionine, three prolines, one glycine and three glutamines, and thymulin carries two glycines and a cyclized glutamine at its N-terminus.

Why do the names sound alike?

"Thymosin alpha-1", "thymosin beta-4" and "thymulin" share a naming root, and nothing more. The Greek letter in a thymosin name is part of a label, not a sign of a common chain: the alpha-1 and beta-4 peptides differ in length by 15 residues and in average mass by about 1855 Da (calculated from the listing weights). The international nonproprietary names make the separation explicit: thymalfasin for thymosin alpha-1 and timbetasin for thymosin beta-4.

Which TB-500 does the question mean?

Two different materials circulate under the name TB-500. The Pepta Labs listing uses it for full-length, N-acetylated thymosin beta-4 (43 residues, CAS 77591-33-4, 4963.44 Da). PubChem resolves the bare name TB-500 to a shorter acetylated fragment, Ac-LKKTETQ (C38H68N10O14, 889.0 g/mol), and the FDA compounding page lists "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500". Neither form is thymosin alpha-1: the fragment shares no run of more than two residues with the thymosin alpha-1 chain. The CAS number and mass on an analytical report settle which TB-500 a vendor means; the TB-500 buying guide covers that listing in detail.

What is thymosin alpha-1, by identifier?

Thymosin alpha-1 is the N-acetylated 28-residue peptide Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN-OH, which corresponds to residues 2-29 of prothymosin alpha. Checking the CAS number, formula and mass together is the fastest way to confirm that a listing or a report describes this compound and not one of the two peptides above.

The composition explains two properties of the chain. Six glutamates and three aspartates, plus the free C-terminal acid, outnumber the four lysines, so the net charge is strongly negative at neutral pH. The chain also has no tryptophan, tyrosine or phenylalanine, so it absorbs weakly near 280 nm, and HPLC methods for it read the peptide bond near 214-220 nm. Identifier conventions for the whole catalog are collected in the CAS number and molecular weight table, and the sequence notation guide explains the Ac-, pE- and -OH prefixes and suffixes used above.

What does the INN thymalfasin refer to?

Thymalfasin is the international nonproprietary name (INN) for synthetic thymosin alpha-1. Both names refer to the same N-acetylated 28-residue peptide, CAS 62304-98-7, formula C129H215N33O55, molecular weight 3108.26 Da. A document that says thymalfasin and a document that says thymosin alpha-1 describe one compound, provided the CAS number and formula match.

What does the Ac- prefix change in the mass?

The Ac- prefix means the N-terminal serine carries an acetyl group, so the peptide has no free N-terminal amine. The acetyl group adds 42.011 Da to the monoisotopic mass (C2H2O), and it is already included in the formula C129H215N33O55. The des-acetyl 28-residue peptide is a different compound with a calculated monoisotopic mass of 3064.49 Da and an average mass of about 3066.24 Da, so a deconvoluted mass near 3064-3066 Da on a report does not describe thymosin alpha-1.

What is thymalfasin acetate: is thymosin alpha-1 an acetate or TFA salt?

Synthetic thymosin alpha-1 is typically supplied as an acetate or a trifluoroacetate (TFA) salt, and the Pepta Labs listing records exactly that. "Thymalfasin acetate" names the acetate salt of the same peptide: the peptide chain, sequence and CAS number of the free peptide do not change, but the salt carries counter-ions and residual water in addition to the peptide. The counter-ion appears on the analytical report only when the testing laboratory reported it.

Because the N-terminus is acetylated and the chain has no arginine or histidine, the four lysine side-chain amines are the only basic groups on thymosin alpha-1. The number of counter-ions is not stated on the listing. Lyophilized peptide salts contain the counter-ion and residual water in addition to the peptide, so the net peptide content is lower than the total solid mass. The TFA versus acetate guide sets out that arithmetic, and how net peptide content is determined explains why that figure is separate from HPLC purity.

How do the masses tell thymosin alpha-1, TB-500 and thymulin apart?

An electrospray mass spectrum separates the three compounds at a glance, because their masses differ by hundreds to thousands of daltons. A peptide of about 3.1 kDa such as thymosin alpha-1 typically shows multiply charged ions that deconvolute to the expected mass; the report states the observed value. The figures below are calculated from each formula (monoisotopic isotope masses, proton 1.00728 Da), not copied from a Pepta Labs report.

For peptides of this size the monoisotopic peak is small and instruments often report the average or most abundant mass instead, so compare a deconvoluted value with the listing weight (3108.26 Da for thymosin alpha-1) as well as with the monoisotopic figure. A satellite peak about 114 Da above the main mass is a trifluoroacetate adduct, common for TFA salts, not a second peptide. The expected mass table gives the same calculated columns for the rest of the catalog, and HPLC versus mass spectrometry explains why a report needs both methods.

Where are the price, report and ordering details for thymosin alpha-1?

The Pepta Labs listing is Thymosin Alpha-1 10mg from $50.86, a supplier-stated 10 mg of lyophilized powder per sealed glass vial, offered singly and in 3-vial and 5-vial packs. Pack prices are shown on the product page, and the price per mg of every listing sits on the research peptide price per mg page, so this article types no price.

• Analytical report: Independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house. The reported RP-HPLC purity for the sourced production run is 99.1%, as shown on the listing on 2026-09-24. The report is sent by email on request, before you order if you want to see it first: use the report request form or email support@peptalabs.com. The CoA verification guide walks through each field.
• What the report does not claim: Sterility, endotoxin or LAL, potency, pharmaceutical-grade, cGMP, contaminant-panel and vial fill-quantity testing are not claimed. Vial labels identify the compound, quantity and research-use designation. They do not carry lot or batch numbers. The report covers the sampled production run, not an individual vial. The full arrangement is on the Testing and Documentation page.
• Ordering: Orders are placed from a free account with a confirmed email, so every order traces back to a real customer. Buyers must be 21 or older and complete an electronic research-use certification at checkout. Payment methods are shown at checkout. The cart shows the payment method for each item.
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• Storage: The dry powder ships at ambient temperature in a sealed vial. Storage conditions for sealed vials of lyophilized peptide are in the peptide storage guide.

Switching from a closed vendor?

Eternal Peptides listed Thymosin Alpha 1 before it closed its website on September 15, 2026; Pepta Labs lists thymosin alpha-1 under the same compound name. Matched listings and the current switching offer are on the switching page, and the background is in Eternal Peptides shut down and Peptide Sciences shut down. If the research peptide category itself is new to you, what research peptides are explains how these materials are sold and documented.

What is the US regulatory listing for thymosin alpha-1?

On an FDA compounding web page (content current as of April 22, 2026), "Thymosin-alpha 1 (Ta1)" appears in the list of bulk substances nominated but withdrawn by the nominators, which the FDA introduces as substances previously in category 2 of the interim policies; the dated row and its context are on the FDA Category 2 peptides 2026 page.

Frequently asked questions

Is thymosin alpha-1 the same as TB-500?

No. Thymosin alpha-1 is an N-acetylated 28-residue peptide, C129H215N33O55, 3108.26 Da, CAS 62304-98-7. TB-500 as listed by Pepta Labs is N-acetylated 43-residue thymosin beta-4, C212H350N56O78S, 4963.44 Da, CAS 77591-33-4. The two chains begin with the same two residues, Ac-Ser-Asp, share no run of more than two consecutive residues, and have only the historical name thymosin in common.

Is thymalfasin the same as thymosin alpha-1?

Yes. Thymalfasin is the INN for synthetic thymosin alpha-1; both names refer to CAS 62304-98-7, formula C129H215N33O55 and molecular weight 3108.26 Da. "Thymalfasin acetate" is the acetate salt of the same peptide.

Is thymulin the same as thymosin alpha-1?

No. Thymulin is a 9-residue peptide with a pyroglutamate N-terminus, pE-AKSQGGSN-OH, C33H54N12O15, 858.86 Da, CAS 78922-62-0 on the Pepta Labs listing. Thymosin alpha-1 is a 28-residue acetylated peptide of 3108.26 Da, CAS 62304-98-7. The two share the thym- name root only, not two consecutive residues.

How many residues does thymosin alpha-1 have?

Thymosin alpha-1 has 28 amino-acid residues: Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN-OH. Its composition is six glutamates, four lysines, three each of aspartate, alanine, serine, threonine and valine, and one each of isoleucine, leucine and asparagine.

Is thymosin alpha-1 acetylated at the N-terminus?

Yes. The N-terminal serine carries an acetyl group (the Ac- prefix), so the peptide has no free N-terminal amine. The acetyl group adds 42.011 Da and is included in the formula C129H215N33O55; the des-acetyl peptide, at a calculated 3064.49 Da monoisotopic, is a different compound.

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