TB-500 / Thymosin Beta-4

1 Identity

The declared article on this record is full-length thymosin beta-4: a 43-residue, N-terminally acetylated, intrinsically disordered peptide with a C-terminal free acid, one methionine, no cysteine, no disulfide, no tyrosine and no tryptophan anywhere in the chain. The material most commonly supplied in the retail channel under the same name is instead the synthetic seven-residue fragment Tb4(17-23), Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH. Both are all-L with no non-proteinogenic residues. At 43 residues the full-length peptide is above the 40-residue threshold in the statutory definition of a protein and the biologics analysis has to be run on it; at seven residues the fragment is a drug. Whichever is cataloged is declared by sequence and residue count on the label and on the certificate, because a name that does not determine a molecular identity cannot be released against a specification. The fragment is carried on the separate ABP-7 / TB-500 Fragment 17-23 record, so that the catalog holds one molecule per record. The last two name the seven-residue fragment, which is a different molecule from the full-length peptide and is carried on its own record.

Sequence

(a) full-length thymosin beta-4, the declared article: Ac-Ser-Asp-Lys-Pro-Asp-Met-Ala-Glu-Ile-Glu-Lys-Phe-Asp-Lys-Ser-Lys-Leu-Lys-Lys-Thr-Glu-Thr-Gln-Glu-Lys-Asn-Pro-Leu-Pro-Ser-Lys-Glu-Thr-Ile-Glu-Gln-Glu-Lys-Gln-Ala-Gly-Glu-Ser-OH; one-letter Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES-OH. Forty-three residues, all L; Met6 is the single oxidation-labile residue; no cysteine, no disulfide; no aromatic residue other than Phe12, which is why the deletion-peptide mass ceiling on this chain is 147.18 Da and not the 163 Da figure a template would supply. (b) the fragment most commonly supplied under the same name, Tb4(17-23): Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH; one-letter Ac-LKKTETQ-OH. Seven residues, all L, N-acetylated, C-terminal free acid. Note that residues 1 to 4 of chain (a) are the entire molecule of the separate Ac-SDKP record.

Molecular formula

C212H350N56O78S (acetylated 43-mer, free acid); C38H68N10O14 (acetylated heptapeptide, free acid). Both normally supplied as acetate salts.

Average mass

4963.506 Da (C212H350N56O78S, acetylated 43-mer) and 889.018 Da (C38H68N10O14, acetylated heptapeptide) — one name, two molecules, a 5.58-to-1 mass ratio. The unacetylated 43-mer, which is what a recombinant lot usually shows and what general reference works usually print, is 4921.469.

Monoisotopic mass

43-mer 4960.48632 Da acetylated, to be used with a deconvoluted charge envelope rather than a single-ion match — for example [M+4H]4+ 1241.12886, [M+5H]5+ 993.10454. Heptapeptide 888.49165 Da acetylated; [M+H]+ 889.49892.

Salt / variant note

The two-molecule pattern shows up in sellers' own listings: a research supplier and a retail seller both use the name TB-500 for molecules 4,074.49 Da apart, and 5.58 heptapeptides fit inside one 43-mer by mass. Acetylated 43-mer C212H350N56O78S 4963.506 / 4960.48632. Unacetylated 43-mer C210H348N56O77S 4921.469 / 4918.47575, 42.04 Da light — the figure general reference works print for the bare sequence and what recombinant material will usually show. Acetylated heptapeptide C38H68N10O14 889.018 / 888.49165. Unacetylated heptapeptide C36H66N10O13 846.981 / 846.48108. Ac-LKKTETQ-NH2 C38H69N11O13 888.034 / 887.50763, 0.98 Da light, an amide where a free acid was declared. Methionine-6 sulfoxide on the 43-mer C212H350N56O79S 4979.505 / 4976.48123, +16.00, the dominant degradant and invisible inside an intact-mass window wider than a few ppm. The shorter actin-motif constructs also circulate: LKKTET-OH C31H58N8O11 718.85 / 718.42250 and Ac-LKKTET-OH C33H60N8O12 760.89 / 760.43306. Ac-LKKTE, the metabolite named in the 2024 metabolism literature, is C29H53N7O10 659.782 / 659.38539. Material in this channel is commonly supplied as an acetate salt, so gross mass and net peptide content differ from the free acid. Deletion peptides in the 43-mer span 57.05 Da (Gly) to 147.18 Da (Phe12) — the ceiling is phenylalanine, not the 163 Da a template supplies, because this sequence contains no tyrosine and no tryptophan at all. The isobaric trap that sits inside this catalog: acetyl hexapeptide-8 (Argireline), C34H60N14O12S, 889.000 / 888.42358, is 0.02 Da from the heptapeptide on average mass and 0.068 Da — 77 ppm — on monoisotopic. Trivially resolved by HRMS at 10 ppm; completely invisible at unit resolution; and both are sold in the same catalogs at similar per-milligram prices. Ac-SDKP at 487.510 is this same parent's residues 1 to 4 and is sold under overlapping names in both directions. BPC-157 C62H98N16O22 1419.556 / 1418.70416 is the commonest blend partner and the commonest co-contaminant.

2 Class & testing panel

Form

Single article

Testing panel

P1 — panel definitionfor either synthetic article. Any lot offered as recombinant material moves the record to P4 in full: expression host declared, host-cell protein ELISA, residual host-cell DNA, and SE-HPLC for aggregates. The certificate for the 43-mer carries an aggregate limit by SE-HPLC, because aggregation is the specific hazard FDA names for this substance

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

The two molecules do not share an evidence base and the citation index will not let one borrow the other's references. Full-length thymosin beta-4 has the credible human data, and it is ophthalmic: Sosne et al., Cornea 2015, randomized Phase 2 in severe dry eye, DOI 10.1097/ico.0000000000000379; Sosne et al., Int J Mol Sci 2022, randomized placebo-controlled double-masked Phase 3 of 0.1 percent RGN-259 in neurotrophic keratopathy, DOI 10.3390/ijms24010554; ARISE-3, NCT03937882, registered Phase 3 in dry eye; Kim et al., Sci Rep 2018, animal dry-eye model, DOI 10.1038/s41598-018-28861-5. Sosne et al., FASEB J 2010, PMID 20179146, is the review identifying LKKTETQ (residues 17-23) as the central actin-binding domain; Shah et al., Expert Opin Biol Ther 2018, PMID 30063851, is in vitro in human hepatic stellate cells. The fragment has no human trial at all: none was identified in any registry or publication. Its base is in vitro, rodent and equine — Esposito 2012 on synthesis and analytical characterization as a doping-control reference standard; Ho 2012 and Kwok 2013 on detection and quantification in equine plasma and urine. And the attribution is contested in the primary literature: Rahaman 2024, an analytical and metabolic study in vitro and in rat, reported that the metabolite Ac-LKKTE — not the administered parent heptapeptide — showed the fibroblast activity, suggesting previously observed effects were attributable to metabolites rather than to LKKTETQ itself. There is no established receptor for the fragment, and even the assumption that the sold molecule is the active species is disputed. Every ophthalmology result above belongs to the parent peptide, and citing it in fragment marketing is misattribution.

4 Storage & specification

Storage

The labeled condition is: lyophilized solid in the sealed original vial, -20 degrees C plus or minus 5 degrees C, desiccated, protected from light, headspace displaced with nitrogen and re-blanketed after any subdivision. Both candidate molecules are hygroscopic and the 43-mer is intrinsically disordered with no disulfide to hold structure, so neither is stored above -20 degrees C at any point in the chain and the container is equilibrated to room temperature inside the desiccator before opening. The nitrogen blanket and light protection are the specific controls for Met6 oxidation on the 43-mer and the label states that reason. An intrinsically disordered 4.9 kDa peptide adsorbs to glass at low concentration, so container-closure qualification includes a measured recovery check on the actual vial and stopper rather than an assumption carried from another product. Reconstituted material is treated as single-use and freeze-thaw cycling is recorded rather than assumed harmless, because aggregation is the named regulatory concern on this substance. Shipped on dry ice or in a validated shipper with a single-use electronic temperature logger in every shipper.

Shelf life

Provisional retest interval 24 months at -20 degrees C plus or minus 5 degrees C, provisional pending this company's own stability data. The protocol is long-term at -20 degrees C with pulls at 0, 3, 6, 9, 12, 18, 24 and 36 months, a parallel arm at 5 plus or minus 3 degrees C, and for the 43-mer a named high-molecular-weight-species trend by SE-HPLC alongside total oxidized species; either trend breaching its protocol trigger shortens the interval without notice. The label carries a retest date, not an expiry date. Material reaching retest is re-assayed and either re-dated for 12 months or destroyed and recorded on the public failed-lot ledger.