TB-4 Fragment (Ac-SDKP)

1 Identity

Synthetic N-acetylated tetrapeptide, all-L, supplied as the free acid or as an acetate or trifluoroacetate salt. It is the natural N-terminal tetrapeptide of thymosin beta-4, released in vivo by prolyl oligopeptidase; that is a statement about where the sequence comes from, not about anything it does. The N-terminal acetyl group is part of the molecule rather than a synthesis protecting group left in place. No cysteine, no disulfide, no non-proteinogenic residue, no D-residue, no metal. All of those names denote one molecule.

Sequence

Ac-Ser-Asp-Lys-Pro-OH; written with the cap, Ac-SDKP-OH. Four residues, all L. The N-terminus is acetylated on the serine alpha-amino nitrogen and the acetyl is localized by MS/MS b1/b2 assignment rather than inferred from intact mass. The C-terminus is the free acid of proline, not an amide. No cysteine, no disulfide. The Asp2-Lys3 amide is the succinimide-forming bond and is the analytical center of gravity of this whole record: aspartyl-to-iso-aspartyl rearrangement there is exactly mass-silent. This is thymosin beta-4 residues 1 to 4, and the same molecule carries the development names goralatide and Seraspenide.

Molecular formula

C20H33N5O9 (free acid). Acetate salt C22H37N5O11; trifluoroacetate salt C22H34F3N5O11.

Average mass

Average mass 487.510 Da for the free acid, C20H33N5O9. Research-supplier listings publish MW 487.5 for the same formula. Mono-acetate salt gross 547.562; mono-TFA salt gross 601.533.

Monoisotopic mass

487.22783 Da free acid; [M+H]+ 488.23510; [M-H]- 486.22055. Mono-acetate 547.24896; mono-TFA 601.22069. A nominal '487' distinguishes nothing on this record and must not be accepted as identity.

Salt / variant note

Free acid C20H33N5O9 487.510 / 487.22783 is the parent, and research-supplier catalogs list the TFA and acetate forms as separate articles - three distinct powders under one product name. Mono-acetate salt gross C22H37N5O11 547.562 / 547.24896, free acid 89.0 percent of the powder. Mono-TFA salt gross C22H34F3N5O11 601.533 / 601.22069, free acid only 81.0 percent - so an HPLC area percent of 99 says nothing whatever about the other 19 percent of the vial. Des-acetyl SDKP C18H31N5O8 445.473 / 445.21726 is 42.04 Da light: a different molecule, a simpler synthesis, and '445.5' under this product name IS that molecule. Ac-SDKP-NH2 C20H34N6O8 486.526 / 486.24381 is 0.98 Da light - an amide where a free acid was declared, invisible to any nominal-mass instrument reporting '487'. N-formyl-SDKP C19H31N5O9 473.483 / 473.21218 is 14.03 Da light and arises from DMF-borne formate. N-trifluoroacetyl-SDKP is C20H30F3N5O9 at 541.481 / 541.19956, i.e. +53.97 Da over Ac-SDKP; a figure of 542.49 / 542.207 for that variant is still in circulation and is 1.01 Da high. The most dangerous variant carries no mass difference at all: the iso-aspartyl and D-iso-aspartyl rearrangement products at the Asp2-Lys3 bond are exactly isobaric at 487.510 / 487.22783 and are detectable only chromatographically; the succinimide intermediate is 18.02 Da light at 469.49, and a stability-indicating gradient that resolves the rearranged acid but not the cyclic intermediate is only half-validated. Finally the name collision inside this catalog, which is the commonest real-world error on this record: material sold as 'TB-4 fragment' is frequently the heptapeptide Ac-LKKTETQ-OH at 889.018 / 888.49165 or the acetylated 43-mer at 4963.506 / 4960.48632, and either number under this product name is a different molecule in a vial with this label on it.

2 Class & testing panel

Form

Single article

Testing panel

P1 — panel definition

3 Primary sources & evidence

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

Substantial in-vitro and rodent literature accumulated over roughly three decades, concentrated in a small number of academic groups working on angiotensin-converting-enzyme N-domain substrate biology and on cardiac and renal fibrosis models. Human evidence is limited to a small number of early-phase studies run in the 1990s under the development name goralatide in a hematology program that was discontinued; the citation index carries those with their study type, phase and outcome, including the discontinuation, in the same typeface as everything else. No controlled human trial addresses the uses that drive current research-market interest, and no product containing this molecule is approved anywhere. Absences are stated as absences: no registered active clinical trial under either the goralatide or Ac-SDKP name was located; no modern pharmacokinetic characterization was located; and no formal toxicology or genotoxicity package was located. Legitimate open work follows from the gap, and the one this record is analytically equipped for is the iso-aspartyl question: the rearrangement rate at Asp2-Lys3 under defined pH and temperature has not been published for this tetrapeptide and is directly measurable with the stability-indicating method written into the specification.

4 Storage & specification

Storage

Lyophilized white powder in the sealed original vial, supplied as the free acid or the acetate salt with the form stated on the label and on the certificate. Labeled storage -20 degrees C plus or minus 5 degrees C, desiccated, protected from light. Hygroscopic: the SOP requires the container to be equilibrated to room temperature inside the desiccator before opening, because condensation on a hygroscopic short peptide is the most common way a good lot is ruined after release. Reconstituted solutions are held at 2-8 degrees C, buffered rather than left in unbuffered water, and treated as single-use - the Asp2-Lys3 succinimide route is pH- and temperature-driven and an aqueous stock left at room temperature is the practical way to generate the one degradant that no mass measurement will find. Shipped on gel packs with a data logger in every shipper; excursion limits are set in the shipping validation, not by the courier.

Shelf life

Provisional retest interval 24 months at -20 degrees C plus or minus 5 degrees C in the sealed original container, stated as provisional pending this company's own stability data. The confirming study is long-term at -20 degrees C plus or minus 5 degrees C with pulls at 0, 3, 6, 9, 12, 18 and 24 months, accelerated at 5 plus or minus 3 degrees C and at 25 degrees C / 60 percent relative humidity, assayed by a stability-indicating RP-HPLC gradient validated to resolve the iso-aspartyl degradant. Total iso-aspartyl is the named trended attribute and the interval is shortened if it exceeds the protocol trigger; water content is trended alongside it because the salt is hygroscopic and water is what drives the rearrangement. The label carries a retest date, not an expiry date, and the interval is shortened without notice if a pull fails. Material reaching retest is re-assayed for purity, water, counterion and net peptide content and is either re-dated for a further 12 months or destroyed and recorded on the public failed-lot ledger.