SS-31 (Elamipretide, MTP-131): D-Arg-Dmt-Lys-Phe-NH2 Sequence, CAS and Counter-Ions

Yes: SS-31, elamipretide and MTP-131 are the same compound, the synthetic tetrapeptide amide H-D-Arg-Dmt-Lys-Phe-NH2, formula C32H49N9O5, molecular weight 639.78 g/mol. Elamipretide is its INN and USAN, SS-31 and MTP-131 are research codes, and all three names resolve to one set of registry identifiers: CAS 736992-21-5, PubChem CID 11764719 and FDA UNII 87GWG91S09. This page sets out that equivalence against the registries, explains the Dmt residue and the D-arginine in the sequence, and gives the counter-ion arithmetic for the acetate and trifluoroacetate salts.

Is SS-31 the same as elamipretide and MTP-131?

Yes. SS-31 is the same compound as elamipretide: SS-31 is a research code and elamipretide the INN and USAN for the tetrapeptide amide H-D-Arg-Dmt-Lys-Phe-NH2, C32H49N9O5, 639.78 Da, CAS 736992-21-5. MTP-131, RX-31 and Szeto-Schiller peptide 31 are three more names for the same molecule, five names in all. The test of an alias is the registry record: every name below maps to the same structure, the same formula and the same CAS, PubChem and UNII numbers, so a listing or certificate under any of these names should print the same values.

The identifiers below are the ones to match. The formula, CAS number and PubChem CID printed on the Pepta Labs listing match the registry records, and the listing's 639.78 g/mol rounds to PubChem's 639.8, so for SS-31 the listing and the registries agree.

CAS 736992-21-5 identifies the free peptide. Salt forms are generally registered separately from the parent structure, so a certificate that prints CAS 736992-21-5 beside the word acetate is naming the parent peptide and its counter-ion, which is normal practice. Registry numbers for the rest of the catalog are collected in the CAS number and molecular weight table, and the research peptides overview explains how a compound is identified by name, sequence and registry number.

What does Dmt mean in the SS-31 sequence?

Dmt is the three-letter abbreviation for 2',6'-dimethyl-L-tyrosine, a tyrosine analogue with two methyl groups on its aromatic ring, and it is the second residue of SS-31. The hydroxyl sits at ring position 4', and the methyl groups occupy positions 2' and 6', the two ring carbons on either side of the carbon that joins the ring to the side-chain CH2. Dmt is not one of the 20 standard amino acids and has no one-letter code, which is why SS-31 is written in three-letter form.

The two methyl groups add C2H4 to tyrosine, 28.031 Da monoisotopic. That is the whole difference between SS-31 (C32H49N9O5, 639.386 Da monoisotopic) and Arg-Tyr-Lys-Phe-NH2 (C30H45N9O5, 611.354 Da monoisotopic), and it is the first figure to check when a report or listing gives a formula.

The D- prefix records configuration at the alpha carbon only: D-arginine has the same formula and mass as L-arginine. The -NH2 suffix does change the formula; the same four residues ending in a free acid would be C32H48N8O6, 640.370 Da monoisotopic, 0.984 Da heavier than SS-31. The peptide sequence notation guide covers these prefixes and suffixes across the catalog. In solid-phase synthesis, D-arginine and Dmt each enter the chain as a protected amino-acid building block, and the C-terminal amide is set by the resin chosen at the start; how research peptides are made walks through those steps.

Is SS-31 sold as an acetate or a TFA salt?

SS-31 is isolated as a salt, usually the acetate or the trifluoroacetate (TFA), because the molecule has three basic groups: the free N-terminal amine, the arginine guanidine and the lysine side-chain amine. Each protonated group pairs with one anion, so a fully occupied salt carries three counter-ions per peptide. Trifluoroacetate comes from the cleavage and reversed-phase purification steps of synthesis; an acetate salt is produced by exchanging it. The name SS-31 does not by itself say which salt form is meant.

The counter-ions add mass. Using 60.052 g/mol for acetic acid and 114.022 g/mol for trifluoroacetic acid, a fully occupied tri-acetate has a formula weight of about 819.9 g/mol and a tri-trifluoroacetate about 981.8 g/mol, against 639.78 g/mol for the free base. The table gives the calculated free-base share of the salt mass at each level of occupancy, before any bound water is counted.

This is why net peptide content is a separate figure from HPLC purity. Purity is the main peak's share of the chromatogram's total peak area and does not see counter-ion or water, so a high purity figure and a free-base share between about 65% and 78% describe different things and do not contradict each other. Net peptide content is not part of the HPLC and mass-spectrometry reporting for Pepta Labs listings. The TFA vs acetate guide sets out the same arithmetic for other catalog compounds.

What mass should an SS-31 mass spectrum show?

An electrospray mass spectrum of SS-31 should show the protonated molecule [M+H]+ at a calculated m/z of 640.393, the monoisotopic mass 639.386 Da plus one proton. Because the molecule has three basic sites, doubly and triply charged ions are also common. All values below are calculated from the formula, not measured; the observed mass for the sampled production run is the value printed on the report.

An observed [M+H]+ within about 0.02 of 640.39 on a high-resolution instrument, or within about 0.5 on a unit-resolution one, is consistent with C32H49N9O5. Mass spectrometry cannot show two things: the configuration of the arginine, because D- and L-arginine have identical masses, and the counter-ion or water content of the powder. The expected mass table lists the same calculation for the other catalog compounds with verified identity records.

Which SS-31 listings does Pepta Labs carry?

Pepta Labs lists SS-31 in two sizes, a supplier-stated 25 mg and 50 mg of lyophilized powder per sealed glass vial, both in the BPC and thymosin category. Each product page shows the reported RP-HPLC purity for the sampled production run.

Each SS-31 size with a live listing:

Prices are read from the catalog when this page loads; price per mg divides the single-unit price by the supplier-stated quantity. Multi-unit pack prices are shown on each product page.

Price per mg for both sizes is also tabulated, next to every other non-incretin listing, on the research peptide price per mg page.

What is tested: 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 report covers the most recent sampled run on file; vials carry no lot number. It is sent by email on request, before you order if you want to see it first, through the report request form.

Where are storage, verification and FDA list status covered?

• Storage: sealed vials ship dry at ambient temperature and are kept at 2-8 C on arrival or -20 C for long-term storage, protected from light and moisture. See the peptide storage guide.
• Reading the report: the COA verification guide and HPLC vs mass spec explain each field, including why a report describes a sampled run rather than an individual vial.
• FDA compounding lists: SS-31 does not appear in the FDA section 503A bulk-substance nominations document (updated May 14, 2026) or on the FDA Category 2 bulk-substance list. The wider US framework is in Peptide Legal Status 2026 and FDA Category 2 peptides.

Frequently asked questions

Is SS-31 the same as elamipretide?

Yes. SS-31 is the research code and elamipretide the INN and USAN for one compound, H-D-Arg-Dmt-Lys-Phe-NH2, C32H49N9O5, 639.78 Da, CAS 736992-21-5, PubChem CID 11764719, UNII 87GWG91S09; a listing or certificate under either name should print the same values.

Are SS-31, elamipretide and MTP-131 the same compound?

Yes. Elamipretide is the INN and USAN, and SS-31, MTP-131 and RX-31 are research codes; all refer to H-D-Arg-Dmt-Lys-Phe-NH2, C32H49N9O5, 639.78 g/mol, CAS 736992-21-5, PubChem CID 11764719, UNII 87GWG91S09.

What is MTP-131?

MTP-131 is a research code for elamipretide, the tetrapeptide amide H-D-Arg-Dmt-Lys-Phe-NH2 also coded SS-31. It names the same structure, formula (C32H49N9O5) and CAS number (736992-21-5), not a different salt or sequence.

What is the CAS number of SS-31?

The CAS Registry Number of SS-31 (elamipretide) is 736992-21-5, for the free peptide C32H49N9O5 at 639.78 g/mol. The same compound is PubChem CID 11764719 and FDA UNII 87GWG91S09.

What does Dmt mean in the SS-31 sequence?

Dmt is 2',6'-dimethyl-L-tyrosine, a non-standard residue with two methyl groups on the tyrosine ring at positions 2' and 6'. It adds C2H4 (28.03 Da) to tyrosine, which is why SS-31 is C32H49N9O5 while Arg-Tyr-Lys-Phe-NH2 is C30H45N9O5.

Why does SS-31 carry several counter-ions per molecule?

SS-31 has three basic sites, the N-terminal amine, the arginine guanidine and the lysine side-chain amine, and each pairs with one anion in the isolated salt. A fully occupied tri-acetate is about 819.9 g/mol and a tri-trifluoroacetate about 981.8 g/mol, against 639.78 g/mol for the free base.

Can mass spectrometry confirm the D-arginine?

No. D- and L-arginine have identical masses, so mass spectrometry confirms the formula C32H49N9O5 (calculated [M+H]+ 640.393) but not the configuration of the arginine. Configuration is established by chiral amino-acid analysis, which is not part of the HPLC and mass-spectrometry reporting for Pepta Labs listings.

All products are supplied for laboratory research use only and are not for human or animal use.