SS-31

What is SS-31?

SS-31, also known as elamipretide and by the research code MTP-131, is a synthetic tetrapeptide with the sequence D-Arg-dimethylTyr-Lys-Phe-NH2. It belongs to the Szeto-Schiller (SS) family of cell-permeable, mitochondria-targeted peptides, whose defining property is that they concentrate in the inner mitochondrial membrane without requiring a membrane potential to get there.

The alternating cationic and aromatic residue pattern is what drives that localization, and the D-arginine and dimethyltyrosine modifications confer resistance to peptidase cleavage. The molecule is unusually short for a signaling peptide, which makes it comparatively easy to synthesize to high purity but also means that a single truncation or epimerization is a proportionally large structural change.

PX1 Research supplies lyophilized SS-31 as a reference compound for mitochondrial bioenergetics, oxidative-stress and cardiolipin research, released against a batch-specific certificate of analysis. Research use only — not for human or veterinary use.

Mechanism of action

The described mechanism centers on cardiolipin, the signature phospholipid of the inner mitochondrial membrane. SS-31 associates with cardiolipin and, in published models, stabilizes the curvature and organization of cristae membranes, which in turn is associated with improved efficiency of the electron transport chain supercomplexes that depend on that membrane architecture.

Downstream, reports describe reduced electron leak and reduced reactive oxygen species generation, along with preserved ATP synthesis under stress conditions. Importantly the literature frames SS-31 as acting on membrane structure rather than as a conventional antioxidant scavenger — the reduction in oxidative species is presented as a consequence of a better-organized respiratory chain.

Because the mechanism is structural and local, SS-31 effects in cell models are typically read out through respirometry (oxygen consumption rate), membrane-potential dyes, cristae morphology by electron microscopy, and cardiolipin-binding assays rather than through receptor pharmacology.

SS-31 vs MOTS-c

SS-31 and MOTS-c are frequently stocked together and just as frequently conflated, but they act at opposite ends of the mitochondrial signaling problem. SS-31 is an exogenous synthetic tetrapeptide that concentrates inside the organelle and acts on membrane and respiratory-chain integrity. MOTS-c is an endogenous mitochondrially encoded 16-mer that translocates outward to the nucleus and alters nuclear gene expression under metabolic stress.

A study pairing them is usually testing local structural protection against global transcriptional adaptation, and the readouts differ accordingly — respirometry and cristae imaging for SS-31, AMPK signaling and transcript panels for MOTS-c.

Their analytical profiles also differ. The four-residue SS-31 sequence with non-canonical residues requires a method that resolves D/L epimers, while the longer MOTS-c sequence is most vulnerable to truncation impurities. Neither is adequately characterized by a generic method.

Analytical characterization

Laboratories that work with SS-31 typically characterize incoming material on three axes before it enters a study: identity, purity and content. Identity is established by high-resolution mass spectrometry against the theoretical monoisotopic mass (639.8 Da for the tetrapeptide amide), usually supported by MS/MS fragmentation that walks the backbone and confirms the sequence rather than just the total mass. A matching intact mass alone can be satisfied by a scrambled or partially epimerized sequence, which is why fragmentation data is the stronger identity evidence.

Purity is quantified by reversed-phase HPLC with UV detection, integrating every resolved peak in the chromatogram and expressing the main peak as a percentage of total area. The gradient matters more than the headline number: a shallow, well-optimized gradient resolves closely eluting process impurities such as deamidation products, oxidation variants, truncated sequences and acetate adducts, while an aggressive gradient can co-elute them under the main peak and inflate the reported purity. PX1 publishes the chromatogram itself, not only the integrated figure, so the resolution behind the number is auditable.

Content — how much peptide is actually in the vial once counter-ions and residual water are subtracted — is the axis most often skipped by low-cost suppliers. Net peptide content is a function of the labeled mass, the water content measured by Karl Fischer titration, and the counter-ion (typically trifluoroacetate or acetate) load. A vial that is 99% pure by HPLC can still under-deliver on content if it carries a high salt and moisture fraction, which is why the COA reports both.

Solubility, reconstitution and stability behavior

SS-31 is supplied as a lyophilized white powder. The lyophilized cake is the most stable form of the molecule and should be kept sealed at −20°C or below, protected from light, until the study begins. The single most common handling error in a research setting is opening a cold vial: atmospheric moisture condenses onto the cake the moment the stopper is broken, and that water starts hydrolytic degradation before reconstitution has even happened. Always equilibrate the sealed vial to room temperature first.

Reconstitution should be performed aseptically with bacteriostatic or sterile water for research reconstitution, introduced slowly down the inner wall of the vial rather than streamed directly onto the cake. Swirl — never shake. Peptides are surface-active, and vigorous agitation drives them to the air-liquid interface where they unfold and aggregate; visible foaming is a sign that material has already been lost to interfacial denaturation. Full dissolution to a clear, particle-free solution normally takes under a minute of gentle swirling.

Once in solution the molecule is far more labile than it was as a powder. Reconstituted SS-31 should be held at 2–8°C, protected from light, and aliquoted immediately into single-use volumes so that the working stock is never subjected to repeated freeze-thaw cycling. Each freeze-thaw cycle contributes measurable loss through aggregation and adsorption to container surfaces. Low-binding polypropylene tubes reduce adsorptive loss at dilute concentrations, and a carrier protein is commonly added to very dilute working solutions for the same reason.

Purity and Certificate of Analysis (COA)

PX1 SS-31 is USA-manufactured and released at ≥99% purity by reversed-phase HPLC with high-resolution LC-MS identity confirmation, endotoxin by kinetic chromogenic LAL, residual solvents by GC and water content by Karl Fischer titration. Because the sequence contains non-canonical residues, identity confirmation by fragmentation — not intact mass alone — is the meaningful check, and the batch report published on this page reflects that.

Sourcing, provenance and what separates lab-grade material

The research-peptide market is unusually wide in quality. The same nominal SS-31 listing can represent USA-manufactured material released against a documented specification, or repackaged bulk of unknown origin with a generic certificate that was never generated from the lot in the vial. The distinction is invisible from the product photo and only becomes visible in the paperwork.

The practical test is traceability: the lot number printed on the vial label should appear on the certificate of analysis, and that certificate should show the actual chromatogram and mass spectrum for that lot rather than a representative example. A COA without a lot number, without instrument traces, or dated years before the vial was filled is a document, not evidence. PX1 publishes the batch-specific report directly on the product page so the chain from manufacturing to vial is checkable before purchase.

Beyond the certificate, consistent lab-grade supply depends on synthesis and release happening under one controlled process: domestic solid-phase manufacturing, preparative HPLC purification, lyophilization under validated cycle parameters, and third-party confirmation of purity and endotoxin. SS-31 sold by PX1 Research is produced and released on that pathway and is supplied strictly for laboratory research use — not for human or veterinary use.

Study design considerations

Respirometry is the anchor readout. Extracellular flux analysis giving basal respiration, ATP-linked respiration, maximal capacity and proton leak provides a mechanistically interpretable profile, and the proton-leak term in particular speaks directly to the membrane-organization hypothesis. A single ATP measurement, by contrast, cannot distinguish improved coupling from increased substrate supply.

Model state matters more here than in most compound classes. The described benefit concerns damaged or stressed mitochondria, so a study run in healthy, unstressed cells may show little because there is little to restore. Including an injury or stress arm — ischemia-reperfusion, oxidative challenge or a genetic mitochondrial defect — is generally necessary for the design to be capable of detecting the reported effect.

Cardiolipin should be measured where the mechanism is being tested rather than assumed. Cardiolipin content and oxidation state by lipidomics, together with cristae morphology by transmission electron microscopy, are the endpoints that distinguish the structural mechanism from a generic antioxidant effect. Pairing SS-31 with a conventional scavenging antioxidant as a comparator arm sharpens that distinction further.

Because the tetrapeptide accumulates in mitochondria, exposure inside the organelle can differ greatly from concentration in the medium, and nominal medium concentration is therefore a poor proxy for target exposure. Studies that quantify intracellular or mitochondrial accumulation produce results that transfer between laboratories far better than those that do not.

On the chemistry side, the non-canonical residues mean that identity confirmation should be part of study documentation rather than left to the supplier's summary. Stating the lot, the measured purity, the confirmation method and the counter-ion in the methods section is the practical minimum for a compound whose stereochemistry cannot be checked by mass alone.

Common research questions about SS-31

Is SS-31 an antioxidant? Not in the conventional scavenging sense, and the distinction matters for study design. The published mechanism is structural: association with cardiolipin stabilizes inner-membrane and cristae organization, which improves the efficiency of the respiratory chain supercomplexes embedded there, and the reduction in reactive oxygen species follows as a consequence of less electron leak. Comparing SS-31 against a scavenging antioxidant therefore tests two different hypotheses.

Why does SS-31 concentrate in mitochondria without a membrane potential? The alternating cationic and aromatic residue pattern drives partitioning into the inner membrane independent of the potential gradient, which is the property that distinguishes the Szeto-Schiller family from potential-dependent targeting strategies such as triphenylphosphonium conjugation. In a depolarized or damaged mitochondrion — precisely the condition of interest in most models — potential-dependent targeting fails and this approach does not.

What is elamipretide, and is it a different compound? Elamipretide is the assigned nonproprietary name for the same molecule; MTP-131 and Bendavia are development codes for it. All four names appear in the literature and refer to the identical D-Arg-dimethylTyr-Lys-Phe-NH2 tetrapeptide.

What analytical issue is specific to this sequence? Non-canonical residues. The molecule contains a D-amino acid, a dimethylated tyrosine and a C-terminal amide, and each is a point where synthesis can go subtly wrong in a way that intact mass will not reveal — an L-arginine epimer has the same mass as the D form. Chiral or fragmentation-based confirmation is the check that matters here, not the mass number alone.

What readouts are standard? Oxygen consumption rate by extracellular flux respirometry, ATP output, membrane-potential dyes, cardiolipin-binding assays, and cristae morphology by transmission electron microscopy. Cross-linking mass spectrometry has also been used to map the protein interaction landscape of the peptide inside the organelle.

Where SS-31 sits in the PX1 catalog

SS-31 anchors the mitochondrial group. MOTS-c is its natural counterpart — local membrane protection versus global transcriptional adaptation — and NAD+ completes the trio by addressing redox cofactor availability. Study panels covering mitochondrial function usually include all three because each reports on a different failure mode.

For aging-phenotype work the panel extends to Epithalon and the bio-regulator peptides, which bring telomere and gene-expression endpoints. Sharing a common set of senescence markers across the panel is what makes compounds with unrelated mechanisms comparable on the same scale.

SS-31 is listed in several vial strengths. Because the sequence contains non-canonical residues, identity confirmation on every lot is done by fragmentation rather than intact mass alone, and that data appears on the batch certificate published on the product page.