SS-31 storage and stability depend on keeping the lyophilized powder sealed, desiccated, cold and dark. The salt form is hygroscopic, meaning it pulls water from the air, and the dimethyltyrosine residue oxidizes. So moisture and light do more damage than freezer temperature. Let a frozen vial reach room temperature before the seal is broken.
- SS-31 is the tetrapeptide D-Arg-Dmt-Lys-Phe-NH2, also known as elamipretide, MTP-131 and Bendavia.
- The salt form is hygroscopic, so cold-vial condensation and humid weighing rooms do more damage than freezer setting.
- The dimethyltyrosine phenol is the oxidation-susceptible site; there is no methionine, tryptophan or cysteine in the sequence.
- No asparagine or glutamine is present, so the deamidation route that dominates most peptide stability advice does not apply.
- As a cationic peptide, SS-31 adsorbs to bare glass, whose surface silanols are negatively charged.
- Any mass arithmetic must use the certificate peptide content figure, not the labeled gross fill weight.
Why does SS-31 not follow the usual peptide storage advice?
Most handling guidance circulating for research peptides was written with large lipidated analogs in mind. Those molecules are interface-active, they aggregate, and the advice written for them reflects that. SS-31 is a different animal. It is a tetrapeptide of roughly 640 daltons in its free-base form, freely water soluble, and carrying substantial positive charge near neutral pH. Aggregation is a minor concern for a molecule of that description.
Two other things matter more. The first is water, which the material takes up readily because it is normally supplied as a hygroscopic salt of a highly basic peptide. The second is oxidation, because the second residue is a phenol and phenols oxidize in the presence of light, oxygen and trace metal ions. Neither of these announces itself visually.
A note on names before anything else. The same molecule appears in the literature and on supplier labels as SS-31, elamipretide, MTP-131 and Bendavia. The research-chemical designation and the clinical development names refer to one structure with four different documentation lineages behind them, and confusing the lineages is how people end up quoting a shelf life derived from a formulated sterile product for a vial of powder.
- Moisture — taken up fast by a hygroscopic peptide salt
- Light and oxygen — oxidation at the dimethyltyrosine phenol
- Water — backbone and C-terminal amide hydrolysis
- Glass surfaces — adsorption of a cationic peptide at low concentration
- Temperature — a rate modifier across all of the above
What does the SS-31 structure predict?
SS-31 is D-Arg-Dmt-Lys-Phe-NH2, where Dmt is 2',6'-dimethyltyrosine. The C-terminus is a primary amide. The free base corresponds to C32H49N9O5 with a monoisotopic mass near 639.4 and an average mass near 639.8; PubChem carries the record under elamipretide and is the reference worth checking against a supplier's stated formula. Commercial material is almost always an acetate or trifluoroacetate salt, and the salt contributes mass that the label may or may not have subtracted.
Each structural feature points at a specific handling consequence.
- The D-arginine at position one resists aminopeptidase cleavage. That is enzymatic resistance and nothing more. It does not slow chemical hydrolysis, and treating protease stability as general stability is the most common misreading of this sequence.
- The dimethyltyrosine phenol is the oxidizable site. There is no methionine, tryptophan or cysteine in the molecule, so the classic oxidation targets are absent and this residue carries the risk instead.
- Lysine and arginine side chains give a net charge around +3 at neutral pH. That explains the high aqueous solubility and it also explains adsorption to bare borosilicate glass, whose surface silanols are negatively charged.
- The C-terminal amide can hydrolyze to the free acid. The mass difference is about +1 Da, which LC-MS resolves comfortably and reversed-phase HPLC often does not, since the acid can elute very close to the parent.
- There is no asparagine or glutamine, so the deamidation route that dominates many peptide stability discussions does not apply here.
Which degradation routes matter, and what controls each?
| Route | Driver | Site | Control |
|---|---|---|---|
| Phenol oxidation | Light, headspace oxygen, trace Fe and Cu, alkaline pH | Dmt residue | Amber or boxed storage, cold, minimal headspace |
| C-terminal amide hydrolysis | Water activity, heat, acidic conditions | C-terminal amide | Keep the solid dry and sealed |
| Backbone hydrolysis | Water, temperature, pH extremes | Any amide bond | Dry storage against desiccant |
| N-terminal cyclization | Heat and moisture in short sequences generally | First two residues | Dry, cold storage. Whether this route is significant for this specific sequence is not established in published work |
| Water uptake by the salt | Ambient humidity, cold-vial condensation | Bulk solid | Equilibrate sealed vials to room temperature before opening; reseal with desiccant |
Freezer temperature is absent from the driver column on purpose. Cold slows every route in the table, which is why cold storage is the default, but temperature modifies rates rather than causing failure. Moisture and light cause failure. A vial kept dry at 4 °C in a closed box will usually outperform one cycled through a frost-free freezer and opened cold in a humid room.
What SS-31 storage conditions suit each physical state?
| State | Temperature | Light | Relative horizon | Why |
|---|---|---|---|---|
| Sealed powder, unopened | Minus 20 °C or below, desiccated | Dark | Longest | Water excluded and every route slowed |
| Sealed powder, in-use stock | 2 to 8 °C, desiccated | Dark | Shorter | Acceptable where the vial is consumed inside a defined window and openings are logged |
| Powder in transit | Ambient | Dark, insulated | Days | A dry solid tolerates short excursions |
The horizons stay relative because assigning months to a research powder requires a stability study on that lot, in that container closure, under those conditions. Very little such data exists in public for research-grade SS-31. ICH Q1A(R2) describes what a real stability program looks like, and a supplier quoting a confident expiry without one is quoting a convention. Where a certificate of analysis carries a retest date, ask what supports it. Our own position on what a lot record should contain is set out in the quality standard.
Why does condensation matter for a hygroscopic salt?
Take a vial from a minus twenty freezer into a room at 22 °C and sixty percent relative humidity. The glass sits far below the dew point. Break the seal and water condenses on the inside surfaces and on the cake itself within seconds. An acetate or trifluoroacetate salt of a strongly basic peptide is a good enough desiccant to take that water up immediately and hold it.
Nothing about the result looks wrong. A slightly damp cake looks like a cake. It may go faintly translucent or slump at the edge, and it may not. The vial then returns to the freezer with water inside it, and the next analyst repeats the cycle.
The control is dull and works: let the sealed vial reach room temperature before the seal is broken, every single time. Twenty to thirty minutes on the bench suits a small vial. Work quickly once open, reseal with desiccant, and log the opening if the protocol tracks excursions. Where a vial is opened repeatedly, weighing out the whole contents once into single-use portions under low humidity costs an hour and removes the problem entirely.
One consequence of hygroscopicity worth stating plainly: a gravimetric weighing of hygroscopic powder in an uncontrolled room is not an accurate measurement of peptide. If the arithmetic downstream depends on mass, either weigh in a glove box or work from the full vial contents and the stated fill weight.
How does net peptide content change the arithmetic?
Suppose a vial is labeled 10 mg and the certificate states peptide content of 84 percent, the balance being counterion and residual water. The peptide present is:
10 mg × 0.84 = 8.4 mg net peptide
That is sixteen percent less peptide than the label mass would suggest, and the difference propagates into every downstream figure. It covers laboratory measurement only.
Whether the peptide content figure on the certificate was determined by nitrogen analysis, by quantitative HPLC against a reference standard, or simply assumed from a typical salt stoichiometry, is a question worth asking. The three answers carry very different uncertainties.
What should the storage record say?
Storage discipline that is not written down is not reproducible. For a received lot the minimum record carries the source lot identifier, the peptide content figure from the certificate, the storage temperature, the date of each opening, and the initials of whoever opened the vial.
The reason is practical. When a result looks wrong, the first question is whether the material was what the protocol assumed. A chain that runs from vial to lot to certificate answers that in a minute. A chain with a gap in it means repeating the work. The same logic applied to a different molecule is set out in the note on tirzepatide storage and stability.
What is the regulatory position?
SS-31 is the research designation for elamipretide, also seen as MTP-131 and Bendavia, a mitochondrially targeted tetrapeptide developed by Stealth BioTherapeutics and studied in primary mitochondrial myopathy and in Barth syndrome. It received FDA orphan drug designation for Barth syndrome, and an accelerated approval for that indication was granted in September 2025 under the brand name Forzinity. The review history involved several extensions, and accelerated approvals carry confirmatory obligations, so anyone relying on the current status should verify it directly against the FDA approval databases rather than against this page.
A prescription medicine manufactured under pharmaceutical quality systems, with sterility, endotoxin, formulation and lot-release controls, and a research powder supplied with a certificate covering identity and purity are different articles. The certificate makes a narrower claim about a different kind of material. Research-grade SS-31 has never been a lawful route to human use at any point in that development timeline.
Status verified August 26, 2026.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. NOT FOR PERSONAL, MEDICAL, DIAGNOSTIC, THERAPEUTIC, OR RECREATIONAL USE.
Bottom line
Keep the sealed SS-31 powder dry, dark and cold, and let a cold vial reach room temperature before breaking the seal. The hygroscopic salt and the dimethyltyrosine phenol are the weak points, so moisture and light do more harm than the freezer setting.
Use the certificate peptide content figure in any mass arithmetic, and record every opening.
Common questions
What are the right SS-31 storage conditions?
Does lyophilized SS-31 have to be kept frozen?
Why does the salt form matter for storage?
What degradation products should an analytical check look for?
Is research-grade SS-31 the same thing as elamipretide the medicine?
More handling guides
Sources
- Szeto and colleagues, Journal of Biological Chemistry, 2004. Original description of the cell-permeable SS peptide series, including the SS-31 sequence D-Arg-Dmt-Lys-Phe-NH2; supports the structure and nomenclature used here.
- Birk and colleagues, peer-reviewed nephrology and pharmacology literature, 2013 onward. Characterization of SS-31 interaction with cardiolipin in mitochondrial membranes; establishes the mechanistic context in which the compound is studied in vitro.
- PubChem compound record for elamipretide. Reference for molecular formula, monoisotopic and average mass, and synonym list; used to check a supplier stated formula and salt correction.
- ICH Q1A(R2) and USP General Chapter <621>. ICH Q1A(R2) defines what a genuine stability program requires before a retest date can be assigned; USP <621> sets the chromatographic system suitability frame for any purity or degradation check.
