SS-31 is a short synthetic peptide studied in laboratory research for its affinity for the inner mitochondrial membrane. It belongs to the Szeto-Schiller family of aromatic-cationic peptides, a group of small molecules designed around an alternating pattern of aromatic and positively charged residues. The compound is also discussed in published literature under the names elamipretide and MTP-131, and those names appear in structural and cell-biology papers describing the same tetrapeptide scaffold.
The catalog specification for SS-31 at Cowboy Chems lists a molecular formula of C32H49N9O5, a molecular weight of 639.80 g/mol, and the CAS registry number 736992-21-5. It is supplied as a lyophilized powder with a stated purity of 99% or higher by HPLC, stored at -20 °C. Like every compound in the catalog, it is sold for research purposes only and is not intended for human use.
What makes the peptide interesting to bench researchers is not size but behavior. At roughly 640 g/mol it is far smaller than most peptides used in cell-signaling work, yet published studies describe it as localizing to a specific organelle membrane without relying on a classical receptor. That combination, a small molecule with an unusual subcellular address, is the reason it keeps appearing in mitochondrial biology research.
What is the molecular architecture of SS-31?
SS-31 is a tetrapeptide. Its design principle is the defining feature of the whole class: aromatic residues alternate with basic, positively charged residues, and the sequence includes a non-natural amino acid, dimethyltyrosine, in place of a standard tyrosine. The dimethyl substitution is described in the design literature as a way to increase resistance to enzymatic breakdown and to tune the aromatic character of the second position. The C-terminus is amidated, a common modification that removes a negative charge and changes how short peptides behave in solution and at membranes.
The net positive charge at physiological pH is central to its reported behavior. Published biophysical work describes a charge of roughly +3 for this peptide class under neutral conditions, which is relatively high for a molecule this small. The aromatic side chains contribute hydrophobic character, so the molecule is amphipathic in a compact way. That balance lets it sit at the interface of a lipid bilayer rather than passing straight through or staying fully dissolved in water.
Because the sequence contains a non-standard residue, researchers confirming identity usually rely on mass spectrometry and HPLC retention comparisons rather than simple amino acid analysis alone. The catalog molecular weight is the figure a certificate of analysis should reproduce when intact peptide is the main species in the sample.
How does SS-31 interact with mitochondrial membranes?
The most cited mechanistic observation is an association with cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane. Cardiolipin carries two negative charges and four acyl chains, and it plays a structural role in organizing respiratory chain complexes into supercomplexes. Published binding studies describe electrostatic attraction between the positively charged residues of SS-31 and the anionic headgroup of cardiolipin, with the aromatic side chains interacting with the hydrophobic region nearby.
Several lines of in vitro evidence are reported in the literature. Isothermal calorimetry and fluorescence experiments on model liposomes show selective association with cardiolipin-containing vesicles compared with vesicles built from neutral lipids. Spectroscopy and molecular dynamics work describes the peptide inserting shallowly into the bilayer interface rather than spanning it. Because the uptake appears to depend on membrane potential and lipid composition more than on a transporter, researchers have used fluorescently labeled analogs to study how small cationic peptides reach the mitochondrial compartment in cultured cells.
It is worth being careful about what these observations establish. They describe where the molecule goes and what it binds in model systems. They do not by themselves demonstrate any functional outcome in a whole organism, and this overview does not make that claim. Any statement about downstream biology has to be tied to the specific assay, cell type, and conditions reported in the original paper.
What model systems have been used in SS-31 research?
Published work spans a wide range of preparations. Isolated mitochondria are the cleanest system, because they let a researcher measure oxygen consumption, membrane potential, and reactive oxygen species production without the confounding effects of other cellular compartments. Cultured cell lines, including cardiomyocyte and neuronal models, are used to look at mitochondrial morphology, membrane potential dyes, and stress-response markers under conditions that challenge the respiratory chain. Ex vivo tissue preparations and animal models appear in the broader literature as well, and those papers should be read for their own design details rather than summarized loosely.
A recurring theme in these studies is the use of metabolic or oxidative stress as the experimental perturbation, with SS-31 included as a variable to see whether mitochondrial readouts shift. Researchers reading this body of work should pay attention to a few practical points:
Which readout is reported: respiration rate, ATP content, membrane potential, ROS fluorescence, or cristae ultrastructure by electron microscopy.
Whether the preparation contains intact cardiolipin, since depletion or oxidation of cardiolipin changes the proposed binding partner.
Which control peptide was used, because charge-matched and aromatic-scrambled controls help separate sequence-specific effects from simple electrostatic ones.
How the peptide was handled and stored before the assay, since degraded material gives results that are hard to interpret.
Structural studies using cryo-electron microscopy of respiratory complexes in recent years have also been cited in discussions of how a small peptide might influence supercomplex organization, though interpretation of that work is still an active research question.
How does SS-31 differ from other mitochondrial research compounds?
Mitochondrial research uses several distinct tool classes, and SS-31 occupies a particular niche among them. Its mechanism is membrane association driven by charge and lipid composition. That is different from compounds that act as enzyme cofactors, from signaling peptides encoded in mitochondrial DNA, and from small-molecule modulators of specific metabolic enzymes.
MOTS-c, for example, is a mitochondrial-derived peptide of roughly 16 amino acids studied for its role in metabolic signaling and nuclear gene regulation. Its formula in the catalog is C100H152N28O22S2 with a molecular weight of 2174.59 g/mol, more than three times the mass of SS-31. The two are not interchangeable: one is a larger, sequence-encoded signaling peptide, the other a compact designed tetrapeptide whose proposed mode of action is physical association with a lipid.
NAD+ is a coenzyme rather than a peptide at all, with catalog formula C21H27N7O14P2 and molecular weight 663.43 g/mol. Studies using it focus on redox cofactor pools and sirtuin-linked pathways, not on membrane binding. A researcher designing a mitochondrial study will often choose among these tools based on which layer of the system is under investigation: membrane structure, signaling, or cofactor availability.
What are the main limitations and open questions?
Honest reading of the SS-31 literature turns up several limitations. Binding to cardiolipin is well supported in model membranes, but how much that interaction explains in intact cells is debated, and some authors argue that additional mechanisms contribute. Reports vary in the concentration ranges used in vitro, and direct comparison between papers is difficult when assay formats differ. The peptide's reported effects on respiration depend heavily on the metabolic state of the preparation, which means results from stressed and unstressed systems should not be pooled.
There is also a reproducibility dimension that applies to every research peptide. Identity and purity of the material matter as much as the experimental design. A tetrapeptide with a non-natural residue can carry synthesis-related impurities, such as deletion sequences or incompletely deprotected material, that are hard to spot without proper analytical data. Researchers are better served by requesting the batch analytical documentation and checking that the mass and retention time match the expected values than by assuming a label is accurate.
Open questions in the field include how membrane potential and cardiolipin remodeling interact over time, whether the peptide's behavior differs between tissue types with different mitochondrial lipid compositions, and how best to design control peptides that isolate sequence-specific effects. These are questions for the primary literature, and this overview is only a map of where to look.
How should researchers source and verify SS-31?
For laboratory work, documentation is the practical foundation. A usable record includes the lot-specific HPLC trace, a mass spectrum confirming the expected molecular weight, and clear storage information. The catalog entry for SS-31 lists storage at -20 °C as a lyophilized powder, which is the standard condition for short peptides held in solid form. Researchers should record lot numbers in their notebooks so any later discrepancy can be traced to a specific batch.
Cross-checking identifiers is a low-cost habit. The formula, molecular weight, and CAS number in a product listing should agree with each other and with the registry entry. When they do not, the discrepancy is worth resolving before any experiment is run.
Research-use disclaimer
SS-31 and all other compounds referenced here are supplied by Cowboy Chems for in vitro and laboratory research purposes only. They are not drugs, supplements, or foods, and they are not intended for human or veterinary use. Nothing in this article describes a use, a recommendation, or an expected outcome in people or animals. The summaries above describe published laboratory findings in general terms and are not a substitute for reading the original papers.
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