Most molecules that drift into a cell end up scattered more or less everywhere. SS-31 is the exception worth paying attention to. It's the laboratory designation for the compound also known as elamipretide, and it was built to do the opposite of scatter: to gather at a single address inside the cell. That address is the mitochondrion — more precisely, its inner membrane. SS-31 is a synthetic four-amino-acid peptide offered strictly for research use only, and this article looks at the chemistry behind its targeting rather than at any use in people or animals. Research-grade SS-31 is a chemical for laboratory study; it is not equivalent to, and should not be confused with, any approved pharmaceutical product bearing the elamipretide name.
Understanding how a peptide is "targeted" turns out to explain most of why its structure looks the way it does. If you're researching this compound, the targeting story is the thread that ties its odd-looking sequence to where it ends up in a cell.
What "mitochondria-targeted" actually means
In one line: a targeted molecule binds something that exists in only one place, so it piles up there instead of spreading out.
Mitochondria are the small compartments inside most of our cells where energy from food is converted into ATP, the molecule cells spend to do work. Each one has two membranes. The inner membrane is folded into pleats called cristae, and that's where the energy-conversion machinery sits — so it's the part of the mitochondrion most of this chemistry cares about.
What sets that inner membrane apart is a chemical signature the rest of the cell mostly lacks: a phospholipid called cardiolipin. Cardiolipin carries a net negative charge and sits almost exclusively in the inner mitochondrial membrane, where it helps hold the cristae and their embedded protein complexes in working order. Since cardiolipin lives in essentially one location, a molecule that binds it effectively carries a postal code for the inner membrane. A 2025 review of the compound's mechanism describes exactly this cardiolipin-directed localization as the heart of how SS-31 behaves (Marcucci et al., 2025). The same logic — a peptide that keys on the mitochondrion — turns up elsewhere in the research literature; for a different example, see our explainer on another peptide tied to the mitochondria.
The four amino acids that make up SS-31
In one line: SS-31 is a tiny peptide whose four building blocks are split evenly between "charged" and "aromatic," and that split is the whole design.
The sequence is written D-Arg–Dmt–Lys–Phe–NH2, and the molecule weighs roughly 639.8 g/mol (Allen & Schenkel, 2025). A few of those notations deserve unpacking in plain terms. The "D-Arg" is a mirror-image (D-form) version of the amino acid arginine; using the D-form makes the peptide harder for the body's ordinary enzymes to chew up, so it stays intact longer in an experimental setting. "Dmt" is dimethyltyrosine, a modified aromatic amino acid. And the "-NH2" at the tail end means the peptide is capped (amidated) — one more tweak that adds stability.
Now the design itself. Two of the four residues — D-Arg and Lys (lysine) — are basic, which means they carry positive charge. The other two — Dmt and Phe (phenylalanine) — are aromatic: they have flat, ring-shaped, oily side chains that like to slip into membranes. So the peptide is, in a sense, half magnet and half grease. That pairing of charge and aromaticity, as we'll see next, is what lets such a small molecule find and stick to the right membrane. Wringing a function out of four carefully chosen residues is a recurring theme in peptide chemistry; the same economy of design shows up in other research tetrapeptides.