SS-31 is a synthetic tetrapeptide — a chain of four amino acids with the sequence D-Arg-Dmt-Lys-Phe-NH₂, where Dmt is 2′,6′-dimethyltyrosine, a modified form of tyrosine. Its molecular formula is C₃₂H₄₉N₉O₅, its molecular weight is approximately 639.8 g/mol, and its CAS number is 736992-21-5. The peptide belongs to a class of mitochondria-targeted tetrapeptides whose structure follows an alternating aromatic-cationic motif (positive charge, aromatic ring, positive charge, aromatic ring), which gives it membrane-permeable characteristics. SS-31 has a structural affinity for cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane. Supplied as a white to off-white lyophilized powder, it is soluble in water.