SS-31 is a four-residue peptide of the Szeto-Schiller class, composed of D-arginine, 2,6-dimethyltyrosine, lysine and phenylalanine, carrying a C-terminal amide. The alternating cationic and aromatic residues are the design feature: the motif partitions the molecule into the inner mitochondrial membrane and holds it there, and it does so without depending on the membrane potential. That distinguishes it from potential-driven mitochondrial probes such as the triphenylphosphonium conjugates, and it is the reason SS-31 is used in study designs where the potential itself is the variable under measurement.

The characterised binding partner is cardiolipin, the four-chain phospholipid effectively confined to the inner mitochondrial membrane. Published work describes association with cardiolipin rather than catalysis, with the measured consequences being cristae organisation, the stability of respiratory supercomplexes, and the interaction between cardiolipin and cytochrome c that governs whether that protein acts as an electron carrier or as a peroxidase. Endpoints in this work are oxygen consumption rate, ATP synthesis rate, membrane potential, and reactive-oxygen-species production.

Preclinical models span mitochondrial myopathy, cardiac and renal ischaemia-reperfusion, and age-related decline in skeletal muscle bioenergetics. The compound also appears as a pharmacological tool for separating cardiolipin-dependent effects from other mitochondrial interventions in the same preparation. SS-31 is catalogued here as a reference compound for laboratory research. No therapeutic, diagnostic, or clinical conclusion is drawn from any study referenced above, and none should be inferred.