SkQ (10-(6'-Plastoquinonyl)decyltriphenyl-phosphonium) stands for a class of organic molecules composed of a large organic cation (often called "penetrating cation" for the ability to penetrate through lipid bilayer) with antioxidant plastoquinone attached to it. When added to a living cell, penetrating cations are distributed according to the transmembrane electrical potential difference. They traverse across the cell membrane (negatively charged from inside) and accumulate in mitochondria (also negatively charged inside). The concentration of a penetrating cation in mitochondria can be more than 1000-fold higher than its extracellular concentration.
SkQ is a mitochondria-targeted antioxidant
It was proposed that penetrating cations can act as "electric locomotive molecules" and target molecules attached to them in mitochondria. Monitoring the distribution of plastoquinonyl-decyl-rhodamine 19 (SkQR1), a fluorescent SkQ, confirmed that it accumulated almost exclusively in mitochondria. Measurements of mitochondrial reactive oxygen species production revealed that SkQ is a very efficient antioxidant, even when added to the cells in the nanomolar concentration range.
SkQ as a potential anti-aging drug
Production of reactive oxygen species in mitochondria may contribute to senescence. Reactive oxygen species damage mitochondrial DNA and other important cell component, leading to gradual impairment of cellular function. Antioxidants may slow this damage. Several studies indicate that SkQ can efficiently protect the cell from oxidative damage (see  for a review).
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