Mitochondrial accumulation of modified TPP+-decyl series assessed by TPP+ ion-selective electrode measurements. Tetraphenylphosphonium electrode response (dashed trace) was calibrated with 3 × 0.25 μM additions of the investigated compound. A greater TPP electrode voltage corresponds to decreased membrane potential when calculated by the Nernst equation. Mouse hind limb mitochondria (mito) (0.05 mg/mL) were added, and finally, mitochondrial substrate combination (5 mM succinate + 5 mM glutamate + 1 mM malate; SGM) was added to the chamber where indicated. The experiment was terminated when steady-state reading was achieved. Corresponding oxygen consumption per unit mass (solid trace) was recorded with a Clark-type oxygen electrode. (A–J) Compounds tested were (A) tetraphenylphosphonium chloride, (B) 4-OMe-TPP-DC (9), (C) 4-CH3-TPP-DC (10), (D) H-TPP-DC (11), (E) 4-Cl-TPP-DC (12), (F) 4-F-TPP-DC (13), (G) 4-CF3-TPP-DC (14), (H) 3-OMe-TPP-DC (30), (I) 4-F-TPP-DC (31), and (J) decyl tri(naphthalen-1-yl)phosphonium iodide (33).