12-(9-Anthroyl)stearic acid, a fluorescent probe for the ubiquinone region of the mitochondrial membrane.

1. 12-(9-Anthroyl)stearic acid can be incorporated into mitochondrial membranes. 2. The fluorescence properties of the membrane-bound probe are different from those of the free molecule. 3. The intensity of emission and fluorescence life-time of the probe is enhanced when, in the presence of substrate, the electron-transport chain is reduced. 4. This change in intensity has been demonstrated to be a result of collisional quenching by oxidised ubiquinone in the oxidised membrane but not when the respiratory chain is in the reduced state. 5. In pulsing anaerobic mitochondria with oxygen the rate of the fluorescence change is found to be slower than the rate of ubiquinone oxidation, suggesting that the probe detects a structural transition in the mitochondrial inner membrane. 6. This transition results in a constraint on ubiquinone motion in the reduced system. Model experiments, using lipid dispersions, have been carried out to test some of the interpretations.