Some major effects of ring substituents on the partition coefficients of quinone headgroups are described. Attention is drawn to the large differences in partition coefficients in cyclohexane/water of the two major freely diffusing redox forms, the quinone, Q, and the hydroquinone, QH2. Methoxy substituents cause a marked increase of the cyclohexane/water partition coefficient of the hydroquinone, but this effect is absent in the quinone and is also not seen in measurements in octanol/water. The relation between partition coefficients and biochemical specificity of quinone binding sites is explored.
