Quantitative relationship between protonophoric and uncoupling activities of substituted phenols

The protonophoric activity through liposomal membranes was measured and compared with the uncoupling activity with the oxidative phosphorylation of rat-liver mitochondria for 19 substituted phenols. Quantitative analyses of the protonophoric activity of the phenols in terms of physicochemical molecular parameters showed that the activity was mostly decided by two factors: the partition coefficient between the liposome and aqueous buffer phases and the acid dissociation constant. Correlation was excellent between protonophoric and uncoupling activities when the difference in the effect of acidity of phenols between liposomal and mitochondrial membranes was taken into account. The results were further evidence for the shuttle-type of mechanism of weakly acidic uncouplers based on the Mitchell chemiosmotic hypothesis.