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Phenolic compounds are toxic to many organisms in that they may affect the energy production in cells by inhibition of the electron transport or by destroying the electrochemical proton gradient built up across membranes. This latter mode of toxic action is commonly referred to as uncoupling of oxidative phosphorylation or photophosphorylation. In this study, the relationship between uncoupling activity, total concentration, and speciation in the photosynthetic membrane (chromatophores) of the purple bacterium Rhodobacter sphaeroides has been evaluated for 18 nitro- and chlorophenols covering a wide range of hydrophobicity and acidity. The uncoupling activity has been determined by time-resolved spectroscopy and is quantified by a pseudo-first-order rate constant, kobs, which is a measure for the increased decay rate of the membrane potential in the presence of a certain amount of a given phenol. The experimental data can be described by an extended “shuttle mechanism” model in which it is assumed that the rate of diffusion of the phenoxide and/or a phenoxide/phenol-heterodimer species through the lipid bilayer of the membrane determines the rate of decay of the electrochemical proton gradient: kobs = k1 + k ‘2 , where and are the concentrations of the phenoxide and phenol, respectively, in the chromatophores (both estimated from membrane−water partitioning experiments), k1 is a measure of the mobility of the phenoxide in the lipid bilayer; and k‘2 is a lumped parameter describing both the tendency of the compound to form a heterodimer in the membrane as well as the mobility of this heterodimer in the lipid bilayer. To our knowledge, this is the first study in which, for a given class of ionogenic organic compounds, a direct quantitative measure of a specific toxic effect (i.e., uncoupling) has been successfully related to the actual concentration and speciation of the compounds at the target site (i.e., in the membrane). This study demonstrates that it is possible to separate the contributions of uptake, speciation, and actual activity (expressed by k1 and/or k2) to the overall uncoupling potency of a given phenol, which is necessary for the derivation of improved quantitative structure−activity relationships (QSARs). Furthermore, the approach taken in this study offers the possibility to evaluate quantitatively synergistic and antagonistic effects of different phenolic compounds on energy transduction when such compounds are present in mixtures.
Escher et al. (Sun,) studied this question.