Phenolic compounds are toxic to many organisms in that they may affect
the energy production in cells by inhibition of the electron transpor
t or by destroying the electrochemical proton gradient built up across
membranes. This latter mode of toxic action is commonly referred to a
s uncoupling of oxidative phosphorylation or photophosphorylation. In
this study, the relationship between uncoupling activity, total concen
tration, and speciation in the photosynthetic membrane (chromatophores
) of the purple bacterium Rhodobacter sphaeroides has been evaluated f
or 18 nitro- and chlorophenols covering a wide range of hydrophobicity
and acidity. The uncoupling activity has been determined by time-reso
lved spectroscopy and is quantified by a pseudo-first-order rate const
ant, k(obs), which is a measure for the increased decay rate of the me
mbrane potential in the presence of a certain amount of a given phenol
. The experimental data can be described by an extended ''shuttle mech
anism'' model in which it is assumed that the rate of diffusion of the
phenoxide and/or a phenoxide/phenol-heterodimer species through the l
ipid bilayer of the membrane determines the rate of decay of the elect
rochemical proton gradient: k(obs) = k(1)C(cph)(A-) + k(2)'C-cph(A-) C
-cph(HA), where C-cph(A-) and C-cph(HA) are the concentrations of the
phenoxide and phenol, respectively, in the chromatophores (both estima
ted from membrane-water partitioning experiments), k(1) is a measure o
f the mobility of the phenoxide in the lipid bilayer; and K-2 is a lum
ped parameter describing both the tendency of the compound to form a h
eterodimer in the membrane as well as the mobility of this heterodimer
in the lipid bilayer. To our knowledge, this is the first study in wh
ich, for a given class of ionogenic organic compounds, a direct quanti
tative 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 demo
nstrates that it is possible to separate the contributions of uptake,
speciation, and actual activity (expressed by k(1) and/or k(2)) to the
overall uncoupling potency of a given phenol, which is necessary for
the derivation of improved quantitative structure-activity relationshi
ps (QSARs). Furthermore, the approach taken in this study offers the p
ossibility to evaluate quantitatively synergistic and antagonistic eff
ects of different phenolic compounds on energy transduction when such
compounds are present in mixtures.