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The total potential energy profile for hydrophobic ion interactions with lipid bilayers can be written as the sum of four terms: the electrical Born, image and dipole contributions, and a neutral energy term. We introduce a specific model for the membrane dipole potential, treating it as a two-dimensional array of point dipoles located near each membrane-water interface. Together with specific theoretical models for the other energy terms, a total potential profile is developed that successfully describes the complete set of thermodynamic parameters for binding and translocation for the two hydrophobic ion structural analogues, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-). A reasonable fit to the data is possible if the dipole potential energy has a magnitude of 5.5 + 0.5 kcal/mol (240 + 20 mV), positive inside, and if the neutral energy contribution for TPP+ and TPB- is -7.0 + 1.0 kcal/mol. These results may also have important implications for small ion interactions with membranes and the energetics of charged groups in membrane proteins.

The thermodynamic properties for the interaction of the hydrophobic ion tetraphenylphosphonium (TPP+) with egg phosphatidylcholine vesicles were studied in detail by equilibrium dialysis and spin label techniques. A partition coefficient of beta = 4.2 + 0.4 x 10(-6) cm (K congruent to 100) was determined. Electrostatic saturation sets in at approximately 600 microM (about one absorbed TPP+ molecule per 100 lipids), and is not screened by salt. The temperature dependence of binding was determined, which reveals that the binding is entropy-driven with a positive (repulsive) enthalpy of binding, a result to be compared with hydrophobic anions in which the binding enthalpy is negative. The membrane dipole potential may be responsible for this binding difference. Activity coefficients are determined and shown to be significantly different from those of most common salts, an important result that should be considered in all hydrophobic ion studies. Comparison of the TPP+ results with those of its anionic structural analogue, tetraphenylboron (TPB-), permits a general analysis of hydrophobic ion interactions with membranes. A theoretical model consistent with the entire set of data is developed in an accompanying article.

Two spin-labeled derivatives of the hydrophobic anion trinitrophenol have been synthesized and characterized in lipid vesicles. In the presence of lipid vesicles, the electron paramagnetic resonance (EPR) spectra of these probes are a composite of both membrane-bound and aqueous populations; as a result, the membrane-aqueous partitioning can be determined from their electron paramagnetic resonance spectra. The effect of transmembrane potentials on the membrane-aqueous partitioning of these spin-labeled hydrophobic ions was examined in phosphatidylcholine vesicles formed by extrusion. Inside positive membrane potentials promote an increase in the binding of these probes that is quantitatively accounted for by a simple thermodynamic model used previously to describe the partitioning of paramagnetic phosphonium ions. The transmembrane migration rates of these ions are dependent on the dipole potential, indicating that these ions transit the membrane in a charged form. The partitioning of the probe is also sensitive to the membrane surface potential, and this dependence is accurately accounted for using the Gouy-Chapman Stern formalism. As a result of the membrane dipole potential, these probes exhibit a stronger binding and a more rapid transmembrane migration rate compared with positive hydrophobic ion spin labels and provide a new set of negatively charged hydrophobic ion probes to investigate membrane electrostatics.

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