The intrinsic pKa values of the phosphate groups of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) and of the phosphate and carboxyl groups of phosphatidylserine (PS) in self-organized monolayers deposited on a hanging mercury drop electrode were determined by a novel procedure based on measurements of the differential capacity C of this lipid-coated electrode. In view of the Gouy-Chapman theory, plots of 1/C at constant bulk pH and variable KCl concentration against the reciprocal of the calculated diffuse-layer capacity Cd,0 at zero charge exhibit slopes that decrease from an almost unit value to vanishingly low values as the absolute value of the charge density on the lipid increases from zero to approximately 2 microC cm-2. The intrinsic pKa values so determined are 0.5 for PE and 0.8 for PC. The plots of 1/C against 1/Cd,0 for pure PS exhibit slopes that pass from zero to a maximum value and then back to zero as pH is varied from 7.5 to 3, indicating that the charge density of the lipid film passes from slight negative to slight positive values over this pH range. An explanation for this anomalous behavior, which is ascribed to the phosphate group of PS, is provided. Interdispersion of PS and PC molecules in the film decreases the "formal" pKa value of the latter group by about three orders of magnitude.

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A biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer was investigated in aqueous KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy, both in the absence and in the presence of the ionophore valinomycin. Impedance spectra, recorded from 1 x 10(-2) to 1 x 10(5) Hz over a potential range of 0.8 V, are satisfactorily fitted to a series of four RC meshes, which are straightforwardly related to the different substructural elements of the biomimetic membrane. The frequency-independent resistances and conductances of both the lipid bilayer and the hydrophilic spacer show a maximum when plotted against the applied potential. This behavior is interpreted on the basis of a general approximate approach that applies the concepts of impedance spectroscopy to a model of the electrified interphase and to the kinetics of potassium ion transport assisted by valinomycin across the lipid bilayer.

Ubiquinone-10 (UQ) was incorporated at concentrations ranging from 0.5 to 2 mol% in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) deposited on a mercury drop electrode, and its electroreduction to ubiquinol (UQH2) was investigated in phosphate and borate buffers over the pH range from 7 to 9.5 by a computerized chronocoulometric technique. The dependence of the applied potential for a constant value of the faradaic charge due to UQ reduction upon the electrolysis time t at constant pH and upon pH at constant t was examined on the basis of a general kinetion approach. This permitted us to conclude that the reduction of UQ to UQH2 in DOPC monolayers takes place via the reversible uptake of one electron with the formation of the semiubiquinone radical anion UQ.-, followed by the rate-determining protonation of this anion with UQH. formation; this neutral radical is more easily reduced than UQ, yielding the ubiquinol UQH2. In spite of the very low concentration of hydrogen ions as compared with that of the acidic component of the buffer, the only effective proton donor is the proton itself; this strongly suggests that the protonation step takes place inside the polar head region of the DOPC monolayer, which is only accessible to protons.

Gramicidin D was incorporated in a biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer, and its behavior was investigated in aqueous 0.1 M KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy. The impedance spectra, recorded from 0.1 to 1 × 10 5 Hz over a potential range of 0.7 V, were fitted to a series of RC meshes, which were related to the different substructural elements of the biomimetic membrane. These impedance spectra were compared with those obtained by incorporating valinomycin, under otherwise identical conditions. The potential dependence of the stationary currents reported on bilayer lipid membranes by Bamberg and Läuger (Bamberg, E.; Läuger, P. J. Membrane Biol. 1973, 11, 177-194) as well as those extracted from potential-step chronocoulometric measurements was interpreted by relating the increase in gramicidin dimerization to a progressive increase in single-file K + flux along the dimeric channels. An analogous approach was adopted in explaining the difference between the impedance spectra obtained with gramicidin D and those obtained with valinomycin. It is concluded that gramicidin has a low tendency to form dimers in the absence of ionic flux.