Effect of pH and monovalent cations on the ionization state of phosphatidylglycerol in monolayers. An experimental (surface potential) and theoretical (Gouy-Chapman) approach

Model for the electrolytic environment and electrostatic properties of biomembranes

Physical and chemical interactions of ions with biomembranes are described by a model originating from the Stern theory. Equations of the model have analytical solutions only for very simple, often unrealistic situations. The numerical resolution adopted permits a much wider application of the model: Potentials and concentrations can be calculated anywhere from the surface and in any electrolytic environment. The model is applied to biomembranes. Simulations are presented in three-dimensional figures which allow one to use the model as a practical research tool. In particular, the simulations reveal that, in practice, it is possible to induce an increase of the surface charge density simultaneously with a decrease of the surface potential, and, theoretically, that the potential at the exclusion distance (which estimates the diffuse layer thickness) exhibits a remarkably constant value as the composition of the free solution is varied.

Evidence for conduction of protons along the interface between water and a polar lipid monolayer

Movements of H+ along the polar heads of phospholipids spread in monolayers were compared to movements of H+ in the aqueous subphase. The probe for detecting H+ movement along the monolayer was a pH-sensitive fluorescein chromophore covalently bound to the head group of phosphatidylethanolamine. The behavior of this probe was not affected by the electrical properties of the lipid/water interface. Lateral diffusion of H+ along the phospholipid/water interface was then studied by acid-jump experiments in which advantage was taken of the large size of the monolayer. H+ was injected a few centimeters away from the probe observation area. The time needed for H+ diffusion to the probe was monitored by the change in the fluorescence signal, fluorescein being nonfluorescent in an acid medium. Diffusion of H+ in the bulk phase was monitored by the fluorescence change of water-soluble fluorescein isothiocyanate. Diffusion along the lipid monolayer was found to be 20 times faster than in the bulk water phase and required a structured monolayer in order to occur, as revealed by variation of the molecular area occupied by the lipid molecules. The molecular basis of rapid H+ transfer along the lipid monolayer may be the existence of a hydrogen-bond network along the polar heads, capable of supporting a rapid "hop and turn" of H+.

A fluorescence quenching technique for the measurement of paramagnetic ion concentrations at the membrane/water interface. Intrinsic and X537A-mediated cobalt fluxes across lipid bilayer membranes

We have characterized the quenching of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethanolamine by Co2+ in egg phosphatidylcholine (PC) lipid bilayer vesicles. The quenching constant obtained is 59 M-1. We demonstrate one use of this fluorescence quenching technique by measuring intrinsic and X537A-mediated transmembrane Co2+ fluxes in large unilamellar PC vesicles. The intrinsic rate constant for Co2+ flux we measure is 3 X 10(-6) S-1. We confirm that the neutral Co approximately (X537A)2 complex is the main component of the X537A-mediated cobalt flux. Since this method measures the concentration of Co2+ at the site of the fluorophore, it is generally applicable to the measurement of paramagnetic ion concentrations in the region of the membrane/water interface.

Alteration of delta psi-dependent amino acid transports in Streptococcus pneumoniae by the antitumoral drug SOAz

Interactions of the antitumoral drug SOAz with natural and model membranes are described. Biological studies were carried out with the bacterium Streptococcus pneumoniae taken as a model system. They reveal that SOAz is able to reduce delta psi and the delta psi-dependent amino acid transports without being cytotoxic for the bacteria. With respect to model membranes, leakage studies carried out with Na+ and K+ loaded lipid vesicles demonstrated that SOAz exhibits no ionophore activity. In contrast, the drug is shown to decrease the surface potential of monolayers of acidic phospholipids but without penetrating within the film. The possibility that SOAz might alter the delta psi part of the proton motive force by decreasing the outside surface potential of the bacterial membrane is discussed.

A study of the structure and dynamics of complexes between polymyxin B and phosphatidylglycerol in monolayers by fluorescence

Interactions between the antibiotic polymyxin B and monolayers of dipalmitoylglycerophosphoglycerol have been reinvestigated through a study of the structure and dynamics of the complexes by means of an interface fluorimeter of our fabrication. A fluorescence technique has been developed where the use of linearly polarized incident beams gives the simultaneous determination of the orientation and the lateral diffusion rate of a fluorescent probe inserted in the film. The present investigation was carried out with 12-(9-anthroyloxy)-stearic acid, a fluorescent compound which forms non-fluorescent photodimers upon illumination. Orientation of the probe was studied by computing the ratio of the two dimerization constants KD and the ratio of the fluorescence intensities obtained with crossed linearly polarized incident lights. The lateral diffusion rate of the probe was obtained by measuring fluorescence recovery after photobleaching (photodimerization) of the probe. Control experiments, carried out with dimyristoylglycerophosphocholine, a lipid which does not interact with polymyxin B, show that the antibiotic does not significantly modify the behaviour of the probe. Both in terms of orientation and dynamics, with respect to dipalmitoylglycerophosphoglycerol, when the antibiotic is present in the subphase (1 microM, saturating conditions), data indicate that the lipid remains in a liquid-expanded state. This is true even at a high surface pressure (pi approximately equal to 37 mN X m-1), above the apparent 'transition' which can be observed at 30-35 mN X m-1 on its compression isotherm. Computation of the contribution of polymyxin B to the film expansion to the conclusion that this 'transition' would be a structural transition between two models of interaction: one, below the 'transition', where the polypeptide ring penetrates between the film-forming lipid molecules and another one, above the 'transition', were the antibiotic is adsorbed at the lipid-water interface with only its hydrocarbon chain penetrating the film.