Model for the electrolytic environment and electrostatic properties of biomembranes

Determination of binding constants of Ca2+, Na+, and Cl- ions to liposomal membranes of dipalmitoylphosphatidylcholine at gel phase by particle electrophoresis

The zeta potentials of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) liposomes were measured at a gel phase as a function of CaCl2 concentration (0-200 mM) in a solution containing different NaCl concentrations (0-200 mM). The data obtained were analyzed with the diffuse double layer theory including the Graham theory. The intrinsic binding constants of ions to DPPC membranes and the distance of the shear plane have been determined independent of both the concentration of CaCl2 and that of NaCl. The values of the constants were 37 M-1 for Ca2+, 0.28 M-1 for Cl, and 0.25 M-1 for Na+; the distance was 0.24 nm under the assumption of binding stoichiometry of Ca2+/DPPC = 1:1.

Mechanism of electrical enhancement of efficacy of antibiotics in killing biofilm bacteria

The bioelectric effect, in which electric fields are used to enhance the efficacy of biocides and antibiotics in killing biofilm bacteria, has been shown to reduce the very high concentrations of these antibacterial agents needed to kill biofilm bacteria to levels very close to those needed to kill planktonic (floating) bacteria of the same species. In this report, we show that biofilm bacteria are readily killed by an antibiotic on all areas of the active electrodes and on the surfaces of conductive elements that lie within the electric field but do not themselves function as electrodes. Considerations of electrode geometry indicate that very low (< 100 microA/cm2) current densities may be effective in this electrical enhancement of antibiotic efficacy against biofilm bacteria, and flow experiments indicate that this bioelectric effect does not appear to depend entirely on the possible local electrochemical generation of antibacterial molecules or ions. These data are expected to facilitate the use of the bioelectric effect in the prevention and treatment of device-related bacterial infections that are caused by bacteria that grow in biofilms and thereby frustrate antibiotic chemotherapy.

The determination of binding site density and association constants for monovalent cation adsorption onto liposomes made from mixtures of zwitterionic and charged lipids

The Gouy-Chapman-Stern theory predicts that the shape (but not the magnitude) of the surface potential dependence on the electrolyte concentration mainly reflects the screening of the surface charge but not cation adsorption. So this dependence does not allow to determine two parameters of the theory (the surface density of binding sites (S) and the association constant (K)) simultaneously. Therefore the fitting procedure for the determination of S and K was suggested as a test of cation adsorption for the surface or zeta potential measurements at a fixed electrolyte concentration but with a variable ratio of charged and neutral components in the lipid mixture. This procedure was applied to the electrophoretic measurements made by the method of photon correlation spectroscopy in the suspensions of PS/PC or CL/PC liposomes in the monovalent electrolytes. For KCl and NaCl electrolytes it only leads to the different association constants (about 0.2 and 0.8 M-1, respectively) corresponding with data from the literature but to the same value of the surface charge density (about -16 microC/cm2) which is smaller than usually postulated for PS membranes. The fitting of zeta potentials measured in tetraalkylammonium salts shows a small cation adsorption but a large surface charge density (about -21 microC/cm2).

Surface pH controls purple-to-blue transition of bacteriorhodopsin. A theoretical model of purple membrane surface

We have developed a surface model of purple membrane and applied it in an analysis of the purple-to-blue color change of bacteriorhodopsin which is induced by acidification or deionization. The model is based on dissociation and double layer theory and the known membrane structure. We calculated surface pH, ion concentrations, charge density, and potential as a function of bulk pH and concentration of mono- and divalent cations. At low salt concentrations, the surface pH is significantly lower than the bulk pH and it becomes independent of bulk pH in the deionized membrane suspension. Using an experimental acid titration curve for neutral, lipid-depleted membrane, we converted surface pH into absorption values. The calculated bacteriohodopsin color changes for acidification of purple, and titrations of deionized blue membrane with cations or base agree well with experimental results. No chemical binding is required to reproduce the experimental curves. Surface charge and potential changes in acid, base and cation titrations are calculated and their relation to the color change is discussed. Consistent with structural data, 10 primary phosphate and two basic surface groups per bacteriorhodopsin are sufficient to obtain good agreement between all calculated and experimental curves. The results provide a theoretical basis for our earlier conclusion that the purple-to-blue transition must be attributed to surface phenomena and not to cation binding at specific sites in the protein.

Pentachlorophenol-induced change of zeta-potential and gel-to-fluid transition temperature in model lecithin membranes

We have determined zeta-potentials for dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) membranes by measuring the electrophoretic mobility of multilayered vesicles and the temperatures of the gel-to-ripple-to-fluid phase transitions of sonicated vesicles by a photometric method. Some conclusions are: (1) The zeta-potentials of DMPC and DPPC vesicles become negative due to adsorption of ionized pentachlorophenol (PCP), (2) their magnitude changes, step-like, on gel-to-fluid transition and (3) the temperature of the step-like change in zeta-potential decreases with an increase in PCP concentration. (4) PCP exhibits a large effect on membrane structure: It induces an isothermal phase change from the ordered to disordered state, which is enhanced by monovalent salt in the aqueous phase. (5) Both ionized and unionized PCP decrease the melting phase transition temperature and abolish the pretransition, (6) the unionized species increases the melting transition width and (7) the ionized species is more potent in abolishing the pretransition. (8) The shorter chain lipid (DMPC) is more sensitive to the presence of PCP; the maximum decrease in delta Tt is 13 K (DMPC) and 7 K (DPPC) in the presence of ionized PCP. We have shown experimentally, by comparing the delta Tt from photometric studies with the density of adsorbed PCP derived from zeta-potential isotherms, that (9) the shift of the melting phase transition temperature increases linearly with the density of adsorbed PCP. (10) In contrast to membranes made of negatively charged lipids, the transition temperature of DMPC and DPPC membranes in the presence of PCP further decreases in the presence of monovalent salt. The salt effect is due to screening of the membrane surface leading to enhanced adsorption of ionized PCP and a depression in transition temperature. (11) It is shown that both the adsorption and the changes of gel-to-fluid phase transition temperature can be described in terms of the Langmuir-Stern-Grahame model and (12) proposed that future studies of membrane toxicity of PCP should be focused on its pH dependence.