Measurement of phospholipid monolayer surface potentials at a hydrocarbon-electrolyte interface

Calcium adsorption and displacement: characterization of lipid monolayers and their interaction with membrane-active peptides/proteins

BACKGROUND

The first target of antimicrobial peptides (AMPs) is the bacterial membrane. In the case of Gram-negative bacteria this is the outer membrane (OM), the lipid composition of which is extremely asymmetric: Whereas the inner leaflet is composed of a phospholipid mixture, the outer leaflet is made up solely from lipopolysaccharides (LPSs). LPS, therefore, represents the first target of AMPs. The binding and intercalation of polycationic AMPs is driven by the number and position of negatively charged groups of the LPS. Also, proteins other than cationic AMPs can interact with LPS, e.g. leading eventually to a neutralization of the endotoxic effects of LPS. We compared different biophysical techniques to gain insight into the properties of the electrical surface potentials of lipid monolayers and aggregates composed of LPSs and various phospholipids and their interaction with peptides and proteins.

RESULTS

The net negative charge calculated from the chemical structure of the phospholipid and LPS molecules is linearly correlated with the adsorption of calcium to two-dimensional lipid monolayers composed of the respective lipids. However, the zeta-potentials determined by the electrophoretic mobility of LPS aggregates can only be interpreted by assuming a dependence of the plane of shear on the number of saccharides and charged groups. Various peptides and proteins were able to displace calcium adsorbed to monolayers.

CONCLUSION

To characterize the electrical properties of negatively charged phospholipids and LPSs and their electrostatic interaction with various polycationic peptides/proteins, the adsorption of calcium to and displacement from lipid monolayers is a suitable parameter. Using the calcium displacement method, the binding of peptides to monolayers can be determined even if they do not intercalate. The interpretation of zeta-potential data is difficulty for LPS aggregates, because of the complex three-dimensional structure of the LPS molecules. However, the influence of peptides/proteins on the zeta-potential can be used to characterize the underlying interaction mechanisms.

Intermolecular forces in spread phospholipid monolayers at oil/water interfaces

The lateral intermolecular forces between phospholipids are of particular relevance to the behavior of biomembranes, and have been approached via studies of monolayer isotherms at aqueous interfaces, mostly restricted to air/water (A/W) systems. For thermodynamic properties, the oil/water (O/W) interface has major advantages but is experimentally more difficult and less studied. A comprehensive reanalysis of the available thermodynamic data on spread monolayers of phosphatidyl cholines (PC) and phosphatidyl ethanolamines (PE) at O/W interfaces is conducted to identify the secure key features that will underpin further development of molecular models. Relevant recourse is made to isotherms of single-chain molecules and of mixed monolayers to identify the contributions of chain-chain interactions and interionic forces. The emphasis is on the properties of the phase transitions for a range of oil phases. Apparent published discrepancies in thermodynamic properties are resolved and substantial agreement emerges on the main features of these phospholipid monolayer systems. In compression to low areas, the forces between the zwitterions of like phospholipids are repulsive. The molecular model for phospholipid headgroup interactions developed by Stigter et al. accounts well for the virial coefficients in expanded phospholipid O/W monolayers. Inclusion of the changes in configuration and orientation of the zwitterion headgroups on compression, which are indicated by the surface potentials in the phase transition region, and inclusion of the energy of chain demixing from the oil phase will be required for molecular modeling of the phase transitions.

Interaction of phloretin with lipid monolayers: relationship between structural changes and dipole potential change

Phloretin is known to adsorb to lipid surfaces and alters the dipole potential of lipid monolayers and bilayers. Its adsorption to biological and artificial membranes results in a change of the membrane permeability for a variety of charged and neutral compounds. In this respect phloretin represents a model substance to study the effect of dipole potentials on membrane permeability. In this investigation we studied the interaction of phloretin with monolayers formed of different lipids in the liquid-expanded and the condensed state. Phloretin integrated into the monolayers as a function of the aqueous concentration of its neutral form, indicated by an increase of the surface pressure in the presence of phloretin. Simultaneous recording of the surface potential of the monolayers allowed us to correlate the degree of phloretin integration and the phloretin-induced dipole potential change. Increasing the surface pressure decreased the phloretin-induced shift of the isotherms, but did not influence the phloretin-induced surface potential change. This means that phloretin adsorption to the lipid surface can occur without affecting the lipid packing. The surface potential effect of phloretin is accompanied by a change of the lipid dipole moment vector dependent on the lipid packing. This means that the relation between the surface potential change and the lipid packing cannot be described by a static model alone. Taking into account the deviations of the surface potential change versus molecular area isotherms of the experimental data to the theoretically predicted course, we propose a model that relates the area change to the dipole moment in a dynamic manner. By using this model the experimental data can be described much better than with a static model.

Dipole potential of lipid membranes

Of the individual potentials which comprise the potential profile of a membrane, the least well understood is the dipole potential. In general, the dipole potential is manifested between the hydrocarbon interior of the membrane and the first few water layers adjacent to the lipid head groups. Changes in dipole potential caused by spreading a lipid at an air- or oil-water interface can be measured directly and the dipole potential of bilayers can be estimated from the conductances of hydrophobic ions. For a typical phospholipid, like phosphatidylcholine, its measured value is approximately 400 mV in monomolecular films and approximately 280 mV in bilayer membranes, with the hydrocarbon region being positive relative to the aqueous phase. The difference between dipole potentials measured in monolayers and bilayer membranes appears to arise from the use of the lipid-free air- or oil-water interface as the reference point for monolayer measurements and can be corrected for. The species-specific correction term is a lipid concentration-independent potential, the existence of which suggests the ability of lipid headgroups to globally reorganize water structure at the interface. The dipole potential arises from the functional group dipoles of the terminal methyl groups of aliphatic chains, the glycerol-ester region of the lipids and the hydrated polar head groups. Classical methods for obtaining partial dipole moments for each of the three contributing regions are all based on questionable assumptions and give conflicting results. More sophisticated mean-field models of dipole potential origin recognize the important role of interfacial water in determining its value but still cannot adequately describe the microscopic nature of the interactions from which it arises. In part this is because the dipole potential develops in a region over which the dielectric constant of the medium is changing from 2 to 80. Despite of our limited understanding of the dipole potential, it is an important regulator of membrane structure and function. Membrane-membrane and membrane-ligand interactions are regulated by the hydration force, the value of which can be related to the dipole potential of the membrane. For thermotropically phase-separated or multicomponent membranes the size and shape of lipid domains is controlled by the balance between the line tension at the domain borders and the difference in dipole density between the domains. Line tension tends to make the domains compact and circular whereas dipole repulsion promotes transitions to complex domain shapes with larger perimeters.(ABSTRACT TRUNCATED AT 400 WORDS)

Phospholipid interactions in model membrane systems. I. Experiments on monolayers

We study the lateral headgroup interactions among phosphatidylcholine (PC) molecules and among phosphatidylethanolamine (PE) molecules in monolayers and extend our previous models. In this paper, we present an extensive set of pressure-area isotherms and surface potential experiments on monolayers of phospholipids ranging from 14 to 22 carbons in length at the n-heptane/water interface, over a wide range of temperature, salt concentration, and pH on the acid side. The pressure data presented here are a considerable extension of previous data (1) to higher surface densities, comprehensively checked for monolayer loss, and include new data on PE molecules. We explore surface densities ranging from extremely low to intermediate, near to the main phase transition, in which range the surface pressures and potentials are found to be independent of the chain length. Thus, these data bear directly on the headgroup interactions. These interactions are observed to be independent of ionic strength. PC and PE molecules differ strongly in two respects: (a) the lateral repulsion among PC molecules is much stronger than for PE, and (b) the lateral repulsion among PC molecules increases strongly with temperature whereas PE interactions are almost independent of temperature. Similarly, the surface potential for PC is found to increase with temperature whereas for PE it does not. In this and the following paper we show that these data from dilute to semidilute monolayers are consistent with a theoretical model that predicts that, independent of coverage, for PC the P-N+ dipole is oriented slightly into the oil phase because of the hydrophobicity of the methyl groups, increasingly so with temperature, whereas for PE the P-N+ dipole is directed into the water phase.

Electrical phenomena at the surface of phospholipid membranes relevant to the sorption of ionic compounds

The change in electrostatical potential-profile across the phospholipid membranes caused by the binding or sorption of ionic compounds (metal cations, ionic surfactants, lipid-soluble ions, and ionophore-metal complexes) can be estimated by the direct measuring method (combination of zeta potential of lipid vesicles and surface potential of lipid monolayer). The analysis of the data by a simple electrical double layer theory reveals the binding/sorption location of these ions. The coexistent effect of metal ions and lipid-soluble ions on the sorption behavior is described from the interfacial electrochemical view point. The coagulation of lipid vesicles caused by those ionic compounds is also discussed.

I. Novel capacitative electrode with a wide frequency range for measurements of flash-induced changes of interface potential at the oil-water interface. Mechanical construction and electrical characteristics of the electrode

The mechanical construction and the electrical properties of a new type of capacitative electrode for the oil-water interface are described. The electrode is designed to detect changes of the interface potential induced by photochemical, photophysical, and photobiological reactions occurring at the interface. The construction is based on capacitative coupling of two aqueous compartments separated by a thin Teflon film. Thereby, the oil-water interface is in horizontal position and the electrode is placed with its planar bottom about 10 micrometer above the interface. A main feature of the electrode is the transparency to visible light which is achieved by having a clear electrolyte solution in the inner compartment of the capacitative electrode. The aqueous subphase and the inner electrolyte are connected with Ag/AgCl electrodes to voltage amplifiers. The capacitative electrode is best operated under open circuit conditions. The frequency range experimentally verified is 500 MHz larger than or equal to 0.03 Hz. The sensitivity is mainly determined by the noise of the electronic amplifiers, typical 50-100 muV.

Phospholipid monolayers at the hydrocarbon-electrolyte interface. The interrelation of film potential and film pressure

Measurements of surface pressure of surface potential are reported for films of distearoyl phosphatidylcholine (density range: 0.15--2.65 . 10(18) molecules/m2) spread at the interface between 2,2,4-trimethylpentane and 100 mM NaCl. Low density behavior of the surface pressure is explained using classical viral theory. The behavior of the surface potential is qualitatively explained for all densities in terms of the dipole moments associated with the carboxyl groups and headgroups of the phosphatidylcholine.