Interaction of electric dipoles with phospholipid head groups. A 2H and 31P NMR study of phloretin and phloretin analogues in phosphatidylcholine membranes

The influence of the chlorine atom on the biological activity of 2'-hydroxychalcone in relation to the lipid phase of biological membranes - Anticancer and antimicrobial activity

The study investigates the effect of the presence of a chlorine atom in the 2'-hydroxychalcone molecule on its interaction with model lipid membranes, in order to discern its potential pharmacological activity. Five chlorine derivatives of 2'-hydroxychalcone were synthesized and evaluated against liposomes composed of POPC and enriched with cationic (DOTAP) or anionic (POPG) lipids. The physicochemical properties of the compounds were initially simulated using SwissAdame software, revealing high lipophilicity (ilogP values: 2.79-2.90). The dynamic light scattering analysis of liposomes showed that chloro chalcones induce minor changes in the diameter of liposomes of different surface charges. Fluorescence quenching assays with a TMA-DPH probe demonstrated the strong ability of the compounds to interact with the lipid bilayer, with varying quenching capacities based on chlorine atom position. FTIR studies indicated alterations in carbonyl, phosphate, and choline groups, suggesting a transition area localization rather than deep penetration into the hydrocarbon chains. Additionally, dipole potential reduction was observed in POPC and POPC-POPG membranes, particularly pronounced by derivatives with a chlorine atom in the B ring. Antibacterial and antibiofilm assays revealed enhanced activity of derivatives with a chlorine atom compared to 2'-hydroxychalcone, especially against Gram-positive bacteria. The MIC and MBIC50 values showed increased efficacy in the presence of chlorine with 3'-5'-dichloro-2'-hydroxychalcone demonstrating optimal antimicrobial and antibiofilm activity. Furthermore, antiproliferative assays against breast cancer cell lines indicated higher activity of B-ring chlorine derivatives, particularly against MDA-MB-231 cells. In general, the presence of a chlorine atom in 2'-hydroxychalcone improves its pharmacological potential, with derivatives showing improved antimicrobial, antibiofilm, and antiproliferative activities, especially against aggressive breast cancer cell lines. These findings underscore the importance of molecular structure in modulating biological activity and highlight chalcones with a chlorine as promising candidates for further drug development studies.

Modulation of the Dipole Potential of Model Lipid Membranes with Phytochemicals: Molecular Mechanisms, Structure-Activity Relationships, and Implications in Reconstituted Ion Channels

Phytochemicals, such as flavonoids, stilbenoids, alkaloids, terpenoids, and related compounds, have a wide range of useful pharmacological properties which cannot be ascribed to binding to a single peptide or protein target alone. Due to the relatively high lipophilicity of phytochemicals, the lipid membrane is thought to mediate their effects via changes in the properties of the lipid matrix, in particular, by modulating the transmembrane distribution of the electrical potential and, consequently, the formation and functioning of the ion channels reconstituted in the lipid bilayers. Therefore, biophysical studies on the interactions between plant metabolites and model lipid membranes are still of interest. This review represents an attempt to provide a critical analysis of a variety of studies on altering membranes and ion channels with phytochemicals via disturbing the potential drop at the membrane-aqueous solution interface. Critical structural motifs and functioning groups in the molecules of plant polyphenols (alkaloids and saponins are identified) and the possible mechanisms of dipole potential modulation with phytochemicals are discussed.

Rotational decoupling between the hydrophilic and hydrophobic regions in lipid membranes

Cells use homeostatic mechanisms to ensure an optimal composition of distinct types of lipids in cellular membranes. The hydrophilic region of biological lipid membranes is mainly composed of several types of phospholipid headgroups that interact with incoming molecules, nanoparticles, and viruses, whereas the hydrophobic region consists of a distribution of acyl chains and sterols affecting membrane fluidity/rigidity related properties and forming an environment for membrane-bound molecules such as transmembrane proteins. A fundamental open question is to what extent the motions of these regions are coupled and, consequently, how strongly the interactions of phospholipid headgroups with other molecules depend on the properties and composition of the membrane hydrophobic core. We combine advanced solid-state nuclear magnetic resonance spectroscopy with high-fidelity molecular dynamics simulations to demonstrate how the rotational dynamics of choline headgroups remain nearly unchanged (slightly faster) with incorporation of cholesterol into a phospholipid membrane, contrasting the well-known extreme slowdown of the other phospholipid segments. Notably, our results suggest a new paradigm in which phospholipid dipole headgroups interact as quasi-freely rotating flexible dipoles at the interface, independent of the properties in the hydrophobic region.

DPepH3, an Improved Peptide Shuttle for Receptor-independent Transport Across the Blood-Brain Barrier

Background:

The use of peptides as drug carriers across the blood-brain barrier (BBB) has increased significantly during the last decades. PepH3, a seven residue sequence (AGILKRW) derived from the α-helical domain of the dengue virus type-2 capsid protein, translocates across the BBB with very low toxicity. Somehow predictably from its size and sequence, PepH3 is degraded in serum relatively fast. Among strategies to increase peptide half-life (t1/2), the use of the enantiomer (wholly made of D-amino acid residues) can be quite successful if the peptide interacts with a target in non-stereospecific fashion.

Methods:

The goal of this work was the development of a more proteolytic-resistant peptide, while keeping the translocation properties. The serum stability, cytotoxicity, in vitro BBB translocation, and internalization mechanism of DPepH3 was assessed and compared to the native peptide.

Results:

DPepH3 demonstrates a much longer t1/2 compared to PepH3. We also confirm that BBB translocation is receptor-independent, which fully validates the enantiomer strategy chosen. In fact, we demonstrate that internalization occurs trough macropinocytosis. In addition, the enantiomer demonstrates to be non-cytotoxic towards endothelial cells as PepH3.

Conclusion:

DPepH3 shows excellent translocation and internalization properties, safety, and improved stability. Taken together, our results place DPepH3 at the forefront of the second generation of BBB shuttles.

Solid-State NMR Approaches to Study Protein Structure and Protein-Lipid Interactions

Solid-state NMR spectroscopy has been developed for the investigation of membrane-associated polypeptides and remains one of the few techniques to reveal high-resolution structural information in liquid-disordered phospholipid bilayers. In particular, oriented samples have been used to investigate the structure, dynamics and topology of membrane polypeptides. Much of the previous solid-state NMR work has been developed and performed on peptides but the technique is constantly expanding towards larger membrane proteins. Here, a number of protocols are presented describing among other the reconstitution of membrane proteins into oriented membranes, monitoring membrane alignment by ³¹P solid-state NMR spectroscopy, investigations of the protein by one- and two-dimensional ¹⁵N solid-state NMR and measurements of the lipid order parameters using ²H solid-state NMR spectroscopy. Using such methods solid-state NMR spectroscopy has revealed a detailed picture of the ensemble of both lipids and proteins and their mutual interdependence in the bilayer environment.

A New Method of Assessing Lipid Mixtures by 31P Magic-Angle Spinning NMR

A variety of lipids that differ by their chains and headgroups are found in biomembranes. In addition to studying the overall membrane phase, determination of the structure, dynamics, and headgroup conformation of individual lipids in the mixture would be of great interest. We have thus developed, to our knowledge, a new approach using solid-state ³¹P NMR, magic-angle spinning, and chemical-shift anisotropy (CSA) recoupling, using an altered version of the recoupling of chemical shift anisotropy (ROCSA) pulse sequence, here penned PROCSA. The resulting two-dimensional spectra allowed the simultaneous measurement of the isotropic chemical shift and CSA of each lipid headgroup, thus providing a valuable measure of its dynamics and structure. PROCSA was applied to mixtures of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) in various relative proportions, to mimic bacterial membranes and assess the respective roles of lipids in shaping these bilayers. The results were interpreted in terms of membrane topology, lipid propensity to adopt various phases or conformations, and lipid-lipid miscibility. Our results showed that PG dictates the lipid behavior when present in a proportion of 20 mol % or more. A small proportion of PG is thus able to impose a bilayer structure to the hexagonal phase forming PE. We discuss the requirement for lipids, such as PE, to be able to adopt non-bilayer phases in a membrane.

pH-Induced Changes in the Surface Viscosity of Unsaturated Phospholipids Monitored Using Active Interfacial Microrheology

Lipid membranes, a major component of cells, are subjected to significant changes in pH depending on their location in the cell: the outer leaflet of the cell membrane is exposed to a pH of 7.4 whereas lipid membranes that make up late endosomes and lysosomes are exposed to a pH of as low as 4.4. The purpose of this study is to evaluate how changes in the environmental pH within cells alter the fluidity of phospholipid membranes. Specifically, we studied pH-induced alterations in the surface arrangement of monounsaturated lipids with zwitterionic headgroups (phosphoethanolamine (PE) and phosphocholine (PC)) that are abundant in plasma membranes as well as anionic lipids (phosphatidylserine (PS) and phosphatidylglycerol (PG)) that are abundant in inner membranes using a combination of techniques including surface tension vs area measurements, interfacial microrheology, and fluorescence/atomic force microscopy. Using an active interfacial microrheology technique, we find that phospholipids with zwitterionic headgroups show a significant increase in their surface viscosity at acidic pH. This increase in surface viscosity is also found to depend on the size of the lipid headgroup, with a smaller headgroup showing a greater increase in viscosity. The observed pH-induced increase in viscosity is also accompanied by an increase in the cohesion pressure between zwitterionic molecules at acidic pH and a decrease in the average molecular area of the lipids, as measured by fitting the surface pressure isotherms to well-established equations of state. Because fluorescent images show no change in the phase of the lipids, we attribute this change in surface viscosity to the pH-induced reorientation of the P^--N⁺ dipoles that form part of the polar lipid headgroup, resulting in increased lipid-lipid interactions. Anionic PG headgroups do not demonstrate this pH-induced change in viscosity, suggesting that the presence of a net negative charge on the headgroup causes electrostatic repulsion between the headgroups. Our results also show that active interfacial microrheology is a sensitive technique for detecting minute changes in the lipid headgroup orientation induced by changes in the local membrane environment, even in unsaturated phospholipids where the surface viscosity is close to the experimental detection limit.

The Dipole Potential Modifies the Clustering and Ligand Binding Affinity of ErbB Proteins and Their Signaling Efficiency

Although activation of the ErbB family of receptor tyrosine kinases (ErbB1-4) is driven by oligomerization mediated by intermolecular interactions between the extracellular, the kinase and the transmembrane domains, the transmembrane domain has been largely neglected in this regard. The largest contributor to the intramembrane electric field, the dipole potential, alters the conformation of transmembrane peptides, but its effect on ErbB proteins is unknown. Here, we show by Förster resonance energy transfer (FRET) and number and brightness (N&B) experiments that the epidermal growth factor (EGF)-induced increase in the homoassociation of ErbB1 and ErbB2 and their heteroassociation are augmented by increasing the dipole potential. These effects were even more pronounced for ErbB2 harboring an activating Val → Glu mutation in the transmembrane domain (NeuT). The signaling capacity of ErbB1 and ErbB2 was also correlated with the dipole potential. Since the dipole potential decreased the affinity of EGF to ErbB1, the augmented growth factor-induced effects at an elevated dipole potential were actually induced at lower receptor occupancy. We conclude that the dipole potential plays a permissive role in the clustering of ErbB receptors and that the effects of lipid rafts on ligand binding and receptor signaling can be partially attributed to the dipole potential.

Red Blood Cell Susceptibility to Pneumolysin: CORRELATION WITH MEMBRANE BIOCHEMICAL AND PHYSICAL PROPERTIES

This study investigated the effect of the biochemical and biophysical properties of the plasma membrane as well as membrane morphology on the susceptibility of human red blood cells to the cholesterol-dependent cytolysin pneumolysin, a key virulence factor of Streptococcus pneumoniae, using single cell studies. We show a correlation between the physical properties of the membrane (bending rigidity and surface and dipole electrostatic potentials) and the susceptibility of red blood cells to pneumolysin-induced hemolysis. We demonstrate that biochemical modifications of the membrane induced by oxidative stress, lipid scrambling, and artificial cell aging modulate the cell response to the toxin. We provide evidence that the diversity of response to pneumolysin in diabetic red blood cells correlates with levels of glycated hemoglobin and that the mechanical properties of the red blood cell plasma membrane are altered in diabetes. Finally, we show that diabetic red blood cells are more resistant to pneumolysin and the related toxin perfringolysin O relative to healthy red blood cells. Taken together, these studies indicate that the diversity of cell response to pneumolysin within a population of human red blood cells is influenced by the biophysical and biochemical status of the plasma membrane and the chemical and/or oxidative stress pre-history of the cell.

Phloretin-induced cytoprotective effects on mammalian cells: A mechanistic view and future directions

Phloretin (C15 H14 O5 ), a dihydrochalcone flavonoid, is mainly found in fruit, leaves, and roots of apple tree. Phloretin exerts antioxidant, anti-inflammatory, and anti-tumor activities in mammalian cells through mechanisms that have been partially elucidated throughout the years. Phloretin bioavailability is well known in humans, but still remains to be better studied in experimental animals, such as mouse and rat. The focus of the present review is to gather information regarding the mechanisms involved in the phloretin-elicited effects in different in vitro and in vivo experimental models. Several manuscripts were analyzed and data raised by authors were described and discussed here in a mechanistic manner. Comparisons between the effects elicited by phloretin and phloridzin were made whenever possible, as well as with other polyphenols, clarifying questions about the use of phloretin as a potential therapeutic agent. Toxicological aspects associated to phloretin exposure were also discussed here. Furthermore, a special section containing future directions was created as a suggestive guide towards the elucidation of phloretin-related actions in mammalian cells and tissues.

Reparameterized United Atom Model for Molecular Dynamics Simulations of Gel and Fluid Phosphatidylcholine Bilayers

A new united atom parametrization of diacyl lipids like dipalmitoylphosphatidylcholine (DPPC) and the dimyristoylphosphatidylcholine (DMPC) has been constructed based on ab initio calculations to obtain fractional charges and the dihedral potential of the hydrocarbon chains, while the Lennard-Jones parameters of the acyl chains were fitted to reproduce the properties of liquid hydrocarbons. The results have been validated against published experimental X-ray and neutron scattering data for fluid and gel phase DPPC. The derived charges of the lipid phosphatidylcholine (PC) headgroup are shown to yield dipole components in the range suggested by experiments. The aim has been to construct a new force field that retains and improves the good agreement for the fluid phase and at the same time produces a gel phase at low temperatures, with properties coherent with experimental findings. The gel phase of diacyl-PC lipids forms a regular triangular lattice in the hydrocarbon region. The global bilayer tilt obtains an azimuthal value of 31° and is aligned between lattice vectors in the bilayer plane. We also show that the model yields a correct heat of melting as well as decent heat capacities in the fluid and gel phase of DPPC.

α-Tocopherols modify the membrane dipole potential leading to modulation of ligand binding by P-glycoprotein

α-Tocopherol (vitamin E) has attracted considerable attention as a potential protective or palliative agent. In vitro, its free radical-scavenging antioxidant action has been widely demonstrated. In vivo, however, vitamin E treatment exhibits negligible benefits against oxidative stress. α-Tocopherol influences lipid ordering within biological membranes and its derivatives have been suggested to inhibit the multi-drug efflux pump, P-glycoprotein (P-gp). This study employs the fluorescent membrane probe, 1-(3-sulfonatopropyl)-4-[β[2-(di-n-octylamino)-6-naphthyl]vinyl] pyridinium betaine, to investigate whether these effects are connected via influences on the membrane dipole potential (MDP), an intrinsic property of biological membranes previously demonstrated to modulate P-gp activity. α-Tocopherol and its non-free radical-scavenging succinate analog induced similar decreases in the MDP of phosphatidylcholine vesicles. α-Tocopherol succinate also reduced the MDP of T-lymphocytes, subsequently decreasing the binding affinity of saquinavir for P-gp. Additionally, α-tocopherol succinate demonstrated a preference for cholesterol-treated (membrane microdomain enriched) cells over membrane cholesterol-depleted cells. Microdomain disruption via cholesterol depletion decreased saquinavir's affinity for P-gp, potentially implicating these structures in the influence of α-tocopherol succinate on P-gp. This study provides evidence of a microdomain dipole potential-dependent mechanism by which α-tocopherol analogs influence P-gp activity. These findings have implications for the use of α-tocopherol derivatives for drug delivery across biological barriers.

The uptake of HIV Tat peptide proceeds via two pathways which differ from macropinocytosis

Cell penetrating peptides (CPPs) have been extensively studied as vectors for cellular delivery of therapeutic molecules, yet the identity of their uptake routes remained unclear and is still under debate. In this study we provide new insights into CPP entry routes by quantitatively measuring the intracellular uptake of FAM-labeled Tat-peptide under rigorous kinetic and thermal conditions. The uptake of Tat-peptide between 4 and 15°C corresponds to Q10=1.1, proceeding through a prompt (<5 min), temperature-independent process, suggesting direct membrane translocation. At longer durations, Tat rate of uptake shows linear dependence on temperature with Q10=1.44, accompanied by activation energy Ea=4.45 Kcal/mole. These values are significantly lower than those we found for the macropinocytosis probe dextran (Q10=2.2 and Ea=7.2 Kcal/mole) which possesses an exponential dependence on temperature, characteristic of endocytosis processes. Tat-peptide and dextran do not interfere with each other's uptake rate and the ratio of Tat-peptide uptake to its extracellular concentration is ~15 times higher than that for dextran. In addition, Phloretin, a modulator of cell membrane dipole potential, is shown to increase dextran uptake but to reduce that of Tat. We conclude that the uptake of Tat differs from that of dextran in all parameters. Tat uptake proceeds by dual entry routes which differ by their energy dependence.

Water behavior in bacterial spores by deuterium NMR spectroscopy

Dormant bacterial spores are able to survive long periods of time without nutrients, withstand harsh environmental conditions, and germinate into metabolically active bacteria when conditions are favorable. Numerous factors influence this hardiness, including the spore structure and the presence of compounds to protect DNA from damage. It is known that the water content of the spore core plays a role in resistance to degradation, but the exact state of water inside the core is a subject of discussion. Two main theories present themselves: either the water in the spore core is mostly immobile and the core and its components are in a glassy state, or the core is a gel with mobile water around components which themselves have limited mobility. Using deuterium solid-state NMR experiments, we examine the nature of the water in the spore core. Our data show the presence of unbound water, bound water, and deuterated biomolecules that also contain labile deuterons. Deuterium–hydrogen exchange experiments show that most of these deuterons are inaccessible by external water. We believe that these unreachable deuterons are in a chemical bonding state that prevents exchange. Variable-temperature NMR results suggest that the spore core is more rigid than would be expected for a gel-like state. However, our rigid core interpretation may only apply to dried spores whereas a gel core may exist in aqueous suspension. Nonetheless, the gel core, if present, is inaccessible to external water.

Molecular dynamic simulation of transmembrane pore growth

A molecular dynamic approach was applied for simulation of dynamics of pore formation and growth in a phospholipid bilayer in the presence of an external electric field. Processing the simulation results permitted recovery of the kinetic coefficients used in the Einstein-Smoluchowski equation describing the dynamics of pore evolution. Two different models of the bilayer membrane were considered: membrane consisting of POPC and POPE lipids. The simulations permitted us to find nonempirical values of the pore energy parameters, which are compared with empirical values. It was found that the parameters are sensitive to membrane type.

Solid-state NMR approaches to study protein structure and protein-lipid interactions

Solid-state NMR spectroscopy has been developed for the investigation of membrane-associated polypeptides and remains one of the few techniques to reveal high-resolution structural information in liquid-disordered phospholipid bilayers. In particular, oriented samples have been used to investigate the structure, dynamics, and topology of membrane polypeptides. Much of the previous solid-state NMR work has been developed and performed on peptides, but the technique is constantly expanding towards larger membrane proteins. Here, a number of protocols are presented describing among other the reconstitution of membrane proteins into oriented membranes, monitoring membrane alignment by (31)P solid-state NMR spectroscopy; investigations of the protein by one- and two-dimensional (15)N solid-state NMR; and measurements of the lipid order parameters using (2)H solid-state NMR spectroscopy. Using such methods solid-state NMR spectroscopy has revealed a detailed picture of the ensemble of both lipids and proteins and their mutual interdependence in the bilayer environment.

Determining the mode of action involved in the antimicrobial activity of synthetic peptides: a solid-state NMR and FTIR study

We have previously shown that leucine to lysine substitution(s) in neutral synthetic crown ether containing 14-mer peptide affect the peptide structure and its ability to permeabilize bilayers. Depending on the substitution position, the peptides adopt mainly either a α-helical structure able to permeabilize dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) vesicles (nonselective peptides) or an intermolecular β-sheet structure only able to permeabilize DMPG vesicles (selective peptides). In this study, we have used a combination of solid-state NMR and Fourier transform infrared spectroscopy to investigate the effects of nonselective α-helical and selective intermolecular β-sheet peptides on both types of bilayers. (31)P NMR results indicate that both types of peptides interact with the headgroups of DMPC and DMPG bilayers. (2)H NMR and Fourier transform infrared results reveal an ordering of the hydrophobic core of bilayers when leakage is noted, i.e., for DMPG vesicles in the presence of both types of peptides and DMPC vesicles in the presence of nonselective peptides. However, selective peptides have no significant effect on the ordering of DMPC acyl chains. The ability of these 14-mer peptides to permeabilize lipid vesicles therefore appears to be related to their ability to increase the order of the bilayer hydrophobic core.

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Solid-state NMR spectroscopic studies on the interaction of sorbic acid with phospholipid membranes at different pH levels

2H, 31P, and 1H-magic-angle-spinning (MAS) solid-state NMR spectroscopic methods were used to elucidate the interaction between sorbic acid, a widely used weak acid food preservative, and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers under both acidic and neutral pH conditions. The linewidth broadening observed in the 31P NMR powder pattern spectra and the changes in the 31P longitudinal relaxation time (T1) indicate interaction with the phospholipid headgroup upon titration of sorbic acid or decanoic acid into DMPC bilayers over the pH range from 3.0 to 7.4. The peak intensities of sorbic acid decrease upon addition of paramagnetic Mn2+ ions in DMPC bilayers as recorded in the 1H MAS NMR spectra, suggesting that sorbic acid molecules are in close proximity with the membrane/aqueous surface. No significant 2H quadrupolar splitting (DeltanuQ) changes are observed in the 2H NMR spectra of DMPC-d54 upon titration of sorbic acid, and the change of pH has a slight effect on DeltanuQ, indicating that sorbic acid has weak influence on the orientation order of the DMPC acyl chains in the fluid phase over the pH range from 3.0 to 7.4. This finding is in contrast to the results of the decanoic acid/DMPC-d54 systems, where DeltanuQ increases as the concentration of decanoic acid increases. Thus, in the membrane association process, sorbic acids are most likely interacting with the headgroups and shallowly embedded near the top of the phospholipid headgroups, rather than inserting deep into the acyl chains. Thus, antimicrobial mode of action for sorbic acid may be different from that of long-chain fatty acids.

Nanoscopic description of biomembrane electrostatics: results of molecular dynamics simulations and fluorescence probing

Electrostatic fields generated on and inside biological membranes are recognized to play a fundamental role in key processes of cell functioning. Their understanding requires an adequate description on the level of elementary charges and the reconstruction of electrostatic potentials by integration over all elementary interactions. Out of all the available research tools, only molecular dynamics simulations are capable of this, extending from the atomic to the mesoscopic level of description on the required time and space scale. A complementary approach is that offered by molecular probe methods, with the application of electrochromic dyes. Highly sensitive to intermolecular interactions, they generate integrated signals arising from electric fields produced by elementary charges at the sites of their location. This review is an attempt to provide a critical analysis of these two approaches and their present and potential applications. The results obtained by both methods are consistent in that they both show an extremely complex profile of the electric field in the membrane. The nanoscopic view, with two-dimensional averaging over the bilayer plane and formal separation of the electrostatic potential into surface (Psi(s)), dipole (Psi(d)) and transmembrane (Psi(t)) potentials, is constructive in the analysis of different functional properties of membranes.

Effect of dipole potential variations on the surface charge potential of lipid membranes

When the dipole potential of dimyristoylphosphatidylcholine (DMPC) monolayers was decreased, either by the insertion of phloretin or by the elimination of carbonyl groups at the interphase, the surface charge potential was displaced to lower negative values. At low ionic strength, the decrease of the negative charge density can be ascribed to a different exposure of the phosphate to water, as there is a good correlation to an increase in the area per lipid. At high ionic strength, the magnitude of the changes in the zeta potential produced by the effects on the dipole potential was found to be dependent on the type of anions present in the subphase. Differences between Cl- and ClO4- were ascribed to the adsorption of anions according to their different hydrations and polarizabilities. The influence of a low dipole potential on the anion adsorption can be ascribed to a less positive image charge at the membrane interior, resulting from an increase in the hydrocarbon core permittivity. This is congruent with the neutralization of interfacial dipoles and the area increase, as well as with the decrease in packing of the hydrocarbon groups. Phloretin did not cause changes in the dipole potential of dimyristoylphosphatidylethanolamine (DMPE), and in consequence, no effects on the zeta potential were measured. It is concluded that changes in the inner water/hydrocarbon plane affect the electrostatic potential measured in the outer plane of the polar headgroup region.

Freezing point depression of water in phospholipid membranes: a solid-state NMR study

Lipid-water interaction plays an important role in the properties of lipid bilayers, cryoprotectants, and membrane-associated peptides and proteins. The temperature at which water bound to lipid bilayers freezes is lower than that of free water. Here, we report a solid-state NMR investigation on the freezing point depression of water in phospholipid bilayers in the presence and absence of cholesterol. Deuterium NMR spectra at different temperatures ranging from -75 to + 10 degrees C were obtained from fully (2)H2O-hydrated POPC (1-palmitoyl-2-oleoylphosphatidylcholine) multilamellar vesicles (MLVs), prepared with and without cholesterol, to determine the freezing temperature of water and the effect of cholesterol on the freezing temperature of water in POPC bilayers. Our 2H NMR experiments reveal the motional behavior of unfrozen water molecules in POPC bilayers even at temperatures significantly below 0 degrees C and show that the presence of cholesterol further lowered the freezing temperature of water in POPC bilayers. These results suggest that in the presence of cholesterol the fluidity and dynamics of lipid bilayers can be retained even at very low temperatures as exist in the liquid crystalline phase of the lipid. Therefore, bilayer samples prepared with a cryoprotectant like cholesterol should enable the performance of multidimensional solid-state NMR experiments to investigate the structure, dynamics, and topology of membrane proteins at a very low temperature with enhanced sample stability and possibly a better sensitivity. Phosphorus-31 NMR data suggest that lipid bilayers can be aligned at low temperatures, while 15N NMR experiments demonstrate that such aligned samples can be used to enhance the signal-to-noise ratio of is 15N chemical shift spectra of a 37-residue human antimicrobial peptide, LL-37.

Disordered proteins: biological membranes as two-dimensional aggregation matrices

Aberrant folded proteins and peptides are hallmarks of amyloidogenic diseases. However, the molecular processes that cause these proteins to adopt non-native structures in vivo and become cytotoxic are still largely unknown, despite intense efforts to establish a general molecular description of their behavior. Clearly, the fate of these proteins is ultimately linked to their immediate biochemical environment in vivo. In this review, we focus on the role of biological membranes, reactive interfaces that not only affect the conformational stability of amyloidogenic proteins, but also their aggregation rates and, probably, their toxicity. We first provide an overview of recent work, starting with findings regarding the amphiphatic amyloid-beta protein (Abeta), which give evidence that membranes can directly promote aggregation, and that the effectiveness in this process can be related to the presence of specific neuronal ganglioside lipids. In addition, we discuss the implications of recent research (medin as an detailed example) regarding putative roles of membranes in the misfolding behavior of soluble, non-amphiphatic proteins, which are attracting increasing interest. The potential role of membranes in exerting the toxic action of misfolded proteins will also be highlighted in a molecular context. In this review, we discuss novel NMR-based approaches for exploring membrane-protein interactions, and findings obtained using them, which we use to develop a molecular concept to describe membrane-mediated protein misfolding as a quasi-two-dimensional process rather than a three-dimensional event in a biochemical environment. The aim of the review is to provide researchers with a general understanding of the involvement of membranes in folding/misfolding processes in vivo, which might be quite universal and important for future research concerning amyloidogenic and misfolding proteins, and possible ways to prevent their toxic actions.

Structural and functional properties of hydration and confined water in membrane interfaces

The scope of the present review focuses on the interfacial properties of cell membranes that may establish a link between the membrane and the cytosolic components. We present evidences that the current view of the membrane as a barrier of permeability that contains an aqueous solution of macromolecules may be replaced by one in which the membrane plays a structural and functional role. Although this idea has been previously suggested, the present is the first systematic work that puts into relevance the relation water-membrane in terms of thermodynamic and structural properties of the interphases that cannot be ignored in the understanding of cell function. To pursue this aim, we introduce a new definition of interphase, in which the water is organized in different levels on the surface with different binding energies. Altogether determines the surface free energy necessary for the structural response to changes in the surrounding media. The physical chemical properties of this region are interpreted in terms of hydration water and confined water, which explain the interaction with proteins and could affect the modulation of enzyme activity. Information provided by several methodologies indicates that the organization of the hydration states is not restricted to the membrane plane albeit to a region extending into the cytoplasm, in which polar head groups play a relevant role. In addition, dynamic properties studied by cyclic voltammetry allow one to deduce the energetics of the conformational changes of the lipid head group in relation to the head-head interactions due to the presence of carbonyls and phosphates at the interphase. These groups are, apparently, surrounded by more than one layer of water molecules: a tightly bound shell, that mostly contributes to the dipole potential, and a second one that may be displaced by proteins and osmotic stress. Hydration water around carbonyl and phosphate groups may change by the presence of polyhydroxylated compounds or by changing the chemical groups esterified to the phosphates, mainly choline, ethanolamine or glycerol. Thus, surface membrane properties, such as the dipole potential and the surface pressure, are modulated by the water at the interphase region by changing the structure of the membrane components. An understanding of the properties of the structural water located at the hydration sites and the functional water confined around the polar head groups modulated by the hydrocarbon chains is helpful to interpret and analyze the consequences of water loss at the membranes of dehydrated cells. In this regard, a correlation between the effects of water activity on cell growth and the lipid composition is discussed in terms of the recovery of the cell volume and their viability. Critical analyses of the properties of water at the interface of lipid membranes merging from these results and others from the literature suggest that the interface links the membrane with the aqueous soluble proteins in a functional unit in which the cell may be considered as a complex structure stabilized by water rather than a water solution of macromolecules surrounded by a semi permeable barrier.

Effects of phloretin and phloridzin on Ca2+ handling, the action potential, and ion currents in rat ventricular myocytes

The effects of the phytoestrogens phloretin and phloridzin on Ca(2+) handling, cell shortening, the action potential, and Ca(2+) and K(+) currents in freshly isolated cardiac myocytes from rat ventricle were examined. Phloretin increased the amplitude and area and decreased the rate of decline of electrically evoked Ca(2+) transients in the myocytes. These effects were accompanied by an increase in the Ca(2+) load of the sarcoplasmic reticulum, as determined by the area of caffeine-evoked Ca(2+) transients. An increase in the extent of shortening of the myocytes in response to electrically evoked action potentials was also observed in the presence of phloretin. To further examine possible mechanisms contributing to the observed changes in Ca(2+) handling and contractility, the effects of phloretin on the cardiac action potential and plasma membrane Ca(2+) and K(+) currents were examined. Phloretin markedly increased the action potential duration in the myocytes, and it inhibited the Ca(2+)-independent transient outward K(+) current (I(to)). The inwardly rectifying K(+) current, the sustained outward delayed rectifier K(+) current, and L-type Ca(2+) currents were not significantly different in the presence and absence of phloretin, nor was there any evidence that the Na(+)/Ca(2+) exchanger was affected. The effects of phloretin on Ca(2+) handling in the myocytes are consistent with its effects on I(to). Phloridzin did not significantly alter the amplitude or area of electrically evoked Ca(2+) transients in the myocytes, nor did it have detectable effects on the sarcoplasmic reticulum Ca(2+) load, cell shortening, or the action potential.

Application of fluorescence polarization microscopy to measure fluorophore orientation in the outer hair cell plasma membrane

The biophysical properties and organization of cell membranes regulate many membrane-based processes, including electromotility in outer hair cells (OHCs) of the cochlea. Studies of the membrane environment can be carried out by measuring the orientation of membrane-bound fluorophores using fluorescence polarization microscopy (FPM). Due to the cylindrical shape of OHCs, existing FPM theory developed for spherical cells is not applicable. We develop a new method for analyzing FPM data suitable for the quasi-cylindrical OHC. We present the theory for this model, as well as a study of the orientation of the fluorescent probe pyridinium, 4-[2-[6-(dioctylamino)-2-naphthalenyl]ethenyl]-1-(3-sulfopropyl) (di-8-ANEPPS) in the OHC membrane. Our results indicate that the absorption transition dipole moment of di-8-ANEPPS orients symmetrically about the membrane normal at 27 deg with respect to the plane of the membrane. The observed agreement between theoretical predictions and experimental measurements establishes the applicability of FPM to study OHC plasma membrane properties.

Impact on lipid membrane organization by free branched-chain fatty acids

Here, we exploit the non-invasive techniques of solid-state NMR (nuclear magnetic resonance) and differential scanning calorimetry (DSC) to study the effect of free iso and ante-iso branched chain fatty acids (BCFAs) on the physicochemical properties of lipid membranes. Free fatty acids are present in biological membranes at low abundance, but can influence the cellular function by modulating the membrane organization. Solid state NMR spectra of dimyristoylphosphatidylcholine (DMPC) lipid membranes containing either free 12-methyltetradecanoic acid (a15:0) or free 13-methyltetradecanoic acid (i15:0), show significant differences in their impact on the lipid bilayer. Chain order profiles obtained by deuterium NMR on fully deuterated DMPC-d(67) bilayers revealed an ordering effect induced by both fatty acids on the hydrophobic membrane core. This behavior was also visible in the corresponding DSC thermograms where the main phase transition of DMPC bilayers-indicative of the hydrophobic membrane region-was shifted to higher temperatures, with the iso isomer triggering more pronounced changes as compared to the ante-iso isomer. This is probably due to a higher packing density in the core of the lipid bilayer, which causes reduced diffusion across membranes. By utilizing the naturally occurring spin reporters nitrogen-14 and phosphorus-31 present in the hydrophilic DMPC headgroup region, even fatty acid induced changes at the membrane interface could be detected, an observation reflecting changes in the lipid headgroup dynamics.

Influence of N-dodecyl-N,N-dimethylamine N-oxide on the activity of sarcoplasmic reticulum Ca2+-transporting ATPase reconstituted into diacylphosphatidylcholine vesicles: Effects of bilayer physical parameters

Sarcoplasmic reticulum Ca-transporting ATPase (EC 3.6.1.38) was isolated from rabbit white muscle, purified and reconstituted into vesicles of synthetic diacylphosphatidylcholines with monounsaturated acyl chains using the cholate dilution method. In fluid bilayers at 37 degrees C, the specific activity of ATPase displays a maximum (31.5+/-0.8 IU/mg) for dioleoylphosphatidylcholine (diC18:1PC) and decreases progressively for both shorter and longer acyl chain lengths. Besides the hydrophobic mismatch between protein and lipid bilayer, changes in the bilayer hydration and lateral interactions detected by small angle neutron scattering (SANS) can contribute to this acyl chain length dependence. When reconstituted into dierucoylphosphatidylcholine (diC22:1PC), the zwitterionic surfactant N-dodecyl-N,N-dimethylamine N-oxide (C12NO) stimulates the ATPase activity from 14.2+/-0.6 to 32.5+/-0.8 IU/mg in the range of molar ratios C12NO:diC22:1PC=0/1.2. In dilauroylphosphatidylcholines (diC12:0PC) and diC18:1PC, the effect of C12NO is twofold-the ATPase activity is stimulated at low and inhibited at high C12NO concentrations. In diC18:1PC, it is observed an increase of activity induced by C12NO in the range of molar ratios C12NO:diC18:1PC< or =1.3 in bilayers, where the bilayer thickness estimated by SANS decreases by 0.4+/-0.1 nm. In this range, the 31P-NMR chemical shift anisotropy increases indicating an effect of C12NO on the orientation of the phosphatidylcholine dipole N(+)-P- accompanied by a variation of the local membrane dipole potential. A decrease of the ATPase activity is observed in the range of molar ratios C12NO:diC18:1PC=1.3/2.5, where mixed tubular micelles are detected by SANS in C12NO+diC18:1PC mixtures. It is concluded that besides hydrophobic thickness changes, the changes in dipole potential and curvature frustration of the bilayer could contribute as well to C12NO effects on Ca(2+)-ATPase activity.

Molecular insight into the electrostatic membrane surface potential by 14n/31p MAS NMR spectroscopy: nociceptin-lipid association

Exploiting naturally abundant (14)N and (31)P nuclei by high-resolution MAS NMR (magic angle spinning nuclear magnetic resonance) provides a molecular view of the electrostatic potential present at the surface of biological model membranes, the electrostatic charge distribution across the membrane interface, and changes that occur upon peptide association. The spectral resolution in (31)P and (14)N MAS NMR spectra is sufficient to probe directly the negatively charged phosphate and positively charged choline segment of the electrostatic P(-)-O-CH(2)-CH(2)-N(+)(CH(3))(3) headgroup dipole of zwitterionic DMPC (dimyristoylphosphatidylcholine) in mixed-lipid systems. The isotropic shifts report on the size of the potential existing at the phosphate and ammonium group within the lipid headgroup while the chemical shielding anisotropy ((31)P) and anisotropic quadrupolar interaction ((14)N) characterize changes in headgroup orientation in response to surface potential. The (31)P/(14)N isotropic chemical shifts for DMPC show opposing systematic changes in response to changing membrane potential, reflecting the size of the electrostatic potential at opposing ends of the P(-)-N(+) dipole. The orientational response of the DMPC lipid headgroup to electrostatic surface variations is visible in the anisotropic features of (14)N and (31)P NMR spectra. These features are analyzed in terms of a modified "molecular voltmeter" model, with changes in dynamic averaging reflecting the tilt of the C(beta)-N(+)(CH)(3) choline and PO(4)(-) segment. These properties have been exploited to characterize the changes in surface potential upon the binding of nociceptin to negatively charged membranes, a process assumed to proceed its agonistic binding to its opoid G-protein coupled receptor.

Physical landscapes in biological membranes: physico-chemical terrains for spatio-temporal control of biomolecular interactions and behaviour

The evolving complexities of biological membranes are discussed from the point of view of potential roles of the physical constitution of the membrane. These include features of the surface and dipole potentials and membrane 'rafts'. These properties are outlined; they emphasize that protein-lipid and specific lipid environments are influential parameters in how biomolecular interactions may take place with and within membranes. Several fluorescence detection technologies directed towards measurement of these properties are also outlined that permit high-resolution experimental determination of intermolecular interactions with membranes by measuring small changes of these potentials. These point to the possibility that the membrane dipole potential in particular is enormously influential in determining the behaviour of receptor and signalling systems within membrane rafts, and offers the means of a novel mechanism for biological control.

The alignment, structure and dynamics of membrane-associated polypeptides by solid-state NMR spectroscopy

Solid-state NMR spectroscopy is being developed at a fast pace for the structural investigation of immobilized and non-crystalline biomolecules. These include proteins and peptides associated with phospholipid bilayers. In contrast to solution NMR spectroscopy, where complete or almost complete averaging leads to isotropic values, the anisotropic character of nuclear interactions is apparent in solid-state NMR spectra. In static samples the orientation dependence of chemical shift, dipolar or quadrupolar interactions, therefore, provides angular constraints when the polypeptides have been reconstituted into oriented membranes. Furthermore, solid-state NMR spectroscopy of aligned samples offers distinct advantages in allowing access to dynamic processes such as topological equilibria or rotational diffusion in membrane environments. Alternatively, magic angle sample spinning (MAS) results in highly resolved NMR spectra, provided that the sample is sufficiently homogenous. MAS spinning solid-state NMR spectra allow to measure distances and dihedral angles with high accuracy. The technique has recently been developed to selectively establish through-space and through-bond correlations between nuclei, similar to the approaches well-established in solution-NMR spectroscopy.

Effect of phloretin on the dipole potential of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol monolayers

The effect of phloretin on the potential of phosphatidylcholine (PC), phosphatidylethanolamine (PE,) and phosphatidylglycerol (PG) monolayers below and above the phase transition in mixtures of different PC/PE ratios with and without cholesterol of ester and ether phospholipids have been determined. The effectiveness of phloretin to decrease the dipole potential of monolayers in the fluid state is lessened by the moieties esterified to the phosphate group in the sequence choline > ethanolamine > glycerol. These effects on the dipole potential of monolayers are independent of the presence of carbonyls. In addition, in the gel state phloretin does not affect the dipole potential on dimyristoylphosphatidylethanolamine, although it is very pronounced in dimyristoylphosphatidylcholine. The changes of the dipole potential induced by phloretin were correlated with the packing of the lipids and with the formation of intermolecular hydrogen bonds between adjacent phospholipid molecules. These results may be indicative of the different distribution of polarized water around the phosphate groups imposed by the surrounding environment.

Influence of phloretin on the skin permeation of lidocaine from semisolid preparations

The purpose of the present study was to determine the significance of phloretin on the topical permeation of lidocaine using different semisolid preparations as delivery systems. One hydrophilic and three lipophilic formulations were used. After estimation of the solubility of phloretin and lidocaine in the vehicles and analysis of the viscoelastic properties, standard diffusion experiments with Franz type diffusion cells through porcine skin were performed. Results indicate a general lidocaine enhancement by phloretin in the tested vehicles. The permeation was enhanced 1.39-fold in the hydrophilic formulation and between 1.25- and 1.76-fold in the lipophilic formulations.

Phloretin and 6-ketocholestanol: membrane interactions studied by a phospholipid/polydiacetylene colorimetric assay and differential scanning calorimetry

The aim of this study was to investigate membrane interactions of phloretin and 6-ketocholestanol using different methods. A previously reported colorimetric assay with phospholipid/polydiacetylene (PDA) vesicles was used to examine a possible interaction of phloretin and 6-ketocholestanol with this target. During this interaction the used aggregates of lipids and conjugated PDA undergo a visible and quantifiable blue to red color transition. A positive result is indicative for a reaction response with membrane lipids of a simplified bilayer structure instead of the complex bilayer system of the stratum corneum. Results of this test confirm previous proposed membrane interactions by skin diffusion studies. Additional differential scanning calorimetry studies with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes confirm a membrane interaction and indicates that phloretin and 6-ketocholestanol interact with the lipid layer and change structural parameters. They strongly decrease the lipid phase transition temperature of DMPC and DPPC liposomes by at least about 6.6 degrees C and maximally about 13.9 degrees C which refers to a higher fluidity of the membrane.

Ultrasensitive two-color fluorescence probes for dipole potential in phospholipid membranes

The principle of electrochromic modulation of excited-state intramolecular proton-transfer reaction was applied for the design of fluorescence probes with high two-color sensitivity to dipole potential, Psid, in phospholipid bilayers. We report on the effect of Psid variation on excitation and fluorescence spectra of two new 3-hydroxyflavone probes, which possess opposite orientations of the fluorescent moiety in the lipid bilayer. The dipole potential in the bilayer was modulated by the addition of 6-ketocholestanol or phloretin and by substitution of dimyristoyl phosphatidylcholine lipid with its ether analog 1,2-di-o-tetradecyl-sn-glycero-3-phosphocholine, and its value was estimated by the reference styryl dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine. We demonstrate that after Psid changes, the probe orienting in the bilayer similarly to the reference dye shows similar shifts in the excitation spectra, whereas the probe with the opposite orientation shows the opposite shifts. The new observation is that the response of 3-hydroxyflavone probes to Psid in excitation spectra is accompanied by and quantitatively correlated with dramatic changes of relative intensities of the two well separated emission bands that belong to the initial normal and the product tautomer forms of the excited-state intramolecular proton-transfer reaction. This provides a strong response to Psid by change in emission color.

Efficacy and irritancy of enhancers on the in-vitro and in-vivo percutaneous absorption of curcumin

Curcumin is a predominant compound derived from the rhizomes of Curcuma longa L., and shows antibacterial, anti-inflammatory and antineoplastic activity. The in-vitro and in-vivo skin absorption of curcumin was investigated after application of enhancers using Wistar rat as an animal model. The enhancers selected in this study included terpenes, flavonoids and cholestanol. The irritant profiles of these enhancers were also established by transepidermal water loss (TEWL) and histological observations. Cyclic monoterpenes generally showed stronger enhancement of curcumin permeation than the other enhancers. Modulation of concentration and pretreatment duration of enhancers possibly indicated that the enhancers have varied ability and mechanisms to enhance curcumin permeation. Terpineol produced the highest TEWL values among the enhancers tested, whereas ketocholestanol produced no, or only a negligible, increase in TEWL as compared with control. The results showed that skin disruption and inflammation did not necessarily correspond to the enhancing efficiency of the enhancers.

Influence of molecular dipoles on human skin permeability: Use of 6-ketocholestanol to enhance the transdermal delivery of bacitracin

In the present work, we report the possibility of modifying the electrostatic properties of the skin by treating human epidermis with compounds whose structures possess a large molecular dipole moment. Data are presented showing that such a modification can be used to enhance dermal drug delivery. Inclusion of such compounds in biological membranes affects the so-called membrane dipole potential, an electrical potential originating from molecular dipoles present on the lipid molecules. Modifications in the magnitude of this potential are known to affect the interaction of hydrophobic ions and peptides with model membranes. Using fluorescein-labeled bacitracin and confocal microscopy, we show that the penetration of the antibiotic peptide bacitracin into the epidermis is enhanced when the skin has been pretreated with liposomes loaded with 30 mol % 6-ketocholestanol, a compound known to increase the magnitude of the membrane dipole potential. Studies using the fluorescent indicators fluoresceinphosphatidylethanolamine and 1-(3-sulfonatopropyl)-4-[beta [2-(di-n-octylamino)-6-naphthyl] vinyl] pyridinium betaine show that the interaction of bacitracin with model membranes is also enhanced by the presence of 6-ketocholestanol in the bilayer and offers some indication to the mechanism of penetration enhancement.

Influence of phloretin and 6-ketocholestanol on the skin permeation of sodium-fluorescein

In the present study, we investigated the feasibility of enhancing the transport of the model drug sodium-fluorescein across rat, porcine and human skin by treating it with phloretin and 6-ketocholestanol. Both 6-ketocholestanol and phloretin were incorporated into unilamellar liposomes and used as a skin pre-treatment. Afterwards standard diffusion experiments with Franz-type diffusion cells were performed. The results indicate a positive effect of 6-ketocholestanol on the sodium-fluorescein diffusion in all skin types, whereas phloretin had no significant influence compared to the control in human skin. In contrast to this, phloretin had a significant positive effect in rat as well as in porcine skin after 30 h of diffusion. In addition to the permeation studies, the human skin samples were analysed by confocal laser scanning microscopy for direct visualisation of the sodium-fluorescein. As expected the results of the diffusion experiments were confirmed by this microscopic method.

The effect of asymmetric surface potentials on the intramembrane electric field measured with voltage-sensitive dyes

Ratiometric imaging of styryl potentiometric dyes can be used to measure the potential gradient inside the membrane (intramembrane potential), which is the sum of contributions from transmembrane potential, dipole potential, and the difference in the surface potentials at both sides of the membrane. Here changes in intramembrane potential of the bilayer membranes in two different preparations, lipid vesicles and individual N1E-115 neuroblastoma cells, are calculated from the fluorescence ratios of di-4-ANEPPS and di-8-ANEPPS as a function of divalent cation concentration. In lipid vesicles formed from the zwitterionic lipid phosphatidylcholine (PC) or from a mixture of the negatively charged lipid phosphatidylserine (PS) and PC, di-4-ANEPPS produces similar spectral changes in response to both divalent cation-induced changes in intramembrane potential and transmembrane potential. The changes in potential on addition of divalent cations measured by the fluorescence ratios of di-4-ANEPPS are consistent with a change in surface potential that can be modeled with the Gouy-Chapman-Stern theory. The derived intrinsic 1:1 association constants of Ba and Mg with PC are 1.0 and 0.4 M(-1); the intrinsic 1:1 association constants of Ba and Mg with PS are 1.9 and 1.8 M(-1). Ratiometric measurements of voltage sensitive dyes also allow monitoring of intramembrane potentials in living cells. In neuroblastoma cells, a tenfold increase of concentration of Ba, Mg, and Ca gives a decrease in intramembrane potential of 22 to 24 mV. The observed changes in potential could also be described by Gouy-Chapman theory. A surface charge density of 1 e(-)/115 A(2) provides the best fit and the intrinsic 1:1 association constants of Ba, Mg, and Ca with acidic group in the surface are 1.7, 6.1, and 25.3 M(-1).