Structural requirement for the rapid movement of charged molecules across membranes. Experiments with tetraphenylborate analogues

Chloroquine-analogues block anthrax protective antigen channels in steady-state and kinetic studies

The tripartite anthrax toxin from Bacillus anthracis represents the prototype of A-B type of toxins, where the effector A (an enzymatic subunit) is transported with the help of a binding component B into a target cell. Anthrax toxin consists of three different molecules, two effectors, lethal factor (LF) and edema factor (EF) and the binding component also known as protective antigen (PA). PA forms heptamers or octamers following binding to host cell's receptors and mediates the translocation of the effectors into the cytosol via the endosomal pathway. The cation-selective PA₆₃-channel is able to reconstitute in lipid membranes and can be blocked by chloroquine and other heterocyclic compounds. This suggests that the PA₆₃-channel contains a binding site for quinolines. In this study, we investigated the structure-function relationship of different quinolines for the block of the PA₆₃-channel. The affinity of the different chloroquine analogues to the PA₆₃-channel as provided by the equilibrium dissociation constant was measured using titrations. Some quinolines had a much higher affinity to the PA₆₃-channel than chloroquine itself. We also performed ligand-induced current noise measurements using fast Fourier transformation to get insight in the kinetics of the binding of some quinolines to the PA₆₃-channel. The on-rate constants of ligand binding were around 10⁸M^-1·s^-1 at 150mM KCl and were only little dependent on the individual quinoline. The off-rates varied between 4s^-1 and 160s^-1 and depended much more on the structure of the molecules than the on-rate constants. The possible use of the 4-aminoquinolines as a therapy is discussed.

Membrane Permeability of Modified Butyltriphenylphosphonium Cations

The alkyltriphenylphosphonium (TPP) group is the most widely used vector targeted to mitochondria. Previously, the length of the alkyl linker was varied as well as structural modifications in the TPP phenyl rings to obtain the optimal therapeutic effect of a pharmacophore conjugated with a lipophilic cation. In the present work, we synthesized butyltriphenylphosphonium cations halogenated and methylated in phenyl rings (C₄TPP-X) and measured electrical current through a planar lipid bilayer in the presence of C₄TPP-X. The permeability of C₄TPP-X varied in the range of 6 orders of magnitude and correlates well with the previously measured translocation rate constant for dodecyltriphenylphosphonium analogues. The partition coefficient of the butyltriphenylphosphonium analogues obtained by calculating the difference in the free energy of cation solvation in water and octane using quantum chemical methods correlates well with the permeability values. Using an ion-selective electrode, a lower degree of accumulation of analogues with halogenated phenyl groups was found on isolated mitochondria of rat liver, which is in agreement with their permeability decrease. Our results indicate the translocation of the butyltriphenylphosphonium cations across the hydrophobic membrane core as rate-limiting stage in the permeability process rather than their binding/release to/from the membrane.

Tetra-arylborate lipophilic anions as targeting groups

Tetraphenylborate (TPB) anions traverse membranes but are excluded from mitochondria by the membrane potential (Δψ). TPB-conjugates also distributed across membranes in response to Δψ, but surprisingly, they rapidly entered cells. They accumulated within lysosomes following endocystosis. This pH-independent targeting of lysosomes makes possible new classes of probe and bioactive molecules.

Environmental Sorption Behavior of Ionic and Ionizable Organic Chemicals

Traditionally our tools for environmental risk assessment of organic chemicals have been developed for neutral chemicals. However, many commercial chemicals are ionic or ionizable and require different tools and approaches for their assessment. In recent years this task starts to obtain increasing attention but our understanding for their environmental fate is still far behind that for neutral chemicals. This review first gives an overview on the principles that govern ionic partitioning in environmental systems which are more complex than the simple partition processes of neutral chemicals. Second, a summary of our current knowledge on various topics such as bioaccumulation, sorption in soils, and nonspecific-toxicity reveals that ionic species can actually be quite hydrophobic contrary to commonly held beliefs. Eventually, we discuss existing models for the quantitative prediction of organic ions' sorption in soils and biota. We have to assert that the available model tools are quite restricted in their application range compared to neutral chemicals which is due to the higher complexity of the various ionic sorption processes. In order to further advance our understanding more high-quality sorption data are needed with a focus on multivalent and zwitterionic ions in all partition systems as well as cations in biological matrices.

Lipophilic ion aromaticity is not important for permeability across lipid membranes

To clarify the contribution of charge delocalization in a lipophilic ion to the efficacy of its permeation through a lipid membrane, we compared the behavior of alkyl derivatives of triphenylphosphonium, tricyclohexylphosphonium and trihexylphosphonium both in natural and artificial membranes. Exploring accumulation of the lipophilic cations in response to inside-negative membrane potential generation in mitochondria by using an ion-selective electrode revealed similar mitochondrial uptake of butyltricyclohexylphosphonium (C₄TCHP) and butyltriphenylphosphonium (C₄TPP). Fluorescence correlation spectroscopy also demonstrated similar membrane potential-dependent accumulation of fluorescein derivatives of tricyclohexyldecylphosphonium and decyltriphenylphosphonium in mitochondria. The rate constant of lipophilic cation translocation across the bilayer lipid membrane (BLM), measured by the current relaxation method, moderately increased in the following sequence: trihexyltetradecylphosphonium ([P_6,6,6,14]) < triphenyltetradecylphosphonium (C₁₄TPP) < tricyclohexyldodecylphosphonium (C₁₂TCHP). In line with these results, measurements of the BLM stationary conductance indicated that membrane permeability for C₄TCHP is 2.5 times higher than that for C₄TPP. Values of the difference in the free energy of ion solvation in water and octane calculated using the density functional theory and the polarizable continuum solvent model were similar for methyltriphenylphosphonium, tricyclohexylmethylphosphonium and trihexylmethylphosphonium. Our results prove that both cyclic and aromatic moieties are not necessary for lipophilic ions to effectively permeate through lipid membranes.

Pyrrolomycins Are Potent Natural Protonophores

The escalating burden of antibiotic drug resistance necessitates research into novel classes of antibiotics and their mechanism of action. Pyrrolomycins are a family of potent natural product antibiotics with nanomolar activity against Gram-positive bacteria, yet with an elusive mechanism of action. In this work, we dissect the apparent Gram-positive specific activity of pyrrolomycins and show that Gram-negative bacteria are equally sensitive to pyrrolomycins when drug efflux transporters are removed and that albumin in medium plays a large role in pyrrolomycin activity. The selection of resistant mutants allowed for the characterization and validation of a number of mechanisms of resistance to pyrrolomycins in both Staphylococcus aureus and an Escherichia coli ΔtolC mutant, all of which appear to affect compound penetration rather than being target associated. Imaging of the impact of pyrrolomycin on the E. coli ΔtolC mutant using scanning electron microscopy showed blebbing of the bacterial cell wall often at the site of bacterial division. Using potentiometric probes and an electrophysiological technique with an artificial bilayer lipid membrane, it was demonstrated that pyrrolomycins C and D are very potent membrane-depolarizing agents, an order of magnitude more active than conventional carbonyl cyanide m-chlorophenylhydrazone (CCCP), specifically disturbing the proton gradient and uncoupling oxidative phosphorylation via protonophoric action. This work clearly unveils the until-now-elusive mechanism of action of pyrrolomycins and explains their antibiotic activity as well as mechanisms of innate and acquired drug resistance in bacteria.

Effect of methyl and halogen substituents on the transmembrane movement of lipophilic ions

The introduction of a halogen or a methyl substituent changes the speed of the flip-flop of the penetrating cations and anions in the opposite way.

Passive Permeability of Planar Lipid Bilayers to Organic Anions

The membrane permeability P of organic ions was reported to be governed by the structure of the permeating molecule. Thus far, it is unclear whether the ion structure alters membrane partition or translocation proper across the membrane. Here, we obtained P values for 24 anionic compounds (18 concrete values, 6 upper limits) measuring the current that they carry through folded planar lipid bilayers. The P values range over more than 10 log units. Our measured permeability values correlate well (r = 0.95; logRMSE 0.74) with the hexadecane/water partition coefficients of the respective chemicals predicted by the COSMO-RS theory. Other attempts to predict P from the partition coefficient of the neutral molecule and from the solvation energy (Born energy) that opposes transfer into the membrane once the molecule is charged were unsuccessful. The uncertainties in assigning an effective radius to nonspherical molecules were much too large. The observation underlines that the actual structure of the molecules needs to be considered to predict partition and thus P by the solubility-diffusion model.

Effect of Alkyl Chain Length on Translocation of Rhodamine B n-Alkyl Esters across Lipid Membranes

Voltage-dependent translocation of a series of cationic rhodamine B derivatives differing in n-alkyl chain length (ethyl, butyl, octyl, dodecyl, octadecyl) from one lipid monolayer to another was studied by measuring electrical current relaxation after a voltage jump on a planar bilayer phosphatidylcholine (PC) membrane. The rate of the translocation decreased in the following series of lipids: diphytanyl-PC > dioleyl-PC > diphytanoyl-PC > dierucoyl-PC. For all the lipids studied, the rate increased with lengthening of the hydrocarbon chain of the rhodamine derivatives, with the increase being most pronounced for the compounds having a short alkyl chain. The results could be well explained by involvement of molecule reorientations in the process of transmembrane flip-flop of the hydrophobic membrane-bound compounds. However, an impact of membrane dipole potential on the translocation rate could not be excluded, because the dipole potential could contribute to the energy barrier for translocation of the compounds located at different depths in the water-membrane interface. Based on the data obtained, a difference in the dipole potential of ester diphytanoyl-PC membranes with respect to ether diphytanyl-PC was estimated to be 108 mV, highlighting the contribution of a layer of oriented carbonyl groups of the lipids to the membrane dipole potential.

Fast flip–flop of halogenated cobalt bis(dicarbollide) anion in a lipid bilayer membrane

Halogenation dramatically affects the flip–flop of cobalt bis(dicarbollide) across the lipid membrane causing acceleration (Cl, Br, I) or deceleration (F).

Boronated derivatives of chlorin e(6) and fluoride-containing porphyrins as penetrating anions: a study using bilayer lipid membranes

Boronated derivatives of porphyrins are studied extensively as promising compounds for boron-neutron capture therapy and photodynamic therapy. Understanding of the mechanism of their permeation across cell membranes is a key step in screening for the most efficient compounds. In the present work, we studied the ability of boronated derivatives of chlorin e(6) and porphyrins, which are mono-, di-, and tetra-anions, to permeate through planar bilayer lipid membranes (BLM). The translocation rate constants through the hydrophobic part of the lipid bilayer were estimated for monocarborane and its conjugate with chlorin e(6) by the method of electrical current relaxation. They were similar, 6.6 and 6.8 sec(-1), respectively. Conjugates of porphyrins carrying two and four carborane groups were shown to permeate efficiently through a BLM although they carry two charges and four charges, respectively. The rate of permeation of the tetraanion estimated by the BLM current had superlinear dependence on the BLM voltage. Because the resting potential of most mammalian cells is negative inside, it can be concluded that the presence of negatively-charged boronated groups in compounds should hinder the accumulation of the porphyrins in cells.

Measurement of dipole potential in bilayer lipid membranes by dielectric spectroscopy

Planar bilayer lipid membranes formed from egg phosphatidylcholine in aqueous media containing the lipophilic anion, dipicrylamine (DPA), were studied by dielectric spectroscopy over a frequency range of 10 Hz-10 MHz. The membranes showed dielectric relaxation due to the translocation of DPA between the membrane interfaces. Incorporating either cholesterol or 6-ketocholestanol into the membranes increased the characteristic frequency of the relaxation, which is proportional to the translocation rate constant of DPA. The results suggested that the sterol dipoles induced positive potential changes within the membrane interior. The changes of the dipole potential were 70 mV for cholesterol and 150 mV for 6-ketocholestanol when the sterol mole fraction was 0.67. The opposite effect was caused by phloretin added to the aqueous media, and the maximum dipole potential change was -90 mV at 100 μM.

In vitro optimization of EtNBS-PDT against hypoxic tumor environments with a tiered, high-content, 3D model optical screening platform

Hypoxia and acidosis are widely recognized as major contributors to the development of treatment resistant cancer. For patients with disseminated metastatic lesions, such as most women with ovarian cancer (OvCa), the progression to treatment resistant disease is almost always fatal. Numerous therapeutic approaches have been developed to eliminate treatment resistant carcinoma, including novel biologic, chemo, radiation, and photodynamic therapy (PDT) regimens. Recently, PDT using the cationic photosensitizer EtNBS was found to be highly effective against therapeutically unresponsive hypoxic and acidic OvCa cellular populations in vitro. To optimize this treatment regimen, we developed a tiered, high-content, image-based screening approach utilizing a biologically relevant OvCa 3D culture model to investigate a small library of side-chain modified EtNBS derivatives. The uptake, localization, and photocytotoxicity of these compounds on both the cellular and nodular levels were observed to be largely mediated by their respective ethyl side chain chemical alterations. In particular, EtNBS and its hydroxyl-terminated derivative (EtNBS-OH) were found to have similar pharmacological parameters, such as their nodular localization patterns and uptake kinetics. Interestingly, these two molecules were found to induce dramatically different therapeutic outcomes: EtNBS was found to be more effective in killing the hypoxic, nodule core cells with superior selectivity, while EtNBS-OH was observed to trigger widespread structural degradation of nodules. This breakdown of the tumor architecture can improve the therapeutic outcome and is known to synergistically enhance the antitumor effects of front-line chemotherapeutic regimens. These results, which would not have been predicted or observed using traditional monolayer or in vivo animal screening techniques, demonstrate the powerful capabilities of 3D in vitro screening approaches for the selection and optimization of therapeutic agents for the targeted destruction of specific cellular subpopulations.

The voltage-dependent step of the chloride transporter of Valonia utricularis encounters a Nernst-Planck and not an Eyring type of potential energy barrier

Voltage-clamp experiments were performed on cells of the giant marine alga Valonia utricularis to study the voltage dependence of the previously postulated chloride transporter (Wang, J., G. Wehner, R. Benz, and U. Zimmermann. 1991. Biophys. J. 59:235-248). Only one exponential current relaxation (apart from the capacitive spike) could be resolved up to a clamp voltage of approximately 120 mV within the time resolution of our experimental instrumentation (100 mus). This means that the rate constants of the heterogeneous complexation, k(R) (association) and k(D) (dissociation), were too fast to be resolved. Therefore, the "Läuger" model for carrier-mediated ion transport with equilibrium heterogeneous surface reaction was used to fit the experimental results. The voltage dependence of the initial membrane conductance was used for the evaluation of the voltage dependence of the translocation rate constant of the complexed carriers, k(AS). The initial conductance was found to be independent on the clamp voltage, which means that the translocation rate constant k(AS) is a linear function of the applied voltage and that the voltage dependence of the translocation of charged carriers through the plasmalemma could be explained by a square-type Nernst-Planck barrier. The movement of the complexed form of the carrier through the membrane may be explained by a diffusion process rather than by simple first-order kinetic jump across an Eyring-type potential well. The current relaxation after a voltage clamp was studied as a function of the external chloride concentration. The results allowed an estimation of the stability constant, K, of the heterogeneous complexation reaction and a calculation of the translocation rate constants of the free and the complexed carriers, k(s) and k(AS), respectively.

Mitochondria-targeted penetrating cations as carriers of hydrophobic anions through lipid membranes

High negative electric potential inside mitochondria provides a driving force for mitochondria-targeted delivery of cargo molecules linked to hydrophobic penetrating cations. This principle is utilized in construction of mitochondria-targeted antioxidants (MTA) carrying quinone moieties which produce a number of health benefitting effects by protecting cells and organisms from oxidative stress. Here, a series of penetrating cations including MTA were shown to induce the release of the liposome-entrapped carboxyfluorescein anion (CF), but not of glucose or ATP. The ability to induce the leakage of CF from liposomes strongly depended on the number of carbon atoms in alkyl chain (n) of alkyltriphenylphosphonium and alkylrhodamine derivatives. In particular, the leakage of CF was maximal at n about 10-12 and substantially decreased at n=16. Organic anions (palmitate, oleate, laurylsulfate) competed with CF for the penetrating cation-induced efflux. The reduced activity of alkylrhodamines with n=16 or n=18 as compared to that with n=12 was ascribed to a lower rate of partitioning of the former into liposomal membranes, because electrical current relaxation studies on planar bilayer lipid membranes showed rather close translocation rate constants for alkylrhodamines with n=18 and n=12. Changes in the alkylrhodamine absorption spectra upon anion addition confirmed direct interaction between alkylrhodamines and the anion. Thus, mitochondria-targeted penetrating cations can serve as carriers of hydrophobic anions across bilayer lipid membranes.

The influence of the membrane on neurosteroid actions at GABA(A) receptors

Modern views of anesthetic neurosteroid interaction with the GABA(A) receptor conceptualize steroid ligands interacting with a protein binding site on the receptor. It has generally been assumed that the steroid interaction/binding site is contained in an extracellular domain of the receptor, and that steroid interactions are of high potency, evidenced by the low aqueous ligand concentrations required to achieve potentiation of channel function. We have been considering implications of the observations that steroids are quite lipophilic and that recently identified putative steroid binding sites are in transmembrane domains of the receptor. Accordingly, we expect that both the effective plasma membrane steroid concentration and steroid pharmacophore properties will contribute to steady-state potency and to the lifetime of steroid actions following removal of free aqueous steroid. Here we review our recent studies that address the evidence that membrane partitioning and intracellular accumulation are non-specific contributors to the effects of anesthetic steroids at GABA(A) receptors. We compare and contrast the profile of anesthetic steroids with that of sulfated steroids that negatively regulate GABA(A) receptor function. These studies give rise to the view that the inherent affinity of anesthetic steroid for GABA(A) receptors is very low; low effective aqueous concentrations are accounted for by lipid partitioning. This yields a very different picture of the interaction of neurosteroids with the GABA(A) receptor than that of steroid interactions with classical intracellular steroid receptors, which exhibit inherently high affinity. These considerations have practical implications for actions of endogenous neurosteroids. Lipophilicity will tend to promote autocrine actions of neurosteroids at GABA(A) receptors within cells that synthesize neurosteroids, and lipophilic retention will limit intercellular diffusion from the source of steroid synthesis. Lipophilicity and steroid access to the receptor binding sites also must be considerations in drug design if drugs are to effectively reach the target GABA(A) receptor site.

In vitrothermal therapy of AT-1 Dunning prostate tumours

To advance the utility of prostate thermal therapy, this study investigated the thermal thresholds (temperature-time) for prostate tissue destruction in vitro. The AT-1 Dunning prostate tumour model was chosen for the study. Three hundred micron thick sections were subjected to controlled temperature-time heating, which ranged from low (40 degrees C, 15 min) to high thermal exposures (70 degrees C, 2 min) (n = 6). After subsequent tissue culture at 37 degrees C, the sections were evaluated for tissue injury at 3, 24 and 72 h by two independent methods: histology and dye uptake. A graded increase in injury was identified between the low and high thermal exposures. Maximum histologic injury occurred above 70 degrees C, 1 min with >95% of the tissue area undergoing significant cell injury and coagulative necrosis. The control and 40 degrees C, 15 min sections showed histologic evidence of apoptosis following 24 and 72 h in culture. Similar signs of apoptosis were minimal or absent at higher thermal histories. Vital-dye uptake quantitatively confirmed complete cell death after 70 degrees C, 2 min. Using the dye data, Arrhenius analysis showed an apparent breakpoint at 50 degrees C, with activation energies of 135.8 kcal/mole below and 4.7 kcal/mole above the threshold after 3 h in culture. These results can be used as a conservative benchmark for thermal injury in the cancerous prostate. Further characterization of the response to thermal therapy in an animal model and in human tissues will be important in establishing the efficacy of the procedure

Curvature-driven pore growth in charged membranes during charge-pulse and voltage-clamp experiments

We find that curvature-driven growth of pores in electrically charged membranes correctly reproduces charge-pulse experiments. Our model, consisting of a Langevin equation for the time dependence of the pore radius coupled to an ordinary differential equation for the number of pores, captures the statistics of the pore population and its effect on the membrane conductance. The calculated pore radius is a linear, and not an exponential, function of time, as observed experimentally. Two other important features of charge-pulse experiments are recovered: pores reseal for low and high voltages but grow irreversibly for intermediate values of the voltage. Our set of coupled ordinary differential equations is equivalent to the partial differential equation used previously to study pore dynamics, but permits the study of longer timescales necessary for the simulations of voltage-clamp experiments. An effective phase diagram for such experiments is obtained.

Effects on membrane capacitance of steroids with antagonist properties at GABAA receptors

We investigated the electrophysiological signature of neuroactive steroid interactions with the plasma membrane. We found that charged, sulfated neuroactive steroids, those that exhibit noncompetitive antagonism of GABA(A) receptors, altered capacitive charge movement in response to voltage pulses in cells lacking GABA receptors. Uncharged steroids, some of which are potent enhancers of GABA(A) receptor activity, produced no alteration in membrane capacitance. We hypothesized that the charge movements might result from physical translocation of the charged steroid through the transmembrane voltage, as has been observed previously with several hydrophobic anions. However, the charge movements and relaxation time constants of capacitive currents did not exhibit the Boltzmann-type voltage dependence predicted by a single barrier model. Further, a fluorescently tagged analog of a sulfated neurosteroid altered membrane capacitance similar to the parent compound but produced no voltage-dependent fluorescence change, a result inconsistent with a strong change in the polar environment of the fluorophore during depolarization. These findings suggest that negatively charged sulfated steroids alter the plasma membrane capacitance without physical movement of the molecule through the electric field.

A combined patch-clamp and electrorotation study of the voltage- and frequency-dependent membrane capacitance caused by structurally dissimilar lipophilic anions

Interactions of structurally dissimilar anionic compounds with the plasma membrane of HEK293 cells were analyzed by patch clamp and electrorotation. The combined approach provides complementary information on the lipophilicity, preferential affinity of the anions to the inner/outer membrane leaflet, adsorption depth and transmembrane mobility. The anionic species studied here included the well-known lipophilic anions dipicrylamine (DPA⁻), tetraphenylborate (TPB⁻) and [W₂(CO)₁₀(S₂CH)]⁻, the putative lipophilic anion \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{B}}{\left( {{\text{CF}}_{3} } \right)}^{ - }_{4} $$\end{document} and three new heterocyclic W(CO)₅ derivatives. All tested anions partitioned strongly into the cell membrane, as indicated by the capacitance increase in patch-clamped cells. The capacitance increment exhibited a bell-shaped dependence on membrane voltage. The midpoint potentials of the maximum capacitance increment were negative, indicating the exclusion of lipophilic anions from the outer membrane leaflet. The adsorption depth of the large organic anions DPA⁻, TPB⁻ and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{B}}{\left( {{\text{CF}}_{3} } \right)}^{ - }_{4} $$\end{document} increased and that of W(CO)₅ derivatives decreased with increasing concentration of mobile charges. In agreement with the patch-clamp data, electrorotation of cells treated with DPA⁻ and W(CO)₅ derivatives revealed a large dispersion of membrane capacitance in the kilohertz to megahertz range due to the translocation of mobile charges. In contrast, in the presence of TPB⁻ and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{B}}{\left( {{\text{CF}}_{3} } \right)}^{ - }_{4} $$\end{document} no mobile charges could be detected by electrorotation, despite their strong membrane adsorption. Our data suggest that the presence of oxygen atoms in the outer molecular shell is an important factor for the fast translocation ability of lipophilic anions.

Energy-landscape-model analysis for irreversibility and its pulse-width dependence in cells subjected to a high-intensity ultrashort electric pulse

We provide a simple, but physical analysis for cell irreversibility and apoptosis in response to an ultrashort (nanosecond), high-intensity electric pulse. Our approach is based on an energy landscape model for determining the temporal evolution of the configurational probability function p(q). The primary focus is on obtaining qualitative predictions of a pulse width dependence to apoptotic cell irreversibility that has been observed experimentally. The analysis couples a distributed electrical model for current flow with the Smoluchowski equation to provide self-consistent, time-dependent transmembrane voltages. The model captures the essence of the experimentally observed pulse-width dependence, and provides a possible physical picture that depends only on the electrical trigger. A number of interesting features are predicted.

Hydrophobic ion hydration and the magnitude of the dipole potential

The magnitude of the dipole potential of lipid membranes is often estimated from the difference in conductance between the hydrophobic ions, tetraphenylborate, and tetraphenylarsonium or tetraphenylphosphonium. The calculation is based on the tetraphenylarsonium-tetraphenylborate hypothesis that the magnitude of the hydration energies of the anions and cations are equal (i.e., charge independent), so that their different rates of transport across the membrane are solely due to differential interactions with the membrane phase. Here we investigate the validity of this assumption by quantum mechanical calculations of the hydration energies. Tetraphenylborate (Delta G(hydr) = -168 kJ mol(-1)) was found to have a significantly stronger interaction with water than either tetraphenylarsonium (Delta G(hydr) = -145 kJ mol(-1)) or tetraphenylphosphonium (Delta G(hydr) = -157 kJ mol(-1)). Taking these differences into account, literature conductance data were recalculated to yield values of the dipole potential 57 to 119 mV more positive in the membrane interior than previous estimates. This may partly account for the discrepancy of at least 100 mV generally observed between dipole potential values calculated from lipid monolayers and those determined on bilayers.

Effect of ethylene oxide and propylene oxide block copolymers on the permeability of bilayer lipid membranes to small solutes including doxorubicin

The effects of ethylene oxide and propylene oxide block copolymers (pluronics) on the permeability of several weak acids and bases through bilayer lipid membranes have been studied by the methods of monitoring (1) pH shifts near planar bilayers, (2) doxorubicin fluorescence quenching inside liposomes, and (3) current transients in the presence of hydrophobic anions. It has been shown that pluronics facilitate the permeation of comparatively large molecules (such as 2-n-undecylmalonic acid and doxorubicin) across lipid bilayers, while the permeation of small solutes (such as ammonium and acetic acid) remains unaffected. Pluronics also accelerate the translocation of large hydrophobic anions (tetraphenylborate). The effect of pluronics correlates with the content of propylene oxide units: it is enhanced when the portion of polypropylene oxide block in the copolymer is increased. The action of the pluronic on lipid membrane permeability differs from the effect of the conventional detergent Triton X-100, which does not affect doxorubicin transport if added at concentrations similar to those used for pluronics. It has been proposed that pluronics accelerate the processes of solute diffusion within lipid bilayers (in a structure-dependent manner) rather than influencing the rate of solute adsorption/desorption on the membrane surface. We suppose that the effect of pluronics on doxorubicin permeation across lipid bilayers along with the known effect on the multidrug resistance protein determines its influence on the therapeutic activity of anthracycline drugs.

Effect of fluorine substitution on the interaction of lipophilic ions with the plasma membrane of mammalian cells

The effects of the anionic tungsten carbonyl complex [W(CO)(5)SC(6)H(5)](-) and its fluorinated analog [W(CO)(5)SC(6)F(5)](-) on the electrical properties of the plasma membrane of mouse myeloma cells were studied by the single-cell electrorotation technique. At micromolar concentrations, both compounds gave rise to an additional antifield peak in the rotational spectra of cells, indicating that the plasma membrane displayed a strong dielectric dispersion. This means that both tungsten derivatives act as lipophilic ions that are able to introduce large amounts of mobile charges into the plasma membrane. The analysis of the rotational spectra allowed the evaluation not only of the passive electric properties of the plasma membrane and cytoplasm, but also of the ion transport parameters, such as the surface concentration, partition coefficient, and translocation rate constant of the lipophilic anions dissolved in the plasma membrane. Comparison of the membrane transport parameters for the two anions showed that the fluorine-substituted analog was more lipophilic, but its translocation across the plasma membrane was slower by at least one order of magnitude than that of the parent hydrogenated anion.

Supraphysiological thermal injury in Dunning AT-1 prostate tumor cells

To investigate the potential application of thermal therapy in the treatment of prostate cancer, the effects of supraphysiological temperatures (40-70 degrees C) for clinically relevant time periods (approximately 15 minutes) were experimentally studied on attached Dunning AT-1 rat prostate cancer cells using multiple assays. The membrane and reproductive machinery were the targets of injury selected for this study. In order to assess membrane injury, the leakage of calcein was measured dynamically, and the uptake of PI was measured postheating (1-3 hours). Clonogenicity was used as a measure of injury to the reproductive machinery 7 days post-injury after comparable thermal insults. Experimental results from all three assays show a broad trend of increasing injury with an increase in temperature and time of insult. Membrane injury, as measured by the fluorescent dye assays, does not correlate with clonogenic survival for many of the thermal histories investigated. In particular, the calcein assay at temperatures of < or = 40 degrees C led to measurable injury accumulation (dye leakage), which was considered sublethal, as shown by significant survival for comparable insult in the clonogenic assay. Additionally, the PI uptake assay used to measure injury post-thermal insult shows that membrane injury continues to accumulate after thermal insult at temperatures > or = 50 degrees C and may not always correlate with clonogenicity at hyperthermic temperatures such as 45 degrees C. Last, although the clonogenic assay yields the most accurate cell survival data, it is difficult to acquire these data at temperatures > or = 50 degrees C because the thermal transients in the experimental setup are significant as compared to the time scale of the experiment. To improve prediction and understanding of thermal injury in this prostate cancer cell line, a first-order rate process model of injury accumulation (the Arrhenius model) was fit to the experimental results. The activation energy (E) obtained using the Arrhenius model for an injury criterion of 30 percent for all three assays revealed that the mechanism of thermal injury measured is likely different for each of the three assays: clonogenics (526.39 kJ/mole), PI (244.8 kJ/mole), and calcein (81.33 kJ/mole). Moreover, the sensitivity of the rate of injury accumulation (d omega/dt) to temperature was highest for the clonogenic assay, lowest for calcein leakage, and intermediate for PI uptake, indicating the strong influence of E value on d omega/dt. Since the clonogenic assay is linked to the ultimate survival of the cell and accounts for all lethal mechanisms of cellular injury, the E and A values obtained from clonogenic study are the best values to apply to predict thermal injury in cells. For higher temperatures (> or = 50 degrees C) indicative of thermal therapies, the results of PI uptake can be used as a conservative estimate of cell death (underprediction). This is useful until better experimental protocols are available to account for thermal transients at high temperature to assess clonogenic ability. These results provide further insights into the mechanisms of thermal injury in single cell systems and may be useful for designing optimal protocols for clinical thermal therapy.

Interaction of lipophilic ions with the plasma membrane of mammalian cells studies by electrorotation

The electrical properties of biological and artificial membranes were studied in the presence of a number of negatively charged tungsten carbonyl complexes, such as [W(CO)5(CN)]- , [W(CO)5(NCS)]-, [W2(CO)10(CN)]-, and [W(CO)5(SCH2C6H5)]-, using the single-cell electrorotation and the charge-pulse relaxation techniques. Most of the negatively charged tungsten complexes were able to introduce mobile charges into the membranes, as judged from electrorotation spectra and relaxation experiments. This means that the tungsten derivatives act as lipophilic anions. They greatly contributed to the polarizability of the membranes and led to a marked dielectric dispersion (frequency dependence of the membrane capacitance and conductance). The increment and characteristic frequency of the dispersion reflect the structure, environment, and mobility of the charged probe molecule in electrorotation experiments with biological membranes. The partition coefficients and the translocation rate constants derived from the electrorotation spectra of cells agreed well with the corresponding data obtained from charge-pulse experiments on artificial lipid bilayers.

The adsorption of phloretin to lipid monolayers and bilayers cannot be explained by langmuir adsorption isotherms alone

Phloretin and its analogs adsorb to the surfaces of lipid monolayers and bilayers and decrease the dipole potential. This reduces the conductance for anions and increases that for cations on artificial and biological membranes. The relationship between the change in the dipole potential and the aqueous concentration of phloretin has been explained previously by a Langmuir adsorption isotherm and a weak and therefore negligible contribution of the dipole-dipole interactions in the lipid surface. We demonstrate here that the Langmuir adsorption isotherm alone is not able to properly describe the effects of dipole molecule binding to lipid surfaces--we found significant deviations between experimental data and the fit with the Langmuir adsorption isotherm. We present here an alternative theoretical treatment that takes into account the strong interaction between membrane (monolayer) dipole field and the dipole moment of the adsorbed molecule. This treatment provides a much better fit of the experimental results derived from the measurements of surface potentials of lipid monolayers in the presence of phloretin. Similarly, the theory provides a much better fit of the phloretin-induced changes in the dipole potential of lipid bilayers, as assessed by the transport kinetics of the lipophilic ion dipicrylamine.

Improved indicators of cell membrane potential that use fluorescence resonance energy transfer

BACKGROUND

Fluorescence detection of cell membrane potentials is an important technique in neurobiology, cell physiology and pharmaceutical screening, but traditional one-fluorophore indicators either respond too slowly or have limited sensitivity. Recently, we introduced two-component sensors based on the transfer of fluorescence resonance energy from fluorescent lectins bound on one side of the plasma membrane to highly fluorescent oxonol acceptors that electrophorese from one face of the membrane to the other in response to membrane potential.

RESULTS

We have found that fluorescent lectins can often be advantageously replaced in such sensors by fluorescently labeled phospholipids. A coumarinlabeled phosphatidylethanolamine donor and a bis(1,3-dihexyl-2-thiobarbiturate)trimethineoxonol acceptor gave the largest sensitivity of fluorescence ratio (>50% per 100 mV) ever reported. The response was also speeded several-fold by lengthening the mobile dye to the pentamethineoxonol analog, the <0.4 ms time constant of which was shorter than action potential durations. Photodynamic damage due to singlet oxygen was reduced by administering a natural carotenoid, astaxanthin.

CONCLUSIONS

Voltage-sensitive fluorescence resonance energy transfer already gives record-setting performance on single cells and will continue to be rationally improvable.

Voltage-dependent translocation of R18 and DiI across lipid bilayers leads to fluorescence changes

We show that the lipophilic, cationic fluorescent dyes R18 and Dil translocate from one monolayer of a phospholipid bilayer membrane to the other in a concentration and voltage-dependent manner. When the probes were incorporated into voltage-clamped planar membranes and potentials were applied, displacement currents resulted. The charged probes sensed a large fraction of the applied field. When these probes were added to only one monolayer, displacement currents were symmetrical around 0 mV, indicating that the probes distributed equally between the two monolayers. Charge translocation required that the bilayer be fluid. When membranes were in a condensed gel phase, displacement currents were not observed; raising the temperature to above the gel-liquid crystalline transition restored the currents. Translocation of R18 was also shown by fluorescence measurements. When R18 was in the bilayer at high, self-quenching concentrations, voltage pulses led to voltage-dependent fluorescence changes. The kinetics of the fluorescence changes and charge translocations correlated. Adding the quencher I- to one aqueous phase caused fluorescence to decrease or increase when voltage moved R18 toward or away from the quencher at low, nonquenching concentrations of R18. In contrast to R18, Dil incorporated into bilayers was a carrier fo I-, and hence I- altered Dil currents. Voltage-driven translocations allow R18 and Dil to be used to probe membrane potential changes.

Evidence for ion chain mechanism of the nonlinear charge transport of hydrophobic ions across lipid bilayers

The conductivity across a lipid bilayer by tetraphenylborate anion is increased 10-fold on the photoformation of lipophilic porphyrin cations. The cations alone have negligible conductivity. This nonlinear photogenerated increase of ion conductivity is termed the photogating effect. Substitution of H by Cl in the para position of tetraphenylborate leads to a 100-fold enhancement of conductivity, whereas the dark conductivities for this and other substituted borates are the same. Moreover, the halo-substituted borates show a large enhancement of conductivity in the low concentration range (10(-8) M), whereas that of tetraphenylborate is small and space charge is negligible. The enhanced ion conductivity has great structural sensitivity to the structure of the anion, the cation, and the lipid, whereas the partition coefficient of all the borates and the concentration of photoformed cations are only slightly affected. The photogated ion transport has a twofold larger activation energy than transport in the dark. Time-resolved photocurrents and voltages demonstrate that the translocation rate of the porphyrin cation is also enhanced 100-fold by the Cl-borate anion but only 10-fold by the H-borate anion. For these reasons the nonlinear gating effect cannot be explained by electrostatics alone, but requires an ion chain or ion aggregate mechanism. Kinetic modeling of the photoinduced current with a mixed cation-anion ion chain can fit the data well. The photogating effect allows the direct study of ion interactions within the bilayer.

Voltage sensing by fluorescence resonance energy transfer in single cells

A new mechanism has been developed for achieving fast ratiometric voltage-sensitive fluorescence changes in single cells using fluorescence resonance energy transfer. The mechanism is based on hydrophobic fluorescent anions that rapidly redistribute from one face of the plasma membrane to the other according to the Nernst equation. A voltage-sensitive fluorescent readout is created by labeling the extracellular surface of the cell with a second fluorophore, here a fluorescently labeled lectin, that can undergo energy transfer with the membrane-bound sensor. Fluorescence resonance energy transfer between the two fluorophores is disrupted when the membrane potential is depolarized, because the anion is pulled to the intracellular surface of the plasma membrane far from the lectin. Bis-(1,3-dialkyl-2-thiobarbiturate)-trimethineoxonols, where alkyl is n-hexyl and n-decyl (DiSBA-C6-(3) and DiSBA-C10-(3), respectively) can function as donors to Texas Red labeled wheat germ agglutinin (TR-WGA) and acceptors from fluorescein-labeled lectin (FI-WGA). In voltage-clamped fibroblasts, the translocation of these oxonols is measured as a displacement current with a time constant of approximately 2 ms for 100 mV depolarization at 20 degrees C, which equals the speed of the fluorescence changes. Fluorescence ratio changes of between 4% and 34% were observed for a 100-mV depolarization in fibroblasts, astrocytoma cells, beating cardiac myocytes, and B104 neuroblastoma cells. The large fluorescence changes allow high-speed confocal imaging.

Shape of the potential energy barrier of the iodine-mediated halide transport

Voltage-clamp experiments were performed on lipid bilayer membranes to study the voltage dependence of the iodine-mediated halide transport. Under all experimental conditions only one exponential current relaxation, apart from the capacitive spike, could be resolved up to a clamp voltage of 200 mV. The current relaxation could be described by an initial conductance, G0, the relaxation time constant, tau, and the relaxation amplitude, alpha, that is the difference between the initial current, I0, and the steady state current, I chi, divided by the steady state current. The occurrence of one single exponential relaxation suggested that one of the different transport steps involved in the carrier-mediated ion transport according to the Lüger-model is always in equilibrium. This is most probably the transport of the free carriers across the membrane. The voltage dependence of G0, tau, and of alpha were used to determine the voltage dependence of the translocation rate constants of the complexed carriers, kAS. In the case of the iodine-mediated iodide transport, G0, tau and alpha were only mediate voltage-dependent, which means the voltage dependent translocation of the complex encounters a trapezoidal barrier shape. For the iodine-mediated bromide translocation G0, tau and alpha exhibited no dependence on the applied clamp-voltage, which suggested that a square Nernst-Planck barrier limits the transport of the corresponding complex.

Application of a fast charge-pulse technique to study the effect of the dipolar substance 2,4-dichlorophenoxyacetic acid on the kinetics of valinomycin mediated K(+)-transport across monoolein membranes

We report on a modified charge-pulse relaxation technique applied at planar lipid membranes. The method has an improved time resolution of 20-30 ns. It is based on the capacitive coupling of a voltage-jump to the membrane. The method was used to study the fast relaxation processes induced by valinomycin/K+ in the presence of 2,4-dichlorophenoxyacetic acid (2,4-D). The change of the rate constants of the ion carrier valinomycin was analysed as a consequence of the adsorption of the dipolar substance 2,4-D to the membrane/water interface of monoolein membranes. The effect of 2,4-D can be explained solely via the influence of the introduced dipole potential, VD. The latter was found to act (primarily) on the inner membrane barrier experienced by the positively charged carrier-ion complex and on the interfacial barriers responsible for complex formation and dissociation. No evidence for a change of the microviscosity of the membrane interior was obtained.

Voltage sensitivity of the fluorescent probe RH421 in a model membrane system

The voltage sensitivity of the fluorescent styrylpyridinium dye RH421 has been investigated in dimyristoylphosphatidylcholine vesicles by inducing an intramembrane electric field through the binding of the hydrophobic ion tetraphenylborate (TPB). To assess the probability of electrochromic and solvatochromic mechanisms for the dye response, the ground-state dipole moment of the dye in chloroform solution was determined from dielectric constant measurements to be 12 (+/- 1) Debye, and the change in dipole moment upon excitation was calculated from measurements of the Stokes shift in solvents of varying polarity to be 25 (+/- 11) Debye. As well as causing absorbance and fluorescence changes of membrane-bound dye, the TPB-induced electrical field was found to reduce significantly the pKa of the dye. The pH at which experiments are carried out is, thus, an important factor in determining the amplitude of the voltage-induced absorbance and fluorescence changes. The observed absorbance changes induced by the field are inconsistent with a pure electrochromic mechanism. A reorientation/solvatochromic mechanism, whereby the electrical field reorients the dye molecules so that they experience a change in polarity of their lipid environment is likely to make a significant contribution to both the spectral changes and to the field effect on the acid-base properties of the dye.

Activation energy for permeation of phosphonium cations through phospholipid bilayer membrane

The conductance caused by translocation of various phosphonium cations across phospholipid (from soybean) bilayer membrane was measured. Phosphonium cations used were tetraphenylphosphonium (TPP+) and triphenylalkylphosphonium cations formulated as (Phe)3-P(+)-(CH2)nCH3 (n = 0-5). The conductance was dependent on voltage applied externally to the membrane in accordance with a theory developed by previous authors. Using the theory, values of beta K(i) were determined, where beta and k(i) are a linear partition coefficient and a rate constant of transmembrane ion transport, respectively. Since beta k(i) depended on the phosphonium ion concentration, values extrapolated to infinite dilution, (beta k(i))0, were determined. Temperature dependence of (beta k(i))0 allowed us to estimate the activation energy of transport, Ea. For TPP+ thermodynamic values obtained were consistent with values calculated by Flewelling and Hubbell [(1986) Biophys. J. 49, 541-552]. When (Phe)3-P(+)-(CH2)nCH3 (n = 0-5) were used, E(a) depended on the odd or even of n. This "odd and even" pattern was observed in a variety of phenomena such as solubility in water, equivalent ionic conductivity in water, and 31P NMR chemical shift.

Kinetics of the iodine- and bromine-mediated transport of halide ions: demonstration of an interfacial complexation mechanism

Stationary and kinetic experiments were performed on lipid bilayer membranes to study the mechanism of iodine- and bromine-mediated halide transport in detail. The stationary conductance data suggested that four different 1:1 complexes between I2 and Br2 and the halides I- and Br- were responsible for the observed conductance increase by iodine and bromine (I3-, I2Br-, Br2I-, and Br3-). Charge pulse experiments allowed the further elucidation of the transport mechanism. Only two of three exponential voltage relaxations predicted by the Läuger model could be resolved under all experimental conditions. This means that either the heterogeneous complexation reactions kR (association) and kD (dissociation) were too fast to be resolved or that the neutral carriers were always in equilibrium within the membrane. Experiments at different carrier and halide concentrations suggested that the translocation of the neutral carrier is much faster than the other processes involved in carrier-mediated ion transport. The model was modified accordingly. From the charge pulse data at different halide concentrations, the translocation rate constant of the complexed carriers, kAS, the dissociation constant, kD, and the total surface concentration of charged carriers, NAS, could be evaluated from one single charge pulse experiment. The association rate of the complex, kR, could be obtained in some cases from the plot of the stationary conductance data as a function of the halide concentration in the aqueous phase. The translocation rate constant, kAS, of the different complexes is a function of the image force and of the Born charging energy. It increases 5000-fold from Br3- to I3- because of an enlarged ion radius.

AHA- heterodimer of a class-2 uncoupler: pentachlorophenol

AHA- heterodimers formed by association of neutral molecules of weak acid (HA) with its conjugate anion (A-) have been proposed to be the charged membrane-permeable species of class-2 uncouplers. Past attempts to extract and identify AHA- heterodimers failed. We have measured optical spectra of HA+A- (1:1) solutions of pentachlorophenol (PCP) in various solvents and in the presence of PC liposomes. Optical studies were supplemented by nuclear magnetic resonance measurements of HA+A- (1:1) solutions of PCP in dichloroethane to gain insight into the formation of AHA- species in lipid membranes. From these experiments, we found evidence for AHA- formation in non-hydrogen-bonding solvents, then reported the AHA- formation constant Kf and the molar absorptivity epsilon AHA-(lambda). Kf decreases with increasing dielectric constant, kappa, from 1210 +/- 130 M-1 for dichloroethane (kappa 10.7), to 340 +/- 34 M-1 for acetonitrile (kappa 37.5); Kf also decreases with increasing concentration of water. In hydrogen-bonding solvents, octanol (kappa 10.3) and methanol (kappa 33.5) and in liposomes, AHA- heterodimers are not observed optically. We estimate Kf for PCP in lipid bilayers from a combination of data on membrane electrical conductivity and surface density of adsorbed PCP. Our estimate for lipid bilayer, 0.005 < Kf < 0.5 M-1, is consistent with our inability to detect the AHA- species optically in liposome suspensions. We propose that penetration of water into the membrane inhibits formation of AHA- in lipid bilayers.

Kinetics of pore size during irreversible electrical breakdown of lipid bilayer membranes

The kinetics of pore formation followed by mechanical rupture of lipid bilayer membranes were investigated in detail by using the charge-pulse method. Membranes of various compositions were charged to a sufficiently high voltage to induce mechanical breakdown. The subsequent decrease of membrane voltage was used to calculate the conductance. During mechanical breakdown, which was probably caused by the widening of one single pore, the membrane conductance was a linear and not exponential function of time after the initial starting process. In a large number of experiments using various lipids and electrolytes, the characteristic opening process of the pore turned out to be independent of the actual membrane potential and electrolyte concentration. Our theoretical analysis of the pore formation suggested that the voltage-induced irreversible breakdown is due to a decrease in edge energy when the pore had formed. After initiation of the pore, the electrical contribution to surface tension is negligible. The time course of the increase of pore size shows that our model of the irreversible breakdown is in good agreement with mechanical properties of membranes reported elsewhere.

Photogating of ionic currents across lipid bilayers. Hydrophobic ion conductance by an ion chain mechanism

The photogating of hydrophobic ion currents across the lipid bilayer membrane allows the direct study of their kinetics by symmetrically forming charge within the membrane and across each interface, rather than across the membrane. We find that the photoinduced conductance continues to increase beyond the region where the tetraphenylboride charge density in the membrane exceeds the estimated porphyrin cation density. This photoconductance is proportional to the tetraphenylboride charge density raised to the second to third power. The risetime of the photogating effect increases with increasing concentration of tetraphenyl boride. The porphyrin cation mobility is increased when the tetraphenylboride anion is present, and low concentrations of tetraphenylphosphonium cation increase the dark conductivity while inhibiting the photoconductivity. The activation energy for both the porphyrin and phosphonium cation induced conductance is more positive than that of the tetraphenylboride conductance. From these results we conclude that in addition to some cancellation of space charge within the membrane, the mechanism of increased conductance involves the transport of these hydrophobic anions via an alternating anion-cation chain, analogous to the Grotthuss mechanism for excess proton conduction in water. This ion chain conductance can be viewed as an evolutionary prototype of an ion channel across the membrane. It also underscores the importance of the counter ion in the transport of large ions such as peptides across the lipid bilayer.