Intramembrane Electric Field Research Articles

Retina‐Inspired Nanofluidic Membranes for Underwater Visual Imaging Based on Active Ion Transport

AbstractThrough evolution, biological organisms have developed ways to sense light using ion channels, which holds several advantages, such as energy efficiency and water resistance, over humanmade optoelectronic devices. Herein, a retina‐inspired nanofluidic system is presented with a Janus heterogeneous membrane (J‐HM), which can achieve underwater visual imaging through light‐driven active ion transport. The J‐HMs are obtained through sequentially assembled WS2, and a kind of metal–organic framework nanosheets via the reaction between 2,3,6,7,10,11‐hexahydroxytriphenylene hydrate (HHTP) and Cu2+ (Cu‐HHTP). Due to the formed intramembrane electric field caused by the efficient charge separation under illumination, a photovoltaic driving force is generated for active ion transport from Cu‐HHTP to WS2. Furthermore, the unidirectionally active ion transport can be enhanced by self‐diffusion under a concentration gradient. The J‐HM with a single‐pixel design shows a nearly linear response with light intensity and has enough resolution for basic object recognition as well as a long‐term memory after data processing using a defined pixelated matrix, which can pave an avenue for designing more intelligent sensing systems.

Green Alga-Inspired Underwater Vision Based on Light-Driven Active Ion Transport across Janus Dual-Field Heterostructures.

Natural organisms have evolved various biological ion channels to make timely responses toward different physical and/or chemical stimuli, giving guidance to construct artificial counterparts and expand the corresponding applications. They have also shown promising potential to overcome disadvantages of traditional electronic devices (e.g., energy-consuming operation and adverse humidity interference). Herein, we constructed a green alga-inspired nanofluidic system based on a Janus dual-field heterogeneous membrane (i.e., J-HM), which can function underwater as an artificial visual platform for light perception through enhanced active ion transport. The J-HM was obtained through sequentially assembled MXene and Cu-HHTP (i.e., a metal-organic framework based on the reaction between 2,3,6,7,10,11-hexahydroxytriphenylene hydrate (HHTP) and Cu2+) building units. Due to the formed temperature gradient and intramembrane electric field caused by the localized thermal excitation and efficient charge separation of J-HM under illumination, thermo-osmotic and photo-driven forces are generated for preferential cation transport from Cu-HHTP to MXene. Furthermore, unidirectional active transport can be enhanced by self-diffusion under a concentration gradient. Then, the corresponding underwater light perceptions at various light illumination conditions are explored, showing nearly a linear correlation with the light intensity. Finally, it is demonstrated that the visual platform can achieve object shape, definition, and distance recognition using a defined pixelated matrix, giving impetus to develop ionic signal transmission based sensing systems.

Photo‐Induced Active Ion Transport‐Assisted Efficient Ionic Power Harvesting from Bioinspired Janus Dual‐Field Heterostructures

AbstractNatural organisms have developed various biological ion channels/transporters to maintain normal life activities to adapt to changing environments. Significantly, ion transporters with active ion transport property show much more controllability on these activities due to a variety of external stimuli, giving guidance to construct artificial counterparts for overcoming the issues in simple nanofluidic systems restricted by self‐diffusive ion transport. Herein, a 2D nanofluidic system based on a Janus membrane (i.e., JM) is constructed, which can achieve light‐driven active ion transport and do favor for ionic power harvesting in electrolyte systems with equal concentrations. The JM is obtained through sequentially assembled montmorillonite (MMT) decorated with photoelectric molecules (i.e., PMMT) and MXene nanosheets. Due to the formed intramembrane electric field and temperature gradient caused by the efficient charge separation and localized thermal excitation under light illumination, a photovoltaic‐driven force and thermo‐osmotic are generated for preferential ion transport. In addition, the unidirectional active transport is employed to harvest ionic power, showing an output power density of 2.0 mW m−2 and a high‐performance energy conversion efficiency of 8.3 × 10−4%. The effects of the light intensity, electrolyte concentration and species on energy conversion performance are investigated, showing the university for ionic power harvesting.

Voltage Dependence and Modulation of Bacterial Channel MscL

Channels carrying canonical voltage sensor domains with multiple charges respond to voltage steeply. Non-voltage-gated channels lacking specialized voltage sensors, nevertheless, can also be modulated by voltage acting on other domains. The reason is that a typical channel residing in the polarized membrane represents a non-uniform protein insert characterized by local parameters of dielectric permeability, dipole and charge whose distributions may change as the channel transitions between the states. Here we describe a strong voltage modulation of bacterial mechanosensitive channel MscL and especially its gain-of-function mutant (V23T), which is exerted through a non-specific dielectric mechanism by water present in the pore with some contribution from the dipole moment of the protein itself. Patch-clamp measurements obtained under linear pressure ramp stimulation at different voltages reveal that both versions of MscL have dipole (linear with voltage) and dielectric/capacitive (quadratic with voltage) components that contribute to the transition energy. In contrast to WT, which exhibits a larger dipole and a smaller dielectric contribution, V23T shows a dominating dielectric effect likely related to the more pronounced hydration of the mutant pore. The increasing aqueous volume of the pore and the flattening of the protein complex impart the capacitive/dielectric energy contribution that stabilizes the expanded and subconductive states of V23T MscL. To further explore the nature of the electric field effect on open probability, we have compared the V23T MscL behavior in electrically symmetric and asymmetric droplet interface bilayers where a large intrinsic electric bias was created by using lipids with different interfacial dipole potentials. The results confirm that V23T MscL senses the local intramembrane electric field in the membrane core between the interfacial layers, and thus the voltage imposed by external electrodes is sensed not directly, but rather in combination with the potential imparted by the lipids.

The voltage-dependence of MscL has dipolar and dielectric contributions and is governed by local intramembrane electric field

Channels without canonical voltage sensors can be modulated by voltage acting on other domains. Here we show that besides protein dipoles, pore hydration can be affected by electric fields. In patches, both WT MscL and its V23T mutant show a decrease in the tension midpoint with hyperpolarization. The mutant exhibits a stronger parabolic dependence of transition energy on voltage, highly consistent with the favourable dielectric contribution from water filling the expanding pore. Purified V23T MscL in DPhPC droplet interface bilayers shows a similar voltage dependence. When reconstituted in an asymmetric DOPhPC/DPhPC bilayer carrying a permanent bias of ~130 mV due to a dipole potential difference between the interfaces, the channel behaved as if the local intramembrane electric field sets the tension threshold for gating rather than just the externally applied voltage. The data emphasize the roles of polarized water in the pore and interfacial lipid dipoles in channel gating thermodynamics.

Action potential propagation: ion current or intramembrane electric field?

The established action potential propagation mechanisms do not satisfactorily explain propagation on myelinated axons given the current knowledge of biological channels and membranes. The flow across ion channels presents two possible effects: the electric potential variations across the lipid bilayers (action potential) and the propagation of an electric field through the membrane inner part. The proposed mechanism is based on intra-membrane electric field propagation, this propagation can explain the action potential saltatory propagation and its constant delay independent of distance between Ranvier nodes in myelinated axons.

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.

Identification of Electric-Field-Dependent Steps in the Na+,K+-Pump Cycle

The charge-transporting activity of the Na+,K+-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme’s reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na+,K+-ATPase’s transport sites in competition with Na+ and K+, but is not occluded within the protein. We find that only the occludable ions Na+, K+, Rb+, and Cs+ cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na+ or K+ binding in a high field access channel is a major electrogenic reaction of the Na+,K+-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.

Occult Physiology: Electrical Cross-Talk between Membrane Lipid, Occluded Ions, and the Na-K ATPase

Occult Physiology: Electrical Cross-Talk between Membrane Lipid, Occluded Ions, and the Na-K ATPase

Modeling of coupled enzyme membrane oscillators—Effects of an electric field

Effects of an external electric field on dynamical patterns in a model of two coupled reaction units, each comprising a reservoir–reactor system are studied using numerical bifurcation analysis. A pH-sensitive enzyme reaction involving ionic species is assumed to take place in the stirred reactors mutually coupled by a permeable passively conducting membrane. Reactants are fed to reactors through the same type of membrane from adjacent reservoirs. An external electric voltage is applied to this system. An intra-membrane electric field caused by the applied voltage generates electrophoretic fluxes of each charged reaction component. Dynamical regimes including multiple steady states, oscillations and excitability occur in the absence of electric fields due to a combination of autocatalytic reaction and diffusive transport. We focus on dynamical patterns occurring as a consequence of electric current, either constant or periodically varying, that passes through the system. In the static case we find that the field suppresses oscillatory dynamics and promotes multiplicity of steady states including occurrence of reaction–ionic migration patterns analogous to Turing patterns. Sinusoidal variations of the current cause a rich variety of dynamical responses, ranging from periodic phase-locked patterns to quasiperiodicity to chaos.

Electric field strength of membrane lipids from vertebrate species: membrane lipid composition and Na+-K+-ATPase molecular activity

Intramembrane electric field strength is a very likely determinant of the activity of ion-transporting membrane proteins in living cells. In the absence of any transmembrane electrical potential or surface potential, its magnitude is determined by the dipole potential of the membrane's lipid components and their associated water of hydration. Here we have used a fluorometric method to quantify the dipole potential of vesicles formed from lipids extracted from kidney and brain of 11 different animal species from four different vertebrate classes. The dipole potential was compared with the fatty acid composition and with the Na(+)-K(+)-ATPase molecular activity of each preparation. The magnitude of the dipole potential was found to be relatively constant across all animal species, i.e., 236-334 mV for vesicles prepared from the total membrane lipids and 223-256 mV for phospholipids alone. The significantly lower value for phospholipids alone is potentially related to the removal of cholesterol and/or other common soluble lipid molecules from the membrane. Surprisingly, no significant dependence of the dipole potential on fatty acid composition was found. This may, however, be due to concomitant compensatory variations in lipid head group composition. The molecular activity of the Na(+)-K(+)-ATPase was found to increase with increasing dipole potential. The fact that the dipole potential is maintained at a relatively constant value over a wide range of animal species suggests that it may play a fundamental role in ensuring correct ion pump conformation and function within the membrane.

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 Å 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.

Permeation of phloretin across bilayer lipid membranes monitored by dipole potential and microelectrode measurements

The transmembrane diffusion of phloretin across planar bilayer lipid membranes is studied under steady-state conditions. Diffusion restrictions and adsorption related effects are measured independently. The adsorption of aligned phloretin dipoles generates a change in the intrinsic dipole potential difference between the inner and outer leaflets of the lipid bilayer. It is monitored by capacitive current measurements carried out with a direct current (dc) bias. The variation of the intramembrane electric field indicates a saturation of the binding sites at the membrane interface. In contrast, pH profile measurements undertaken in the immediate membrane vicinity show a constant membrane permeability. If phloretin binding and transmembrane diffusion are treated as two competitive events rather than subsequent steps in the transport queue the contradictory results become explainable. A mathematical model is developed where it is assumed that diffusing phloretin molecules are randomly oriented, i.e., that they do not contribute to the intrinsic membrane potential. Only the dipoles adsorbing onto the membrane are oriented. Based on these theory the membrane permeability is calculated from the capacitive current data. It is found to agree very well with the permeability deduced from the microelectrode measurements.

Faster voltage-dependent activation of Na+ channels in growth cones versus somata of neuroblastoma N1E-115 cells

Kinetics of voltage-gated ionic channels fundamentally reflect the response of the channels to local electric fields. In this report cell-attached patch-clamp studies reveal that the voltage-dependent activation rate of sodium channels residing in the growth cone membrane differs from that of soma sodium channels in differentiating N1E-115 neuroblastoma cells. Because other electrophysiological properties of these channels do not differ, this finding may be a reflection of the difference in intramembrane electric field in these two regions of the cell. This represents a new mechanism for channels to attain a range of activities both within and between cells.

Time-Resolved Fluorescence Investigations of the Interaction of the Voltage-Sensitive Probe RH421 with Lipid Membranes and Proteins

Fluorescence lifetimes and fluorescence anisotropy decays of the voltage-sensitive styryl-pyridinium dye RH421 have been measured in the presence of dimyristoylphosphatidylcholine vesicles, the water soluble enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco), and Na+,K(+)-ATPase-containing membrane fragments. The effect of an intramembrane electric field on the photophysical properties of the dye was investigated by the binding of the hydrophobic ion tetraphenylborate (TPB) to the membrane. TPB was found to significantly increase the average fluorescence lifetime and the order of membrane-bound dye. The increase in fluorescence lifetime is consistent with reorientation of the dye further into the membrane interior. The increase in order may be attributed to an electric field-induced alignment of the dye molecules. From the values of the rotational diffusion constant experimentally determined, the expected response time of the dye to an applied electric field can be calculated for a reorientational mechanism to be on the order of tens of nanoseconds. Experiments with rubisco showed that the dye interacts strongly with the protein. In this case, the dye is so tightly bound that it has almost no independent motion and rotates virtually solely with the protein. The rate of rotational motion of the dye in the presence of Na+,K(+)-ATPase-containing membrane fragments is similar to that in pure lipid membranes. The order parameter of the dye in the Na+,K(+)-ATPase membrane fragments is close to the maximum value. This is most probably due to the high density of protein molecules, which restricts the range of motion of the dye.

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.

Influence of Intramembrane Electric Charge on Na,K-ATPase

Effects of lipophilic ions, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-), on interactions of Na+ and K+ with Na,K-ATPase were studied with membrane-bound enzyme from bovine brain, pig kidney, and shark rectal gland. Na+ and K+ interactions with the inward-facing binding sites, monitored by eosin fluorescence and phosphorylation, were not influenced by lipophilic ions. Phosphoenzyme interactions with extracellular cations were evaluated through K(+)-, ADP-, and Na(+)-dependent dephosphorylation. TPP+ decreased: 1) the rate of transition of ADP-insensitive to ADP-sensitive phosphoenzyme, 2) the K+ affinity and the rate coefficient for dephosphorylation of the K-sensitive phosphoenzyme, 3) the Na+ affinity and the rate coefficient for Na(+)-dependent dephosphorylation. Pre-steady state phosphorylation experiments indicate that the subsequent occlusion of extracellular cations was prevented by TPP+. TPB- had opposite effects. Effects of lipophilic ions on the transition between phosphoenzymes were significantly diminished when Na+ was replaced by N-methyl-D-glucamine or Tris+, but were unaffected by the replacement of Cl- by other anions. Lipophilic ions affected Na-ATPase, Na,K-ATPase, and p-nitrophenylphosphatase activities in accordance with their effects on the partial reactions. Effects of lipophilic ions appear to be due to their charge indicating that Na+ and K+ access to their extracellular binding sites is modified by the intramembrane electric field.

Electrical potential changes in the surface and the central region of chromatophore membranes of photosynthetic bacteria detected by the absorbance changes of ethidium and carotenoid

(1) Changes of local intramembrane electrical field in the surface and central region of the chromatophore membrane during energization were studied both by the measurement of absorbance changes of ethidium, a monovalent cationic dye, and of carotenoid, the intrinsic probe of electrical field. (2) Binding of ethidium to the chromatophore membrane of Rhodopseudomonas sphaeroides was found to be dependent on the energization of membrane as well as on the ionic condition of the medium. The dye was released from the membrane when salt was added to the suspension, indicating the electrostatic interaction between the positive dye and the net negative membrane surface. The result was explained by the surface-potential dependent distribution of the dye to the membrane surface, as seen with other charged dyes (Masamoto, K., Matsuura, K., Itoh, S. and Nishimura, M. (1981) Biophys. Acta 638, 108–115). (3) Energization of chromatophores by flash-light-induced absorbance change of ethidium showing a similar difference spectrum to that induced by the addition of salts. The release of ethidium by a single turn-over flash of saturating intensity was estimated to be 0.22 ethidium per reaction center. Addition of ethidium (at 200 μM) slightly affected the flash-induced absorbance change of carotenoid which responds to the intramembrane electricalfield change, indicating a low-membrane permeability of the dye. The extent of the absorbance change of ethidium was linear to that of carotenoid, and was abolished in the presence of valinomycin plus K +. However, the rise and decay kinetics of the absorbance change of ethidium was different from that of carotenoid. (4) These absorbance changes of ethidium and carotenoid can be explained by a model in which ethidium responds to the potential changes in the surface region and carotenoid in the central hydrophobic region of the chromatophore membrane.

Further Studies of Proton Translocations in Chloroplasts After Single-Turnover Flashes. III. Conditions for the Operation of an Apparent Q-Cycle in Thylakoids

The proton-to-electron ratio in pea thylakoids, considering proton uptake with ferricyanide as electron acceptor, was reconfirmed as 1 in periods of single-turnover (<0.5 �s, 2-3 mJ) flashes delivered at 5-50 Hz and at pH values 6.4-8.3. Addition of valinomycin in the presence of K+ increased proton uptake in a way depending on [val], flash frequency and flash number. A maximal stimulation by valinomycin of up to about 1.8× controls was observed in fresh preparations. Half-maximal stimulation was caused by c. 2 nM valinomycin at 10-20 Hz, at c. 3 Hz with 10 nM valinomycin, and after c. five flashes at 50 Hz with 10 nM valinomycin. The results are discussed in terms of recent models for the Q-cycle. It is suggested that such a cycle operates in chloroplasts only when the intramembrane electric field induced in a series of flashes is kept small by the presence of valinomycin. Preliminary observations of 'P518', the thylakoid component probably indicating the electric field, are consistent with this idea. This field may control the transverse electron flow between the two cytochrome b6 molecules in the b/f complexes.

Theory of electromechanical effects in nerve.

The electromechanical transduction mechanisms operating in nerve membranes are considered theoretically. For mechanical-to-electrical transduction (mechanical generator potentials), a model is proposed in which the surface charge on the membrane mediates stress-induced changes in the intramembrane electric field, thus opening transmembrane ion conductance channels or reducing the ion selectivity of the membrane via leak conductance pathways. For electrical-to-mechanical transduction (axon diameter change with excitation), an investigation into two well-known electrostatic properties of dielectrics, electrostriction and piezoelectricity, in the context of the nerve membrane is undertaken which predicts a few percent change in axon dimensions for voltage- and space-clamped axons.