Ion Current Fluctuations Research Articles

Does the Formation of a Taylor Cone in a Pulsating Electrospray Directly Impact Mass Spectrometry Signals?

Electrospray ionization (ESI) remains the dominant technique in mass spectrometry (MS)-based analyses. Here, we investigated the relationship between a crucial aspect of ESI, the formation of the Taylor cone, and the MS ion current by utilizing a triple quadrupole (QqQ) mass spectrometer coupled with a streaming high-speed camera and a 3-ring electrode system. In one test, ion current over a 30-s plume gate (a ring electrode) opening was compared with the Taylor cone occurrence analyzed offline, with Spearman's correlation coefficients consistently near 0 despite parameter variations. In another test, real-time detection of Taylor cones was synchronized with QqQ-MS, selectively opening (de-energizing) the plume gate based on the Taylor cone status. This approach enabled matching the ion current with the Taylor cone occurrence. There was no apparent difference between the MS signals recorded in the presence and absence of a Taylor cone. Additionally, a Faraday plate was employed as a detector in offline experiments, revealing agreement between the frequency of liquid meniscus (Taylor cone) oscillation (∼1.92 kHz)-measured by high-speed imaging-and the frequency of spray current (∼1.93 kHz). We suggest that the lack of positive correlations in the MS experiments is due to intrinsic ion carryover during transit from the ion source to the detector and due to the insufficient data acquisition rate of the mass spectrometer, which erases short-term fluctuations of ion current.

Mechanistic description of spontaneous loss of memory persistent activity based on neuronal synaptic strength

Persistent neural activity associated with working memory (WM) lasts for a limited time duration. Current theories suggest that its termination is actively obtained via inhibitory currents, and there is currently no theory regarding the possibility of a passive memory-loss mechanism that terminates memory persistent activity. Here, we develop an analytical-framework, based on synaptic strength, and show via simulations and fitting to wet-lab experiments, that passive memory-loss might be a result of an ionic-current long-term plateau, i.e., very slow reduction of memory followed by abrupt loss. We describe analytically the plateau, when the memory state is just below criticality. These results, including the plateau, are supported by experiments performed on rats. Moreover, we show that even just above criticality, forgetfulness can occur due to neuronal noise with ionic-current fluctuations, yielding a plateau, representing memory with very slow decay, and eventually a fast memory decay. Our results could have implications for developing new medications, targeted against memory impairments, through modifying neuronal noise.

TolC-AcrA complex formation monitored by time dependent single-channel electrophysiology

Characterizing protein-protein interaction on a single molecular level is a challenge, experimentally as well as interpretation of the data. For example, Gram-negative bacteria contain protein complexes spanning the outer and inner cell wall devoted to efflux effectively cell toxic substances. Recent seminal work revealed the high-resolution structure of such a tripartic composition TolC-AcrA-AcrB suggesting to design inhibitors preventing efflux of antibiotics. To show that electrophysiology can provide supporting information here, we reconstitute single TolC homotrimer into a planar lipid membrane, apply a transmembrane voltage and follow the assembly of AcrA to TolC using the modulation of the ion current through TolC channel during binding. In particular, the presence of AcrA in solution increases the average ionic current through TolC and, moreover, reduces the ion-current fluctuations caused by flickering of TolC. Here, we show that statistical properties of ion-current fluctuations (the power spectral density) provide a complementary measure of the interaction of the TolC-AcrA complex in presence of putative efflux pump inhibitors. Both characteristics, the average ion current across TolC and the current noise, taken into consideration together, point to a stiffening of the tip of TolC which might reduce the formation of the complex.

Disentangling 1/f noise from confined ion dynamics.

Ion transport through biological and solid-state nanochannels is known to be a highly noisy process. The power spectrum of current fluctuations is empirically known to scale like the inverse of frequency, following the long-standing yet poorly understood Hooge's law. Here, we report measurements of current fluctuations across nanometer-scale two-dimensional channels with different surface properties. The structure of fluctuations is found to depend on the channel's material. While in pristine channels current fluctuations scale like 1/f1+a with a = 0-0.5, the noise power spectrum of activated graphite channels displays different regimes depending on frequency. Based on these observations, we develop a theoretical formalism directly linking ion dynamics and current fluctuations. We predict that the noise power spectrum takes the form 1/f × Schannel(f), where 1/f fluctuations emerge in fluidic reservoirs on both sides of the channel and Schannel describes fluctuations inside it. Deviations to Hooge's law thus allow direct access to the ion transport dynamics of the channel - explaining the entire phenomenology observed in experiments on 2D nanochannels. Our results demonstrate how current fluctuations can be used to characterize nanoscale ion dynamics.

Identification of Single Amino Acid Chiral and Positional Isomers Using an Electrostatically Asymmetric Nanopore.

Chirality is essential in nearly all biological organizations and chemical reactions but is rarely considered due to technical limitations in identifying L/D isomerization. Using OmpF, a membrane channel from Escherichia coli with an electrostatically asymmetric constriction zone, allows discriminating chiral amino acids in a single peptide. The heterogeneous distribution of charged residues in OmpF causes a strong lateral electrostatic field at the constriction. This laterally asymmetric constriction zone forces the sidechains of the peptides to specific orientations within OmpF, causing distinct ionic current fluctuations. Using statistical analysis of the respective ionic current variations allows distinguishing the presence and position of a single amino acid with different chiralities. To explore potential applications, the disease-related peptide β-Amyloid and its d-Asp1 isoform and a mixture of the icatibant peptide drug (HOE 140) and its d-Ser7 mutant have been discriminated. Both chiral isomers were not applicable to be distinguished by mass spectroscopy approaches. These findings highlight a novel sensing mechanism for identifying single amino acids in single peptides and even for achieving single-molecule protein sequencing.

A simple model considering spiking probability during extracellular axon stimulation.

The spiking probability of an electrically stimulated axon as a function of stimulus amplitude increases in a sigmoidal dependency from 0 to 1. However, most computer simulation studies for neuroprosthetic applications calculate thresholds for neural targets with a deterministic model and by reducing the sigmoid curve to a step function, they miss an important information about the control signal, namely how the spiking efficiency increases with stimulus intensity. Here, this spiking efficiency is taken into account in a compartment model of the Hodgkin Huxley type where a noise current is added in every compartment with an active membrane. A key parameter of the model is a common factor knoise which defines the ion current fluctuations across the cell membrane for every compartment by its maximum sodium ion conductance. In the standard model Gaussian signals are changed every 2.5 μs as a compromise of accuracy and computational costs. Additionally, a formula for other noise transmission times is presented and numerically tested. Spiking probability as a function of stimulus intensity can be approximated by the cumulative distribution function of the normal distribution with RS = σ/μ. Relative spread RS, introduced by Verveen, is a measure for the spread (normalized by the threshold intensity μ), that decreases inversely with axon diameter. Dynamic range, a related measure used in neuroprosthetic studies, defines the intensity range between 10% and 90% spiking probability. We show that (i) the dynamic range normalized by threshold is 2.56 times RS, (ii) RS increases with electrode-axon distance and (iii) we present knoise values for myelinated and unmyelinated axon models in agreement with recoded RS data. The presented method is applicable for other membrane models and can be extended to whole neurons that are described by multi-compartment models.

Changes in Salt Concentration Modify the Translocation of Neutral Molecules through a ΔCymA Nanopore in a Non-monotonic Manner

The voltage-dependent transport through biological and artificial nanopores is being used in many applications such as DNA or protein sequencing and sensing. The primary approach to determine the transport has been to measure the temporal ion current fluctuations caused by solutes when applying external voltages. Crossing the nanoscale confinement in the presence of an applied electric field primarily relies on two factors, i.e., the electrophoretic drag and electroosmosis. The electroosmotic flow (EOF) is a voltage-dependent ion-associated flow of solvent molecules, i.e., usually water, and depends on many factors, such as pH, temperature, pore diameter, and also the concentration of ions. The exact interplay between these factors is so far poorly understood. In this joint experimental and computational study, we have investigated the dependence of the EOF on the concentration of the buffer salt by probing the transport of α-cyclodextrin molecules through the ΔCymA channel. For five different KCl concentrations in the range between 0.125 and 2 M, we performed applied-field molecular dynamics simulations and analyzed the ionic flow and the EOF across the ΔCymA pore. To our surprise, the concentration-dependent net ionic flux changes non-monotonically and nonlinearly and the EOF is seen to follow the same pattern. On the basis of these findings, we were able to correlate the concentration-dependent EOF with experimental kinetic constants for the translocation of α-cyclodextrin through the ΔCymA nanopore. Overall, the results further improve our understanding of the EOF-mediated transport through nanopores and show that the EOF needs to seriously be taken into consideration when analyzing the permeation of (neutral) substrates through nanopores.

Impact of electrode position on the dynamic range of a human auditory nerve fiber

Objective. Electrodes of a cochlear implant generate spikes in auditory nerve fibers (ANFs). While the insertion depth of each of the electrodes is linked to a frequency section of the acoustic signal, the amplitude of the stimulating pulses controls the loudness of the related frequency band. However, in comparison to acoustic stimulation the dynamic range (DR) of an electrically stimulated ANF is quite small. Approach. The DR of an electrically stimulated ANF is defined as the interval of stimulus amplitudes that causes firing probabilities between 10% and 90%. A compartment model that includes sodium ion current fluctuations as the stochastic key component for spiking was evaluated for different electrode placements and fiber diameters. Main results. The DR is reversely related to ANF diameter. An increased DR is expected to improve the quality of auditory perception for CI users. Electrodes are often placed as close to the center axis of the cochlea as possible. The analysis of the simulated auditory nerve firing showed that this placement is disadvantageous for the DR of a selected ANF. Significance. Five times larger DRs are expected for electrodes close to the terminal of the dendrite or at mid-dendritic placement as opposed to electrodes close to the modiolus.

Development of an OmpG multiplex biosensor

The rapid and selective detection of disease biomarkers is integral for improving patient outcomes. However, commonly employed diagnostic techniques exhibit drawbacks related to selective biomarker detection from complex samples while maintaining rapid assay time. Nanopore sensing offers one solution to address these limitations, achieving highly selective, single-molecule protein detection by monitoring ionic current fluctuations through an engineered protein pore. Outer membrane protein G (OmpG) is a promising scaffold for the development of these sensors as each of OmpG’s seven flexible extracellular loops can function as a display platform for affinity reagents, either tethered small molecules, or genetically encoded peptide motifs.

Large-Peptide Permeation Through a Membrane Channel: Understanding Protamine Translocation Through CymA from Klebsiella Oxytoca*.

Quantifying the passage of the large peptide protamine (Ptm) across CymA, a passive channel for cyclodextrin uptake, is in the focus of this study. Using a reporter‐pair‐based fluorescence membrane assay we detected the entry of Ptm into liposomes containing CymA. The kinetics of the Ptm entry was independent of its concentration suggesting that the permeation through CymA is the rate‐limiting factor. Furthermore, we reconstituted single CymA channels into planar lipid bilayers and recorded the ion current fluctuations in the presence of Ptm. To this end, we were able to resolve the voltage‐dependent entry of single Ptm peptide molecules into the channel. Extrapolation to zero voltage revealed about 1–2 events per second and long dwell times, in agreement with the liposome study. Applied‐field and steered molecular dynamics simulations added an atomistic view of the permeation events. It can be concluded that a concentration gradient of 1 μm Ptm leads to a translocation rate of about one molecule per second and per channel.

Highly Nonuniform Ionic Transport on Ceramic-Polymer Hybrid Solid Electrolyte By Nanometer-Scale Operando Imaging

The ion conductivity of solid-state electrolytes (SSEs) has been greatly enhanced to a level comparable to that of liquid electrolytes, which is critical for fast charging/discharging. However, ion transport has primarily been characterized by macroscopic measurements such as electrochemical impedance spectroscopy (EIS) and galvanostatic profiles of charging/discharging. These measurements essentially reflect an average of microscopic ion-transport phenomena. Ion transport in SSEs can be far more complicated, but detailed microscopic transport characterization studies are currently lacking. Theoretical studies have proposed that ion transport can be governed by highly crystalline-orientation-dependent ionic hopping through the ceramic lattice, and some SSEs can even act as one-dimensional or two-dimensional ion conductors.[1] [2] Characterization methods are needed that measure in-situ ion transport with nm-scale spatial resolution to improve our understanding and tune the ion conduction in solid-state batteries (SSBs).Here, we report on nanometer-resolution in-situ ionic and electronic transport imaging of two advanced polymer-ceramic hybrid electrolytes: a polyimin/LixPySz (Li2S/P2S5:30/70) bulk hybrid SSE[3] and a Li-containing polyacrylonitrile (PAN)-coated Li-ion conductive glass ceramic (LICGC) with components of Li2O-Al2O3-SiO2-P2O5-TiO2-GeO2.[4] Hybrid SSE is a promising approach that combines high ion conductivity of ceramic SSE and high stability of polymer SSE. We achieved the nm-resolution in-situ transport imaging by an atomic force microscopy (AFM)-based half-cell setup consisting of an AFM-probe/SSE/Li-metal structure (see Fig. 1). The AFM probe size ensures the nm-scale spatial resolutions, and we detected the small current in the pA‒fA (10-12‒10-15 A) range flowing through the AFM probe by using a logarithmic-scale current amplifier that detects both the ionic conduction and electronic leaking currents through the SSE.Concerning the bulk hybrid SSE, we found large ionic current fluctuations—five times to one order of magnitude on the LPS region of the SSE. The electronic leaking current is about 5 orders of magnitude smaller than the ionic current. Both the ionic and electronic transport at the LPS/polymer boundary show sharp transitions with no detectable current flowing through the polymer particles. The average ionic current through the LPS region decreases significantly with decreasing polyimine particle size and with extensive cycling, elucidating the Li-dendrite prevention mechanisms of this bulk hybrid SSE by polymer extension into ceramic interparticles.[3] The large ionic current fluctuation is understood in terms of highly anisotropic transport kinetic barriers along the different crystalline axes due to different grain orientations in the polycrystalline ceramic materials;[1] [2] this is a new factor to be mitigated to eliminate Li dendrite formation and growth in the SSBs.Regarding the polymer-coated LICGC SSE, we found significant ionic current fluctuation of 2‒3× the ion current but that is significantly less than in the LPS SSE. This mitigated current fluctuation is likely due to the more homogeneous ion-transport barriers of the glassy material with smaller grains in its amorphous matrix. The electronic leaking current is slightly larger than LPS, four orders of magnitude smaller than the ionic current. The polymer coating reduced the ionic current by several times compared with the bare LICGC SSE without the PAN coating. The PAN coating clearly prevented SSE degradation due to cycling.[4] Without the coating, the SSE after extensive cycling degraded extensively and nonuniformly, with domain sizes of μm to hundreds of μm. The degradation can be ionic blocking with slight electronic leaking as well as highly electronic shunting, depending on different domains or areas. The polymer coating effectively prevented both ionic and electronic degradation. In fact, ionic conduction improved by about 5 times, but electronic leaking was degraded slightly.Our AFM-based imaging opens up novel characterization of ionic and electronic transport in SSEs. The nm-scale inhomogeneous transport mechanisms as revealed by this operando imaging are far beyond the averaged results provided by macroscopic characterizations.

Fast Blocking Effect of Gallamine on LCC Channels: Analysis Using a Truncated Kinetic Scheme

Modulation of Large-conductance cationic (LCC) channel currents through the inner nuclear membrane of rat Purkinje neurons by gallamine was estimated using analysis of the amplitude histograms. A kinetic scheme consisting of one open state and one blocked state of the channel was used. The search for optimal kinetic parameters of the truncated model was performed for the analysis of the blocking effect. The estimated rate constants of blocking and unblocking by 100 μM gallamine at –40 mV were 17880 ± ± 975 and 12790 ± 922, respectively. The obtained data show that gallamine is a fast blocker of the above channels; blocking is accompanied by ion current fluctuations suitable for the analysis using the two-state model if contribution of the events formed by intrinsic ion channel activity can be neglected.

Nonuniform Ionic and Electronic Transport of Ceramic and Polymer/Ceramic Hybrid Electrolyte by Nanometer‐Scale Operando Imaging for Solid‐State Battery

AbstractReplacing the liquid electrolyte in lithium batteries with solid‐state ion conductor is promising for next‐generation energy storage that is safe and has high energy density. Here, nanometer‐resolution ionic and electronic transport imaging of Li3PS4 (LPS), a solid‐state electrolyte (SSE), is reported. This nm resolution is achieved by using a logarithm‐scale current amplifier that enhances the current sensitivity to the fA range. Large fluctuations of ion current—one to two orders of magnitude on the LPS and on the LPS region of a polymer/LPS bulk hybrid SSE—that must be mitigated to eliminate Li dendrite formation and growth, are found. This ion current fluctuation is understood in terms of highly anisotropic transport kinetic barriers along the different crystalline axes due to different grain orientations in the polycrystalline and glass ceramic materials. The results on the bulk hybrid SSE show a sharp transition of ionic and electronic transport at the LPS/polymer boundary and decreases in average ionic current with decreasing polyimine particle size and with extensive cycling. The results elucidate the mechanism of polyimine extension into interparticles to prevent Li dendrite growth. This work opens up novel characterization of charge transport, which relates to Li plating and stripping for solid‐state‐batteries.

Dynamic interaction of fluoroquinolones with magnesium ions monitored using bacterial outer membrane nanopores.

Divalent ions are known to have a severe effect on the translocation of several antibiotic molecules into (pathogenic) bacteria. In the present study we have investigated the effect of divalent ions on the permeability of norfloxacin across the major outer membrane channels from E. coli (OmpF and OmpC) and E. aerogenes (Omp35 and Omp36) at the single channel level. To understand the rate limiting steps in permeation, we reconstituted single porins into planar lipid bilayers and analyzed the ion current fluctuations caused in the presence of norfloxacin. Moreover, to obtain an atomistic view, we complemented the experiments with millisecond-long free energy calculations based on temperature-accelerated Brownian dynamics simulations to identify the most probable permeation pathways of the antibiotics through the respective pores. Both, the experimental analysis and the computational modelling, suggest that norfloxacin is able to permeate through the larger porins, i.e., OmpF, OmpC, and Omp35, whereas it only binds to the slightly narrower porin Omp36. Moreover, divalent ions can bind to negatively charged residues inside the porin, reversing the ion selectivity of the pore. In addition, the divalent ions can chelate with the fluoroquinolone molecules and alter their physicochemical properties. The results suggest that the conjugation with either pores or molecules must break when the antibiotic molecules pass the lumen of the porin, with the conjugation to the antibiotic being more stable than that to the respective pore. In general, the permeation or binding process of fluoroquinolones in porins occurs irrespective of the presence of divalent ions, but the presence of divalent ions can vary the kinetics significantly. Thus, a detailed investigation of the interplay of divalent ions with antibiotics and pores is of key importance in developing new antimicrobial drugs.

Synthetically Diversified Protein Nanopores: Resolving Click Reaction Mechanisms.

Nanopores are emerging as a powerful tool for the investigation of nanoscale processes at the single-molecule level. Here, we demonstrate the methionine-selective synthetic diversification of α-hemolysin (α-HL) protein nanopores and their exploitation as a platform for investigating reaction mechanisms. A wide range of functionalities, including azides, alkynes, nucleotides, and single-stranded DNA, were incorporated into individual pores in a divergent fashion. The ion currents flowing through the modified pores were used to observe the trajectory of a range of azide-alkyne click reactions and revealed several short-lived intermediates in Cu(I)-catalyzed azide-alkyne [3 + 2] cycloadditions (CuAAC) at the single-molecule level. Analysis of ion-current fluctuations enabled the populations of species involved in rapidly exchanging equilibria to be determined, facilitating the resolution of several transient intermediates in the CuAAC reaction mechanism. The versatile pore-modification chemistry offers a useful approach for enabling future physical organic investigations of reaction mechanisms at the single-molecule level.

Effects of H-bonds on sugar binding to chitoporin from Vibrio harveyi

BackgroundVhChiP is a sugar-specific-porin present in the outer membrane of the marine bacterium Vibrio harveyi and responsible for chitin uptake, with a high selectivity for chitohexaose. MethodsVhChiP and its mutants were expressed and purified from BL21 (DE3) Omp8 Rosetta strain. After reconstitution into planar lipid bilayers, the ion current fluctuations caused by chitohexaose entering the channel were measured in deuterium oxide and in water. ResultsThe role of hydrogen-bonding in sugar binding was investigated by comparing channel occlusion by chitohexaose in buffers containing H2O and D2O. The BLM results revealed the significant contribution of hydrogen bonding to the binding of chitohexaose in the constriction zone of VhChiP. Replacing H2O as solvent by D2O significantly decreased the on- and off-rates of sugar penetration into the channel. The importance of hydrogen bonding inside the channel was more noticeable when the hydrophobicity of the constriction zone was diminished by replacing Trp136 with the charged residues Asp or Arg. The on- and off-rates decreased up to 2.5-fold and 4-fold when Trp136 was replaced by Arg, or 5-fold and 3-fold for Trp136 replacement by Asp, respectively. Measuring the on-rate at different temperatures and for different channel mutants revealed the activation energy for chitohexaose entrance into VhChiP channel. ConclusionsHydrogen-bonds contribute to sugar permeation.

Ampicillin permeation across OmpF, the major outer-membrane channel in Escherichia coli

The outer cell wall of the Gram-negative bacteria is a crucial barrier for antibiotics to reach their target. Here, we show that the chemical stability of the widely used antibiotic ampicillin is a major factor in the permeation across OmpF to reach the target in the periplasm. Using planar lipid bilayers we investigated the interactions and permeation of OmpF with ampicillin, its basic pH-induced primary degradation product (penicilloic acid), and the chemically more stable benzylpenicillin. We found that the solute-induced ion current fluctuation is 10 times higher with penicilloic acid than with ampicillin. Furthermore, we also found that ampicillin can easily permeate through OmpF, at an ampicillin gradient of 10 μm and a conductance of Gamp ≅ 3.8 fS, with a flux rate of roughly 237 molecules/s of ampicillin at Vm = 10 mV. The structurally related benzylpenicillin yields a lower conductance of Gamp ≅ 2 fS, corresponding to a flux rate of ≈120 molecules/s. In contrast, the similar sized penicilloic acid was nearly unable to permeate through OmpF. MD calculations show that, besides their charge difference, the main differences between ampicillin and penicilloic acid are the shape of the molecules, and the strength and direction of the dipole vector. Our results show that OmpF can impose selective permeation on similar sized molecules based on their structure and their dipolar properties.

Protein Nanopore-Based Discrimination between Selected Neutral Amino Acids from Polypeptides.

Nanopore probing of biological polymers has the potential to achieve single-molecule sequencing at low cost, high throughput, portability, and minimal sample preparation and apparatus. In this article, we explore the possibility of discrimination between neutral amino acid residues from the primary structure of 30 amino acids long, engineered peptides, through the analysis of single-molecule ionic current fluctuations accompanying their slowed-down translocation across the wild type α-hemolysin (α-HL) nanopore, and molecular dynamics simulations. We found that the transient presence inside the α-HL of alanine or tryptophan residues from the primary sequence of engineered peptides results in distinct features of the ionic current fluctuation pattern associated with the peptide reversibly blocking the nanopore. We propose that α-HL sensitivity to the molecular exclusion at the most constricted region mediates ionic current blockade events correlated with the volumes that are occluded by at least three alanine or tryptophan residues, and provides the specificity needed to discriminate between groups of neutral amino acids. Further, we find that the pattern of current fluctuations depends on the orientation of the threaded amino acid residues, suggestive of a conformational anisotropy of the ensemble of conformations of the peptide on the restricted nanopore region, related to its relative axial orientation inside the nanopore.

Sensing Single Molecule Penetration into Nanopores: Pushing the Time Resolution to the Diffusion Limit.

To quantify small molecule penetration into and eventually permeation through nanopores, we applied an improved excess-noise analysis of the ion current fluctuation caused by entering molecules. The kinetic parameters of substrate entry and exit are derived from a two-state Markov model, analyzing the substrate concentration dependence of the average ion current and its variance. Including filter corrections allows one to detect the transition rates beyond the cutoff frequency, fc, of the instrumental ion-current filter. As an application of the method, we performed an analysis of the single-channel ion current of Meropenem, an antibiotic of the carbapenem family, interacting with OmpF, the major general outer membrane channel of Escherichia coli bacteria. At 40 °C we detected the residence time of Meropenem inside OmpF of about 500 ns-more than 2 orders of magnitude smaller than fc-1 and close to the diffusion limit of few hundred nanoseconds. We also have established theoretical limit conditions under which the substrate-induced channel blockages can be detected and suggest that submicrosecond-scale gating kinetic parameters are accessible with existing experimental equipment.

Peptide translocation across MOMP, the major outer membrane channel from Campylobacter jejuni

Here we report on translocation of short poly-arginines across the MOMP porin, the major outer membrane protein in the cell wall of Campylobacter jejuni. MOMP was purified to homogeneity from a pathogenic strain of C. jejuni. Its reconstitution in lipid membranes and measuring the ion-current revealed two main distinct populations of protein channels which we interpreted as mono and trimers. Addition of poly-arginines causes concentration and voltage dependent ion-current fluctuations. Increasing the transmembrane potential decreases the residence time of the peptide inside the channel indicating successful translocation. We conclude that poly-arginines can cross the outer membrane of Campylobacter through the MOMP channel.