Channel Conductance Measurements Research Articles

Transport properties of SiO2/AlInN/AlN/GaN metal–oxide–semiconductor high electron mobility transistors on SiC substrate

Unpassivated SiO2/AlInN/AlN/GaN metal–oxide–semiconductor high electron mobility transistors (MOSHEMTs) exhibiting a thin barrier layer are investigated with a particular focus on their dc characteristics dependence on the gate length. The epiwafer exhibits a sheet resistance of ∼250 Ω/□ and a channel charge density of 7.4 × 1012 cm−2 deduced from the 1 MHz capacitance–voltage curves. The results indicate that the thickness of the AlInN barrier can be reduced below 5 nm without degradation of the insulated gate devices performance. For transistors with gate lengths (LG) between 1.8 and 2.0 µm, dc drain saturation currents densities as high as 1.8 A mm−1 are achieved at +4 V gate–source bias (VGS) with very low reverse gate leakage currents. The electron zero-bias drift mobility was determined to be 1670 cm2 V−1 s−1 from the low-field channel conductance measurements. On the other side, using an analytical model it is found that the maximum output current density at VGS = 0 V can be enhanced by ∼23% when LG is scaled from 1.8 µm down to 100 nm. With further improvement of the quality of the gate insulating oxide layer and the implementation of surface passivation, both with the aim of suppressing the observed current collapse, the presented results suggest that these MOSHEMTs could become very attractive for the realization of high-power electronics.

Interface trap density metrology from sub-threshold transport in highly scaled undoped Si n-FinFETs

Channel conductance measurements can be used as a tool to study thermally activated electron transport in the sub-threshold region of state-of-art FinFETs. Together with theoretical tight-binding (TB) calculations, this technique can be used to understand the dependence of the source-to-channel barrier height (Eb) and the active channel area (Saa) on three important parameters: (i) the gate bias (Vgs), (ii) the temperature, and (iii) the FinFET cross-section size. The quantitative difference between experimental and theoretical values that we observe can be attributed to the interface traps present in these FinFETs. Therefore, based on the difference between measured and calculated values of (i) Saa and (ii) |∂Eb/∂Vgs| (channel to gate coupling), two new methods of interface trap density (Dit) metrology are outlined. These two methods are shown to be very consistent and reliable, thereby opening new ways of analyzing in situ state-of-the-art multi-gate FETs down to the few nanometer width limit. Furthermore, theoretical investigation of the spatial current density reveals volume inversion in thinner FinFETs near the threshold voltage.

Conjugal plasmid transfer inStreptomycesresembles bacterial chromosome segregation by FtsK/SpoIIIE

Conjugation is a major route of horizontal gene transfer, the driving force in the evolution of bacterial genomes. Antibiotic producing soil bacteria of the genus Streptomyces transfer DNA in a unique process involving a single plasmid-encoded protein TraB and a double-stranded DNA molecule. However, the molecular function of TraB in directing DNA transfer from a donor into a recipient cell is unknown. Here, we show that TraB constitutes a novel conjugation system that is clearly distinguished from DNA transfer by a type IV secretion system. We demonstrate that TraB specifically recognizes and binds to repeated 8 bp motifs on the conjugative plasmid. The specific DNA recognition is mediated by helix α3 of the C-terminal winged-helix-turn-helix domain of TraB. We show that TraB assembles to a hexameric ring structure with a central ∼3.1 nm channel and forms pores in lipid bilayers. Structure, sequence similarity and DNA binding characteristics of TraB indicate that TraB is derived from an FtsK-like ancestor protein, suggesting that Streptomyces adapted the FtsK/SpoIIIE chromosome segregation system to transfer DNA between two distinct Streptomyces cells.

Tubulin-VDAC Interaction: Salt Dependence of Conductance and Reversal Potential

The recently reported blocking of mitochondrial voltage-dependent anion channel (VDAC) by tubulin [1] has raised new questions about the role of tubulin C terminal tails (CTT) in the low conductance states exhibited by VDAC channel. The voltage sensitive reversible channel closure observed suggests that tubulin is more than an enhancer of VDAC gating. However, there is no conclusive evidence about the nature of the channel interaction with tubulin tails. Experiments have also revealed that VDAC-tubulin interaction is also influenced by the lipid composition of the membrane hosting the channel. While the duration of tubulin induced closure seems to be independent of salt concentration, the opposite is true with VDAC open time between successive blockages. The change in conductance between open and close states is also concentration dependent. By performing single channel conductance measurements as well as reversal potential measurements (in open and closed states) we analyze the importance of electrostatic interactions between VDAC ionizable residues and the CTT negative charge in the binding of tubulin. The anion selectivity of VDAC in its open state is turned into cation selectivity under tubulin blockage. The recently published 3D structure of VDAC [2] seems fully compatible with the partial penetration of alpha or beta tubulin tails into the channel lumen.[1] T. K. Rostovtseva et al. PNAS, 105 (2008) 18746.[2] S. Hiller et al. Trends Biochem. Sci., 35 (2010) 514.

Acid Sensitivity and Amanatadine Block of Influenza a M2 Channels in Folded Bilayers

Understanding of ion channel function involves assignment of channel transport activities to constituents, to their arrangements and to membrane conditions. One strategy for identifying the structure function relationship is reconstitution of protein in planar lipid membranes. Reconstitution of M2 protein has been reported in the past using lipid vesicles and measuring the proton flux at different pH. Also, single channel conductance measurements were reported based on studies with decane inflated planar lipid bilayers at low pH where amantadine sensitivity was low. The present work reports the proton conductance of Influenza A M2(22-62) in a bilayer (275 pF) folded from proteoliposomes (1 mg M2(22-62) in 20 mg POPE:POPC:POPG, 3:1:1). The 250-µm bilayer was estimated to contain 2.9x107 tetramers. The channel conductance was undetectable with symmetrical pH 7.5 at −30 mV (trans relative to cis). Upon acidification of the cis chamber to pH 5.5 (and stirring), there was a smooth increase in current to a level of 633 pS (−19 pA, i.e. current flow from cis to trans, 4.1 protons/tetramer/s). Then, amantadine was injected cis and trans to a concentration of 0.1 mM. Stirring cis produced 79% block over the course of 5.6 minutes. The residual current was constant for the next 5.6 min. Then, cis neutralization eliminated the residual current. Rechallenge with cis acidification (with amantadine present) showed persistence of block. Acidification and amantadine sensitivity of this construct matches with those observed for other Influenza A M2 constructs in Xenopus oocytes and MEL cells. Similar results were observed in two additional folded-bilayer experiments and further studies are in progress.

Omp85 from the Thermophilic Cyanobacterium Thermosynechococcus elongatus Differs from Proteobacterial Omp85 in Structure and Domain Composition

Omp85 proteins are essential proteins located in the bacterial outer membrane. They are involved in outer membrane biogenesis and assist outer membrane protein insertion and folding by an unknown mechanism. Homologous proteins exist in eukaryotes, where they mediate outer membrane assembly in organelles of endosymbiotic origin, the mitochondria and chloroplasts. We set out to explore the homologous relationship between cyanobacteria and chloroplasts, studying the Omp85 protein from the thermophilic cyanobacterium Thermosynechococcus elongatus. Using state-of-the art sequence analysis and clustering methods, we show how this protein is more closely related to its chloroplast homologue Toc75 than to proteobacterial Omp85, a finding supported by single channel conductance measurements. We have solved the structure of the periplasmic part of the protein to 1.97 Å resolution, and we demonstrate that in contrast to Omp85 from Escherichia coli the protein has only three, not five, polypeptide transport-associated (POTRA) domains, which recognize substrates and generally interact with other proteins in bigger complexes. We model how these POTRA domains are attached to the outer membrane, based on the relationship of Omp85 to two-partner secretion system proteins, which we show and analyze. Finally, we discuss how Omp85 proteins with different numbers of POTRA domains evolved, and evolve to this day, to accomplish an increasing number of interactions with substrates and helper proteins.

Truncated beta-amyloid peptide channels provide an alternative mechanism for Alzheimer's Disease and Down syndrome.

Full-length amyloid beta peptides (Abeta(1-40/42)) form neuritic amyloid plaques in Alzheimer's disease (AD) patients and are implicated in AD pathology. However, recent transgenic animal models cast doubt on their direct role in AD pathology. Nonamyloidogenic truncated amyloid-beta fragments (Abeta(11-42) and Abeta(17-42)) are also found in amyloid plaques of AD and in the preamyloid lesions of Down syndrome, a model system for early-onset AD study. Very little is known about the structure and activity of these smaller peptides, although they could be the primary AD and Down syndrome pathological agents. Using complementary techniques of molecular dynamics simulations, atomic force microscopy, channel conductance measurements, calcium imaging, neuritic degeneration, and cell death assays, we show that nonamyloidogenic Abeta(9-42) and Abeta(17-42) peptides form ion channels with loosely attached subunits and elicit single-channel conductances. The subunits appear mobile, suggesting insertion of small oligomers, followed by dynamic channel assembly and dissociation. These channels allow calcium uptake in amyloid precursor protein-deficient cells. The channel mediated calcium uptake induces neurite degeneration in human cortical neurons. Channel conductance, calcium uptake, and neurite degeneration are selectively inhibited by zinc, a blocker of amyloid ion channel activity. Thus, truncated Abeta fragments could account for undefined roles played by full length Abetas and provide a unique mechanism of AD and Down syndrome pathologies. The toxicity of nonamyloidogenic peptides via an ion channel mechanism necessitates a reevaluation of the current therapeutic approaches targeting the nonamyloidogenic pathway as avenue for AD treatment.

A semi-synthetic ion channel platform for detection of phosphatase and protease activity.

Sensitive methods to probe the activity of enzymes are important for clinical assays and for elucidating the role of these proteins in complex biochemical networks. This paper describes a semi-synthetic ion channel platform for detecting the activity of two different classes of enzymes with high sensitivity. In the first case, this method uses single ion channel conductance measurements to follow the enzyme-catalyzed hydrolysis of a phosphate group attached to the C-terminus of gramicidin A (gA, an ion channel-forming peptide) in the presence of alkaline phosphatase (AP). Enzymatic hydrolysis of this phosphate group removes negative charges from the entrance of the gA pore, resulting in a product with measurably reduced single ion channel conductance compared to the original gA-phosphate substrate. This technique employs a standard, commercial bilayer setup and takes advantage of the catalytic turnover of enzymes and the amplification characteristics of ion flux through individual gA pores to detect picomolar concentrations of active AP in solution. Furthermore, this technique makes it possible to study the kinetics of an enzyme and provides an estimate for the observed rate constant (k(cat)) and the Michaelis constant (K(M)) by following the conversion of the gA-phosphate substrate to product over time in the presence of different concentrations of AP. In the second case, modification of gA with a substrate for proteolytic cleavage by anthrax lethal factor (LF) afforded a sensitive method for detection of LF activity, illustrating the utility of ion channel-based sensing for detection of a potential biowarfare agent. This ion channel-based platform represents a powerful, novel approach to monitor the activity of femtomoles to picomoles of two different classes of enzymes in solution. Furthermore, this platform has the potential for realizing miniaturized, cost-effective bioanalytical assays that complement currently established assays.

Simultaneous Alternating and Direct Current Readout of Protein Ion Channel Blocking Events Using Glass Nanopore Membranes

Alternating current (ac) phase-sensitive detection is used to measure the conductance of the ion channel alpha-hemolysin (alphaHL), while simultaneously applying a direct current (dc) bias to electrostatically control the binding affinity and kinetics of charged molecules within the protein lumen. Ion channel conductance was recorded while applying a 10-20 mV rms, 1-2 kHz bias across a single alphaHL protein inserted in a 1,2-diphytanoyl-sn-glycero-3-phosphocholine lipid bilayer that is suspended across the orifice (100-500 nm radius) of a glass nanopore membrane. Step changes in the ac ion channel conductance with a temporal response (t(10-90)) of 1.5 ms and noise amplitude of approximately 2 pA were obtained using a low-noise potentiostat and a lock-in amplifier. These conditions were used to monitor the reversible and stochastic binding of heptakis-(6-O-sulfo)-beta-cyclodextrin and a nine base pair DNA hairpin molecule to the ion channel. Alternating current methodology allows the binding kinetics and affinity between the protein ion channel and analyte to be investigated as a function of the dc bias, including ion channel conductance measurements in the absence of a dc bias.

Interfacial Energetics of Silicon in Contact with 11 M NH4F(aq), Buffered HF(aq), 27 M HF(aq), and 18 M H2SO4

Open-circuit impedance spectra, channel impedance spectroscopy on solution-gated field-effect devices, and differential capacitance vs potential (Mott−Schottky) measurements were used to determine the energetics of n-Si(111), n-Si(100), and p-Si(111) electrodes in contact with aqueous 11 M (40% by weight) NH4F, buffered HF (BHF), 27 M (48%) HF(aq), and concentrated (18 M) H2SO4. A Mott−Schottky analysis of As2Csc-2-vs-E (where As is the interfacial area, and Csc is the differential capacitance as a function of the electrode potential, E) data yielded reliable barrier heights for some silicon/liquid contacts in this work. Performing a Mott−Schottky analysis, however, requires measurement of the differential capacitance under reverse bias, where oxidation or etching can occur for n-Si and where electroplating of metal contaminants can occur for p-Si. Hence, open-circuit methods would offer desirable, complementary approaches to probing the energetics of such contacts. Accordingly, open-circuit, near-surface channel conductance measurements have been performed using solution-gated n+-p-Si(111)-n+ and p+-n-Si(100)-p+ devices. Additionally, open-circuit impedance spectra were obtained for silicon electrodes in contact with these solutions. The combination of the three techniques indicated that the surfaces of n-Si(111) and n-Si(100) were under accumulation when in contact with either 11 M NH4F(aq) or BHF(aq). The barrier heights for n-Si(111) and n-Si(100) in 11 M NH4F(aq) were −0.065 ± 0.084 V and −0.20 ± 0.21 V, respectively, and were −0.03 ± 0.19 V and −0.07 ± 0.24 V, respectively, for these surfaces in contact with buffered HF(aq). Consistently, p-Si(111) surfaces were determined to be in inversion in contact with these electrolytes, exhibiting barrier heights of 0.984 ± 0.078 V in contact with 11 M NH4F(aq) and 0.97 ± 0.22 V in contact with buffered HF(aq). In contact with 27 M HF(aq), n-Si(111) and n-Si(100) were in depletion, with barrier heights of 0.577 ± 0.038 V and 0.400 ± 0.057 V, respectively, and p-Si(111) was under inversion with a barrier height of 0.856 ± 0.076 V. Measurements performed in 18 M H2SO4 revealed barrier heights of 0.75 ± 0.11 V, 0.696 ± 0.043 V, and 0.889 ± 0.018 V for n-Si(111), n-Si(100), and p-Si(111), respectively, demonstrating that in 18 M H2SO4, the band edge positions of Si were different for different doping types. The barrier height data demonstrate that the observed low recombination rates of silicon in contact with 11 M NH4F, BHF, or 18 M H2SO4 cannot necessarily be attributed to a reduction in the number of surface trap states. In part, low surface recombination rates are expected for such systems because the very large or very small barrier height for silicon in contact with these liquids provides a potential barrier that prevents one type of photogenerated carrier (either electrons or holes) from reaching the surface, thereby producing a low steady-state surface recombination rate.

Controlled delivery of proteins into bilayer lipid membranes on chip.

The study and the exploitation of membrane proteins for drug screening applications requires a controllable and reliable method for their delivery into an artificial suspended membrane platform based on lab-on-a-chip technology. In this work, a polymeric device for forming lipid bilayers suitable for electrophysiology studies and biosensor applications is presented. The chip supports a single bilayer and is configured for controlled protein delivery through on-chip microfluidics. In order to demonstrate the principle of protein delivery, the potassium channel KcsA was reconstituted into proteoliposomes, which were then fused with the suspended bilayer on-chip. Fusion of single proteoliposomes with the membrane was identified electrically. Single channel conductance measurements of KcsA in the on-chip bilayer were recorded and these were compared to previously published data obtained with a conventional planar bilayer system.

Ion Channel Conductance Measurements on a Silicon-Based Platform

Conductance measurements of the transmembrane porin protein OmpF as a function of pH and bath concentration have been made with both a microfabricated silicon substrate device and a commercially available polystyrene aperture. Ion transport through the channel was simulated in atomic detail: the measured current was compared with theoretically calculated current, using a Brownian Dynamics kernel coupled to the Poisson equation by a P3M force field. The explicit protein structure and fixed charge distribution in the protein are calculated using the molecular dynamics code, GROMACS. Reasonable agreement is obtained in the simulated versus measured conductance over the range of experimental concentrations studied.

Statistical mechanics model for the interaction between the neurotransmitter gamma-aminobutyric acid and GABAA receptors.

Interactions between the neurotransmitter gamma-aminobutyric acid (GABA) and GABAA receptor ion channels play an important role in the central nervous system. A statistical mechanics model is proposed for the interaction between GABA and GABAA receptors. The model provides good fits to the electrophysiology data as well as an estimation of receptor activation energies, and predicts the temperature dependence consistent with measurements. In addition, the model provides insights into single channel conductance measurements. This model is also applicable to other ligand-gated ion channels with similar pentameric structures.

Use of near-surface channel conductance and differential capacitance versus potential measurements to correlate inversion layer formation with low effective surface recombination velocities at n-Si/liquid contacts

Near-surface channel conductance measurements, differential capacitance versus potential measurements, and surface recombination velocity measurements have been performed on (111)- and (100)-oriented n-type Si samples in contact with nitrogen and/or liquid electrolyte solutions containing I2, I2/I−, ferrocene+/0, or decamethylferrocene+/0 in either methanol or tetrahydrofuran. Si/liquid contacts that displayed a low effective surface recombination velocity S corresponded to those that formed an inversion layer at the solid/liquid contact as indicated by channel conductance measurements or by differential capacitance versus potential measurements. Contacts that did not produce an inversion layer at the Si surface did not produce low effective S values. The observed behavior is consistent with the known energetics of Si/liquid contacts and provides an explanation for the low effective S values observed in these systems.

Refolded Outer Membrane Protein A of Escherichia coliForms Ion Channels with Two Conductance States in Planar Lipid Bilayers

Outer membrane protein A (OmpA), a major structural protein of the outer membrane of Escherichia coli, consists of an N-terminal 8-stranded beta-barrel transmembrane domain and a C-terminal periplasmic domain. OmpA has served as an excellent model for studying the mechanism of insertion, folding, and assembly of constitutive integral membrane proteins in vivo and in vitro. The function of OmpA is currently not well understood. Particularly, the question whether or not OmpA forms an ion channel and/or nonspecific pore for uncharged larger solutes, as some other porins do, has been controversial. We have incorporated detergent-purified OmpA into planar lipid bilayers and studied its permeability to ions by single channel conductance measurements. In 1 M KCl, OmpA formed small (50-80 pS) and large (260-320 pS) channels. These two conductance states were interconvertible, presumably corresponding to two different conformations of OmpA in the membrane. The smaller channels are associated with the N-terminal transmembrane domain, whereas both domains are required to form the larger channels. The two channel activities provide a new functional assay for the refolding in vitro of the two respective domains of OmpA. Wild-type and five single tryptophan mutants of urea-denatured OmpA are shown to refold into functional channels in lipid bilayers.

Differences between porin isolated from Rhodobacter capsulatus B10 and 37b4

Porin was isolated from Rhodobacter (Kb.) capsulatus wild type BIO, as well as from Rb. capsulatus 37b4 as a control. The porin from Rb. capsulatus B10 shows significant differences to that of Rb. capsulatus 37b4 in N-terminal sequence, amino acid composition and molecular mass. The apparent molecular mass of the purified porin from Rb. capsulatus B10 is about 28 kDa by SDS-PAGE, whereas the native porin trimer migrates at 75 kDa. For Rb. capsulatus 37b4 the molecular mass of the momomer is about 30 kDa and for the trimer about 76 kDa. These differences may be related to the morphological differences between the two wild type strains. Rb. capsulatus BIO has an extracellular capsule whereas Rb. capsulatus 37b4 is capsuleless. The native proteins from both wild types showed similar single channel conductance measurements in black lipid membranes. The B10 porin has been crystallised using the detergent ß-d-octyl-gluco-pyranoside. The crystals diffracted to 3 Å and belong to the orthorhombic space-group P2 12 12 1. The crystal packing could be elucidated by molecular replacement.

Characterization of Ca(2+)-activated 86Rb+ fluxes in rat C6 glioma cells: a system for identifying novel IKCa-channel toxins.

1. The pharmacological characteristics of a putative Ca2+ activated K+ channel (IKCa channel) in rat glioma C6 cells were studied in the presence of the Ca2+ ionophore, ionomycin and various K+ channel blockers, 86Rb+ being used as a radioisotopic tracer for K+. 2. The resting 86Rb+ influx into C6 cells was 318 +/- 20 pmol s-1. The threshold for ionomycin activation of 86Rb+ influx was approx. 100 nM. At ionomycin concentrations above the activation threshold, the initial rate of 86Rb+ influx was proportional to ionophore concentration. Ionomycin-activated 86Rb+ flux was saturable (EC50 = 0.62 +/- 0.03 microM) and was not inhibited by ouabain. 3. Intracellular Ca2+ increased within 30 s from a basal level of 42 +/- 2 nM to 233 +/- 17 nM, after addition of 2 microM ionomycin. During this period, intracellular pH fell from 7.03 +/- 0.04 to 6.87 +/- 0.03 and the cell hyperpolarized from -34 +/- 10 mV to -76 +/- 2 mV. 4. Single channel conductance measurements on inside-out patches in physiological K+ solutions identified a 14 +/- 3 pS CA(2+)-activated K+ current between -25 mV and +50 mV. In symmetrical (100 mM) K+, the single channel conductance was 26 pS. 5. Externally applied quinine (IC50 = 0.12 +/- 0.34 mM) and tetraethylammonium chloride (IC50 = 10 +/- 1.9 mM) inhibited 86Rb+ influx into C6 cells in a concentration-dependent manner. Charybdotoxin (IC50 = 0.5 +/- 0.02 nM) and iberiotoxin (IC50 = 800 +/- 150 nM), as well as the crude venoms from the scorpions Leiurus quinquestriatus and Mesobuthus tamulus, also inhibited 86Rb+ influx. In contrast, apamin and toxin I had no inhibitory effects on 86Rb+ flux. A screen of fractions from cation exchange h.p.l.c. of Mesob. tamulus venom revealed the presence of at least four charybdotoxin-like peptides. One of these was iberiotoxin; the other three are novel toxins. 6. The ionomycin-activated 86Rb+ influx into rat C6 glioma cells has proved to be a valuable pharmacological assay for the screening of toxins and crude venoms which modify intermediate conductance, Ca2+ activated K+ channel activity.

The mode of action of SA-FF22, a lantibiotic isolated from Streptococcus pyogenes strain FF22.

SA-FF22 is a lanthionine-containing bacteriocidal peptide isolated from Streptococcus pyogenes strain FF22. The peptide interacts closely with non-energised artificial phospholipid vesicles, as evidenced by a 'blue shift' in the fluorescent emissions associated with a tryptophan residue within the peptide sequence. Furthermore, SA-FF22 induced efflux of radiolabelled amino acids from artificially energised cytoplasmic membrane vesicles and arrested uptake of amino acids by intact cells. By measuring the decrease in membrane potential of both starved and energised SA-FF22-treated cells, and through the use of artificial planar membranes, a potential of approximately 100 mV was deduced as the minimum required to induce pore formation by SA-FF22. This threshold potential is independent of the orientation of the applied voltage (i.e. trans or cis orientations are equally effective). Single channel conductance measurements suggested that the pores formed by SA-FF22 are relatively unstable, short-lived and approximately 0.5-0.6 nm in diameter. This is somewhat smaller than those of the previously described, pore-forming lantibiotics and should not allow significant efflux of large molecules such as ATP. Thus, death of affected cells seems to result from membrane-potential disruption and subsequent exhaustion of the cells.

Biochemical characterization and crystallization of porin from Rhodopseudomonas blastica

Oligomeric porin of the phototrophic bacterium Rhodopseudomonas blastica DSM 2131 was obtained from cell envelopes by differential temperature extraction in the presence of detergent and salt. The isolated porin exhibited strong porin activity after reconstitution into lipid bilayer membranes. The effective channel diameter for the trimer was estimated as 1.5 nm from single channel conductance measurements in the presence of 1 M KCl. Moderate cation-selectivity was observed. Oligomeric porin migrated as a single band (apparent molecular weight 81 kDa) on sodium dodecyl sulfate polyacrylamide gelelectrophoresis when solubilized below 70 °C. The oligomers were converted into monomers on heating to 70 °C or above forming two bands with apparent molecular weight of 36 kDa and 35 kDa. The oligomer was not sensitive to EDTA. Its molecular weight was determined to be 119.3 kDa by analytical ultracentrifugation. The isoelectric point was 5.7. Circular dichroism data indicated a high content of β-sheet structure. Gasphase sequencing of the N-terminal residues revealed the sequence: NH2-Glu-Ile-Ser-Leu-Asn-Gly-Tyr-Gly-Arg-Phe. Crystals with a maximal side length of 300 μm and diffracting to 0.32 nm resolution were obtained with the porin oligomer in the presence of C8E4 and 1,2,3-heptanetriol by using the vapor phase equilibration technique.

Temperature Dependence of Electron Mobility in Si Inversion Layers

Temperature dependence of electron mobility in Si inversion layers is investigated using channel conductance measurements in the temperature range from 15 to 300 K. The electron mobility is analyzed by using the theoretical model in which two-dimensional electron gas confined in the inversion layer interacts with acoustic phonons, intervalley phonons, surface roughness, and ionized impurities. At higher temperatures above 100 K, the electron mobility is dominated by acoustic phonon scattering and intervalley phonon scattering. At temperatures below 100 K, ionized impurity scattering plays an important role in determining the mobility in the case of low electron sheet densities, whereas in the case of higher electron sheet densities, surface roughness scattering becomes important.