Showing papers by "Braunschweig University of Technology published in 2009"

Molybdenum cofactors, enzymes and pathways

Abstract: The trace element molybdenum is essential for nearly all organisms and forms the catalytic centre of a large variety of enzymes such as nitrogenase, nitrate reductases, sulphite oxidase and xanthine oxidoreductases. Nature has developed two scaffolds holding molybdenum in place, the iron-molybdenum cofactor and pterin-based molybdenum cofactors. Despite the different structures and functions of molybdenum-dependent enzymes, there are important similarities, which we highlight here. The biosynthetic pathways leading to both types of cofactor have common mechanistic aspects relating to scaffold formation, metal activation and cofactor insertion into apoenzymes, and have served as an evolutionary 'toolbox' to mediate additional cellular functions in eukaryotic metabolism.

Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films

Abstract: The electronic structures of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of MoO3 are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of MoO3 in hole injection in organic devices. The MoO3 films are strongly n-type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of MoO3.

Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers.

Abstract: Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal?organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour?liquid?solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro-?and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I?V characteristics of ZnO:P nanowire/ZnO:Ga p?n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

THEMIS observations of an earthward-propagating dipolarization front

Abstract: [1] We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = -20.1 R E (THEMIS P1 probe), at X = -16.7 R E (P2), and at X = -11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.

Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus

Abstract: Images from the Cassini spacecraft showed erupting plumes of water vapour and ice particles on Saturn's moon Enceladus, prompting speculation a subsurface ocean might be acting as a source of liquid water. Two groups this week report evidence relevant to the search for this subsurface ocean. The results, at first sight contradictory, leave the ocean a possibility, though still a hypothetical one. Postberg et al. used the Cassini Cosmic Dust Analyser to determine the chemical composition of ice grains in Saturn's E-ring, which consists largely of material from Enceladus. They find a population of E-ring grains rich in sodium salts, which should be possible only if the plumes originate from liquid water. Schneider et al. used Earth-based spectroscopic telescopes to search for sodium emission in the gas plumes erupting from Enceladus and found none. This is inconsistent with a direct supply from a salty ocean and suggests alternative eruption sources such as a deep ocean, a freshwater reservoir or ice. Or if there is a salty reservoir of water, some process not yet determined must be preventing the sodium from escaping into space. Saturn's moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole, raising the possibility of a subsurface ocean. Minor organic or siliceous components, identified in many ice grains, could be evidence of interaction between Enceladus' rocky core and liquid water; however it has been unclear whether the water is still present today or if it has frozen. Now, the identification of a population of E-ring grains that are rich in sodium salts suggests that the plumes originate from liquid water. Saturn's moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole1,2,3,4,5, suggesting the possibility of a subsurface ocean5,6,7. These plume particles are the dominant source of Saturn’s E ring7,8. A previous in situ analysis9 of these particles concluded that the minor organic or siliceous components, identified in many ice grains, could be evidence for interaction between Enceladus’ rocky core and liquid water9,10. It was not clear, however, whether the liquid is still present today or whether it has frozen. Here we report the identification of a population of E-ring grains that are rich in sodium salts (∼0.5–2% by mass), which can arise only if the plumes originate from liquid water. The abundance of various salt components in these particles, as well as the inferred basic pH, exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core11. The plume vapour is expected to be free of atomic sodium. Thus, the absence of sodium from optical spectra12 is in good agreement with our results. In the E ring the upper limit for spectroscopy12 is insufficiently sensitive to detect the concentrations we found.

BRENDA, AMENDA and FRENDA the enzyme information system: new content and tools in 2009

Abstract: The BRENDA (BRaunschweig ENzyme DAtabase) (http://www.brenda-enzymes.org) represents the largest freely available information system containing a huge amount of biochemical and molecular information on all classified enzymes as well as software tools for querying the database and calculating molecular properties. The database covers information on classification and nomenclature, reaction and specificity, functional parameters, occurrence, enzyme structure and stability, mutants and enzyme engineering, preparation and isolation, the application of enzymes, and ligand-related data. The data in BRENDA are manually curated from more than 79 000 primary literature references. Each entry is clearly linked to a literature reference, the origin organism and, where available, to the protein sequence of the enzyme protein. A new search option provides the access to protein-specific data. FRENDA (Full Reference ENzyme DAta) and AMENDA (Automatic Mining of ENzyme DAta) are additional databases created by continuously improved text-mining procedures. These databases ought to provide a complete survey on enzyme data of the literature collection of PubMed. The web service via a SOAP (Simple Object Access Protocol) interface for access to the BRENDA data has been further enhanced.

Reprogramming of murine fibroblasts to induced pluripotent stem cells with chemical complementation of Klf4

Abstract: Ectopic expression of defined transcription factors can reprogram somatic cells to induced pluripotent stem (iPS) cells, but the utility of iPS cells is hampered by the use of viral delivery systems. Small molecules offer an alternative to replace virally transduced transcription factors with chemical signaling cues responsible for reprogramming. In this report we describe a small-molecule screening platform applied to identify compounds that functionally replace the reprogramming factor Klf4. A series of small-molecule scaffolds were identified that activate Nanog expression in mouse fibroblasts transduced with a subset of reprogramming factors lacking Klf4. Application of one such molecule, kenpaullone, in lieu of Klf4 gave rise to iPS cells that are indistinguishable from murine embryonic stem cells. This experimental platform can be used to screen large chemical libraries in search of novel compounds to replace the reprogramming factors that induce pluripotency. Ultimately, such compounds may provide mechanistic insight into the reprogramming process.

MetaboliteDetector: comprehensive analysis tool for targeted and nontargeted GC/MS based metabolome analysis

Abstract: We have developed a new software, MetaboliteDetector, for the efficient and automatic analysis of GC/MS-based metabolomics data. Starting with raw MS data, the program detects and subsequently identifies potential metabolites. Moreover, a comparative analysis of a large number of chromatograms can be performed in either a targeted or nontargeted approach. MetaboliteDetector automatically determines appropriate quantification ions and performs an integration of single ion peaks. The analysis results can directly be visualized with a principal component analysis. Since the manual input is limited to absolutely necessary parameters, the program is also usable for the analysis of high-throughput data. However, the intuitive graphical user interface of MetaboliteDetector additionally allows for a detailed examination of a single GC/MS chromatogram including single ion chromatograms, recorded mass spectra, and identified metabolite spectra in combination with the corresponding reference spectra obtained from a ...

The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments

Abstract: The growth processes from protoplanetary dust to planetesimals are not fully understood. Laboratory experiments and theoretical models have shown that collisions among the dust aggregates can lead to sticking, bouncing, and fragmentation. However, no systematic study on the collisional outcome of protoplanetary dust has been performed so far so that a physical model of the dust evolution in protoplanetary disks is still missing. We intend to map the parameter space for the collisional interaction of arbitrarily porous dust aggregates. This parameter space encompasses the dust-aggregate masses, their porosities and the collision velocity. With such a complete mapping of the collisional outcomes of protoplanetary dust aggregates, it will be possible to follow the collisional evolution of dust in a protoplanetary disk environment. We use literature data, perform own laboratory experiments, and apply simple physical models to get a complete picture of the collisional interaction of protoplanetary dust aggregates. In our study, we found four different types of sticking, two types of bouncing, and three types of fragmentation as possible outcomes in collisions among protoplanetary dust aggregates. We distinguish between eight combinations of porosity and mass ratio. For each of these cases, we present a complete collision model for dust-aggregate masses between 10^-12 and 10^2 g and collision velocities in the range 10^-4 to 10^4 cm/s for arbitrary porosities. This model comprises the collisional outcome, the mass(es) of the resulting aggregate(s) and their porosities. We present the first complete collision model for protoplanetary dust. This collision model can be used for the determination of the dust-growth rate in protoplanetary disks.

Madagascar as a model region of species diversification.

Abstract: Tropical biotas provide excellent settings in which to explore mechanisms of evolutionary diversification, yet these processes remain poorly understood. Pioneering work on biodiversity patterns and diversification processes in other tropical regions has recently been complemented by studies in Madagascar. Here we review diversity models and diversification mechanisms proposed for the fauna of this island and the perspectives for testing them. Madagascar has a diverse biota that has evolved in isolation, and is characterised by regionally pronounced and locally steep environmental gradients, common patterns of microendemism across taxa and numerous evolutionary radiations. These characteristics establish Madagascar as a promising system for the study of pattern and process in species diversification.

Global distribution of whistler‐mode chorus waves observed on the THEMIS spacecraft

Abstract: [1] Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Citation: Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

Al2O3/ZrO2 Nanolaminates as Ultrahigh Gas-Diffusion Barriers—A Strategy for Reliable Encapsulation of Organic Electronics

Abstract: Adv Mater 2009, 21, 1845–1849 2009 WILEY-VCH Verlag G Gas-diffusion barriers play an important role in many applications today, such as flat-panel displays or solar cells grown on plastic substrates In particular, organic light-emitting diodes (OLEDs) need efficient encapsulation because of the high sensitivity of many organic or electrode materials to moisture and oxygen, which has been demonstrated to cause device degradation and limited lifetime Maximum allowable permeation rates for a viable encapsulant to be used in organic optoelectronics are still under debate, but it is common sense that the requirements are far more demanding than those for typical barrier films used in food or pharmacy packaging A very commonly quoted figure for the upper limit of the water-vaportransmission rate (WVTR) in order to reach a minimum OLED lifetime of 10000 h is 10 6 gm 2 day This value originated from an estimate of the amount of water needed to degrade the reactive cathode material For the oxygen-transmission rate (OTR), maximum values for similar OLED lifetimes have been reported in the region of 10 –10 3 cmm 2 day Nowadays, a very common technique is the encapsulation of organic devices with a glass or metal lid, which is applied under nitrogen atmosphere using, for example, an epoxy resin as glue In addition, the residual moisture in the cavity between the organic device and lid is further minimized by an opaque getter material For large-area, flexible, or transparent applications, glass-lid encapsulation is not suitable A promising alternative is thin-film barriers of metal oxides or nitrides, such as Al2O3, SiO2, TiO2, or SiN These materials can be deposited by several techniques Plasma-assisted processes, such as sputtering or plasma-enhanced chemical vapor deposition (PECVD), provide high quality films, but their typically reported OTR and WVTR values of about 05 cmm 2 day 1 and 03 gm 2 day , respectively, limited by imperfections in the films, are not sufficient for OLED applications Moreover, efficient step coverage and conformal coating are not straightforward with these techniques As a workaround, hybrid organic/inorganic multilayer structures have been introduced In these, the polymeric intermediate layer levels the inorganic layer so that diffusion pathways through the entire stack of the barrier can be reduced However, the combination of alternating nonvacuum-based polymer deposition processes and vacuum-based deposition processes is timeconsuming and costly, and therefore not really practical for production An alternative technique that promises highly uniform thin-film coatings is atomic-layer deposition (ALD) ALD relies on the sequential exposure of the surface to be coated to a metal–organic precursor and a reactant (H2O, O3, NH3, etc) The sequential dosing of precursors leads to a self-limiting process with concomitant precise control over thickness and homogeneity The technique is well-established in the processing of insulating films for gate dielectrics and capacitors ALD has the advantage of allowing the deposition of very dense films at low temperatures (<100 8C), and thus appears to be a promising technique suitable for preparing encapsulation layers on top of organic electronic devices There are a few reports dealing with ALD-grown thin films for OLED encapsulation Very-low permeation rates of 65 10 5 gm 2 day 1 at 60 8C have been reported for Al2O3 films that were grown at 120 8C [13] However, deposition temperatures above 100 8C might be critical, because glass-transition temperatures for many functional OLED materials have been reported in this very temperature range In this Communication, we report on thin encapsulation layers prepared by ALD at a temperature of 80 8C Specifically, we study neat Al2O3 films as a reference, and introduce novel highly efficient permeation barriers based on so-called nanolaminate structures A nanolaminate, in this case, is a composite film consisting of repetitively deposited ultrathin (a few nanometers thick) alternating layers of Al2O3 and ZrO2 The alternating nanolayer structure suppresses the formation of both microscopic voids and nanocrystals As a result, the probability of the occurrence of statistical defects in the barrier structure related to permeation paths along connected voids or grain boundaries is significantly reducedWe demonstrate that nanolaminates render pinhole-free thin-film encapsulation layers suitable for large-area organic devices possible First, the quantitative assessment of permeation rates on the order of 10 6 gm 2 day 1 for water and 10 3 cmm 2 day 1 for oxygen is very challenging and, as yet, no commercially available method can be applied in this range Therefore, we used a very sensitive permeation-measurement technique that was introduced by Paetzold et al The method allows the measurement

Maximum unbiased validation (MUV) data sets for virtual screening based on PubChem bioactivity data.

Abstract: Refined nearest neighbor analysis was recently introduced for the analysis of virtual screening benchmark data sets. It constitutes a technique from the field of spatial statistics and provides a mathematical framework for the nonparametric analysis of mapped point patterns. Here, refined nearest neighbor analysis is used to design benchmark data sets for virtual screening based on PubChem bioactivity data. A workflow is devised that purges data sets of compounds active against pharmaceutically relevant targets from unselective hits. Topological optimization using experimental design strategies monitored by refined nearest neighbor analysis functions is applied to generate corresponding data sets of actives and decoys that are unbiased with regard to analogue bias and artificial enrichment. These data sets provide a tool for Maximum Unbiased Validation (MUV) of virtual screening methods. The data sets and a software package implementing the MUV design workflow are freely available at http://www.pharmchem....

The electronic phase diagram of the LaO 1− x F x FeAs superconductor

Abstract: The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mossbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

On the regulation of photosynthesis by excitonic interactions between carotenoids and chlorophylls

Abstract: Selective 2-photon excitation (TPE) of carotenoid dark states, Car S1, shows that in the major light-harvesting complex of photosystem II (LHCII), the extent of electronic interactions between carotenoid dark states (Car S1) and chlorophyll (Chl) states, φCouplingCar S1−Chl, correlates linearly with chlorophyll fluorescence quenching under different experimental conditions. Simultaneously, a linear correlation between both Chl fluorescence quenching and φCouplingCar S1−Chl with the intensity of red-shifted bands in the Chl Qy and carotenoid absorption was also observed. These results suggest quenching excitonic Car S1−Chl states as origin for the observed effects. Furthermore, real time measurements of the light-dependent down- and up-regulation of the photosynthetic activity and φCouplingCar S1−Chl in wild-type and mutant (npq1, npq2, npq4, lut2 and WT+PsbS) Arabidopsis thaliana plants reveal that also in vivo the quenching parameter NPQ correlates always linearly with the extent of electronic Car S1–Chl interactions in any adaptation status. Our in vivo measurements with Arabidopsis variants show that during high light illumination, φCouplingCar S1−Chl depends on the presence of PsbS and zeaxanthin (Zea) in an almost identical way as NPQ. In summary, these results provide clear evidence for a very close link between electronic Car S1–Chl interactions and the regulation of photosynthesis. These findings support a photophysical mechanism in which short-living, low excitonic carotenoid–chlorophyll states serve as traps and dissipation valves for excess excitation energy.

Most quantum States are too entangled to be useful as computational resources.

Abstract: It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs a sufficient amount of entanglement for this speedup to be manifest. In measurement-based quantum computing, the need for a highly entangled initial state is particularly obvious. Defying this intuition, we show that quantum states can be too entangled to be useful for the purpose of computation, in that high values of the geometric measure of entanglement preclude states from offering a universal quantum computational speedup. We prove that this phenomenon occurs for a dramatic majority of all states: the fraction of useful $n$-qubit pure states is less than $\mathrm{exp} (\ensuremath{-}{n}^{2})$. This work highlights a new aspect of the role entanglement plays for quantum computational speedups.

Implementation of a Lattice Boltzmann kernel using the Compute Unified Device Architecture developed by nVIDIA

Abstract: In this article a very efficient implementation of a 2D-Lattice Boltzmann kernel using the Compute Unified Device Architecture (CUDA™) interface developed by nVIDIA® is presented. By exploiting the explicit parallelism exposed in the graphics hardware we obtain more than one order in performance gain compared to standard CPUs. A non-trivial example, the flow through a generic porous medium, shows the performance of the implementation.

Process chain simulation to foster energy efficiency in manufacturing

Abstract: Due to the significant ecological relevance and constantly rising prices, energy consumption more and more gets into the focus of manufacturing companies which strive to consciously consider energy consumption when planning and managing production facilities. Thereby it is important to take into account the interdependencies of all technical processes (production and technical building services). Against this background this paper presents an integrated concept to foster energy efficiency in manufacturing companies on different layers. Important part of the concept is an appropriate simulation approach. The approach enables to derive and evaluate technical as well as organizational measures to increase energy efficiency with respect to both ecological and economic objectives.

OpenFLUX: efficient modelling software for 13C-based metabolic flux analysis.

Abstract: The quantitative analysis of metabolic fluxes, i.e., in vivo activities of intracellular enzymes and pathways, provides key information on biological systems in systems biology and metabolic engineering. It is based on a comprehensive approach combining (i) tracer cultivation on 13C substrates, (ii) 13C labelling analysis by mass spectrometry and (iii) mathematical modelling for experimental design, data processing, flux calculation and statistics. Whereas the cultivation and the analytical part is fairly advanced, a lack of appropriate modelling software solutions for all modelling aspects in flux studies is limiting the application of metabolic flux analysis. We have developed OpenFLUX as a user friendly, yet flexible software application for small and large scale 13C metabolic flux analysis. The application is based on the new Elementary Metabolite Unit (EMU) framework, significantly enhancing computation speed for flux calculation. From simple notation of metabolic reaction networks defined in a spreadsheet, the OpenFLUX parser automatically generates MATLAB-readable metabolite and isotopomer balances, thus strongly facilitating model creation. The model can be used to perform experimental design, parameter estimation and sensitivity analysis either using the built-in gradient-based search or Monte Carlo algorithms or in user-defined algorithms. Exemplified for a microbial flux study with 71 reactions, 8 free flux parameters and mass isotopomer distribution of 10 metabolites, OpenFLUX allowed to automatically compile the EMU-based model from an Excel file containing metabolic reactions and carbon transfer mechanisms, showing it's user-friendliness. It reliably reproduced the published data and optimum flux distributions for the network under study were found quickly (<20 sec). We have developed a fast, accurate application to perform steady-state 13C metabolic flux analysis. OpenFLUX will strongly facilitate and enhance the design, calculation and interpretation of metabolic flux studies. By providing the software open source, we hope it will evolve with the rapidly growing field of fluxomics.

Dynamics of laminar separation bubbles at low-Reynolds-number aerofoils

Abstract: The laminar separation bubble on an SD7003 aerofoil at a Reynolds number Re = 66000 was investigated to determine the dominant frequencies of the transition process and the flapping of the bubble. The measurements were performed with a high-resolution time-resolved particle image velocimetry (TR-PIV) system. Contrary to typical measurements performed through conventional PIV, the different modes can be identified by applying TR-PIV. The interaction between the shed vortices is analysed, and their significance for the production of turbulence is presented. In the shear layer above the bubble the generation and amplification of vortices due to Kelvin–Helmholtz instabilities is observed. It is found that these instabilities have a weak coherence in the spanwise direction. In a later stage of transition these vortices lead to a three-dimensional breakdown to turbulence.

An improved Escherichia coli donor strain for diparental mating

Abstract: We present a new method for diparental mating with the outstanding advantage that counterselection of the Escherichia coli donor strain is not required. This improved method uses a new donor strain, E. coli ST18, a hemA deletion mutant defective in tetrapyrrole biosynthesis. The hemA mutation can be complemented by addition of 5-aminolevulinic acid. Therefore, counterselection is carried out only using standard media and growth conditions optimal for the recipient strain. Consequently, recipient strains are isolated in a significantly shorter period.

Selectivity versus mobility: separation of anode and cathode in microbial bioelectrochemical systems.

Abstract: During the operation of a microbial bioelectrochemical system, charge balance must be maintained between the anode and the cathode. In an ideal scenario, the charge balance would be realized by the unhindered migration of H(+) or OH(-). At the same time, any kind of diffusion (crossover) between both electrode compartments should be avoided. However, as several studies have demonstrated, the experimental reality does not match this ideal picture. Crossover processes occur and H(+)/OH(-) migration only plays an inferior role in the charge-balancing ion transfer, which results in significant losses in the performance of the microbial bioelectrochemical system. This Minireview summarizes the conflict of selectivity versus mobility and discusses principle strategies to cope with the resulting constraints, including pH-static operation and the use of different separator materials and membrane-free systems. Finally, we show that every setup compromises either selectivity or mobility, and no apparent ideal solution currently exists.