Probing Membrane Protein Insertion into Lipid Bilayers by Solid-State NMR

Determination of the environment surrounding a protein is often key to understanding its function and can also be used to infer the structural properties of the protein. By using proton-detected solid-state NMR, we show that reduced spin diffusion within the protein under conditions of fast magic-angle spinning, high magnetic field, and sample deuteration allows the efficient measurement of site-specific exposure to mobile water and lipids. We demonstrate this site specificity on two membrane proteins, the human voltage dependent anion channel, and the alkane transporter AlkL from Pseudomonas putida. Transfer from lipids is observed selectively in the membrane spanning region, and an average lipid-protein transfer rate of 6 s⁻¹ was determined for residues protected from exchange. Transfer within the protein, as tracked in the ¹⁵N-¹H 2D plane, was estimated from initial rates and found to be in a similar range of about 8 to 15 s⁻¹ for several resolved residues, explaining the site specificity.     

Cooperative Self‐Assembly of Pyridine‐2,6‐Diimine‐Linked Macrocycles into Mechanically Robust Nanotubes

Nanotubes assembled from macrocyclic precursors offer a unique combination of low dimensionality, structural rigidity, and distinct interior and exterior microenvironments. Usually the weak stacking energies of macrocycles limit the length and mechanical strength of the resultant nanotubes. Imine‐linked macrocycles were recently found to assemble into high‐aspect ratio (>10³), lyotropic nanotubes in the presence of excess acid. Yet these harsh conditions are incompatible with many functional groups and processing methods, and lower acid loadings instead catalyze macrocycle degradation. Here we report pyridine‐2,6‐diimine‐linked macrocycles that assemble into high‐aspect ratio nanotubes in the presence of less than 1 equiv of CF₃CO₂H per macrocycle. Analysis by gel permeation chromatography and fluorescence spectroscopy revealed a cooperative self‐assembly mechanism. The low acid concentrations needed to induce assembly enabled nanofibers to be obtained by touch‐spinning, which exhibit higher Young's moduli (1.33 GPa) than many synthetic polymers and biological filaments. These findings represent a breakthrough in the design of inverse chromonic liquid crystals, as assembly under such mild conditions will enable the design of structurally diverse and mechanically robust nanotubes from synthetically accessible macrocycles.     

Viability of Using Glycerin as a Co-substrate in Anaerobic Digestion of Sugarcane Stillage (Vinasse): Effect of Diversified Operational Strategies

Vinasse, from sugar and ethanol production, stands out as one of the most problematic agroindustry wastes due to its high chemical oxygen demand, large production volume, and recalcitrant compounds. Therefore, the viability of using glycerin as a co-substrate in vinasse anaerobic digestion was tested, to increase process efficiency and biogas productivity. The effect of feeding strategy, influent concentration, cycle length, and temperature were assessed to optimize methane production. Glycerin (1.53% v/v) proved to be a good co-substrate since it increased the overall methane production in co-digestion assays. CH₄ productivity enhanced exponentially as influent concentration increased, but when temperature was increased to 35 °C, biogas production was impaired. The highest methane productivity and yield were achieved using fed-batch mode, at 30 °C and at an organic loading rate of 10.1 kg COD m⁻³ day⁻¹: 139.32 mol CH₄ m⁻³ day⁻¹, 13.86 mol CH₄ kg COD_applied, and 15.30 mol CH₄ kg COD_removed. Methane was predominantly produced through the hydrogenotrophic route. In order to treat all the vinasse produced by a mid-size sugar and ethanol plant, nine reactors with 7263.4 m³ each would be needed. The energy generated by burning the biogas in boilers would reach approximately 92,000 MW h per season and could save up to US$ 240,000.00 per month in diesel oil demand.

     

Photocatalytic Hydrogen Generation by Vesicle-Embedded [FeFe]Hydrogenase Mimics: A Mechanistic Study

Artificial photosynthesis—the direct photochemical generation of hydrogen from water—is a promising but scientifically challenging future technology. Because nature employs membranes for photodriven reactions, the aim of this work is to elucidate the effect of membranes on artificial photocatalysis. To do so, a combination of electrochemistry, photocatalysis, and time-resolved spectroscopy on vesicle-embedded [FeFe]hydrogenase mimics, driven by a ruthenium tris-2,2′-bipyridine photosensitizer, is reported. The membrane effects encountered can be summarized as follows: the presence of vesicles steers the reactivity of the [FeFe]-benzodithiolate catalyst towards disproportionation, instead of protonation, due to membrane characteristics, such as providing a constant local effective pH, and concentrating and organizing species inside the membrane. The maximum turnover number is limited by photodegradation of the resting state in the catalytic cycle. Understanding these fundamental productive and destructive pathways in complex photochemical systems allows progress towards the development of efficient artificial leaves.     

Large Eddy Simulation of a Novel Gas-Assisted Coal Combustion Chamber

In this work a recently presented combustion chamber that is specifically designed for the investigation of gas-assisted coal combustion and the validation of models is simulated under reactive conditions for the first time. In the configuration coal combustion is assisted and stabilized by a methane flame. In the course of the investigation, the configuration’s complexity is increased successively. Results of the isothermal single-phase flow are discussed first. Subsequently, reproducibility of the single-phase methane flame by means of the applied modeling approach is evaluated. In a further step, coal particles having the same thermal power as the methane flame are injected into the configuration. Particle histories, the conversion of the coal particles as well as its retroactive effect on the gas phase are investigated. Experimental results based on laser diagnostics are provided for all operating points and used for comparison with numerical results. Gas phase velocity fields for all operating points are available. In order to identify the reaction in the reactive single-phase case planar laser induced fluorescence of the OH-radical (OH-PLIF) was applied. Overall good agreement between numerical and experimental results could be obtained. In the Large Eddy Simulation (LES) a Flamelet Generated Manifold (FGM) based model is utilized. The four-dimensional manifold is spanned by two mixture fractions, a reaction progress variable and the enthalpy on which the gas phase chemistry gets mapped onto. Thereby, the model accounts for both, volatiles reaction and char conversion. Furthermore, finite rate chemistry effects as well as non-adiabatic physics are considered.     

A gauge-covariant derivation of the strong-field approximation

A brief discussion of gauge invariance in nonrelativistic quantum mechanics is presented. The strong-field approximation for ionization of an atom by a laser field is rederived in such a way that the resulting ionization amplitude comes out identical in any gauge. This result agrees with the usual length-gauge form.     

Structure of neutron-rich nuclei around the N = 126 closed shell; the yrast structure of 205Au126 up to spin-parity I^{\pi} = (19/2+)

Heavy neutron-rich nuclei have been populated through the relativistic fragmentation of a $\ensuremath ^{208}_{\ 82}{\rm Pb}$ beam at $\ensuremath E/A = 1$ GeV on a $\ensuremath 2.5 {\rm g/cm^2}$ thick Be target. The synthesised nuclei were selected and identified in-flight using the fragment separator at GSI. Approximately 300 ns after production, the selected nuclei were implanted in an $\ensuremath \sim 8$ mm thick perspex stopper, positioned at the centre of the RISING $\ensuremath \gamma$ -ray detector spectrometer array. A previously unreported isomer with a half-life $\ensuremath T_{1/2} = 163(5)$ ns has been observed in the N = 126 closed-shell nucleus $\ensuremath ^{205}_{\ 79}{\rm Au}$ . Through $ \gamma$ -ray singles and $ \gamma$ - $ \gamma$ coincidence analysis a level scheme was established. The comparison with a shell model calculation tentatively identifies the spin-parity of the excited states, including the isomer itself, which is found to be $\ensuremath I^{\pi} = (19/2^+)$ .     

Scaled boundary finite element analysis of stress singularities in piezoelectric multi-material systems

The scaled boundary finite element method (SBFEM) in an extension for piezoelectric materials is used to analyze twoand three-dimensional stress singularities in piezoelectric multi-material systems. It is found to be an efficient tool for the analysis of singularity orders of such situations, that turn out to be rather complex. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

How cold is cold dark matter? Small-scales constraints from the flux power spectrum of the high-redshift lyman-alpha forest

We present constraints on the mass of warm dark matter (WDM) particles derived from the Lyman-alpha flux power spectrum of 55 high-resolution HIRES spectra at 2.0 or approximately 1.2 keV (2sigma) if the WDM consists of early decoupled thermal relics and m(WDM) > or approximately 5.6 keV (2sigma) for sterile neutrinos. Adding the Sloan Digital Sky Survey Lyman-alpha flux power spectrum, we get m(WDM) > or approximately 4 keV and m(WDM) > or approximately 28 keV (2sigma) for thermal relics and sterile neutrinos. These results improve previous constraints by a factor of 2.