Bonding,aromaticity, and planar tetracoordinated carbon in Si<Subscript>2</Subscript>CH<Subscript>2</Subscript> and Ge<Subscript>2</Subscript>CH<Subscript>2</Subscript>

Natural bond orbital (NBO) analyses and dissected nucleus-independent chemical shifts (NICS _{π z z} ) were computed to evaluate the bonding (bond type, electron occupation, hybridization) and aromatic character of the three lowest-lying Si₂CH₂ (1-Si, 2-Si, 3-Si) and Ge₂CH₂ (1-Ge, 2-Ge, 3-Ge) isomers. While their carbon C₃H₂ analogs favor classical alkene, allene, and alkyne type bonding, these Si and Ge derivatives are more polarizable and can favor “highly electron delocalized”? and “non-classical”? structures. The lowest energy Si ₂CH₂ and Ge ₂CH₂ isomers, 1-Si and 1-Ge, exhibit two sets of 3–center 2–electron (3c-2e) bonding; a π-3c-2e bond involving the heavy atoms (C–Si–Si and C–Ge–Ge), and a σ-3c-2e bond (Si–H–Si, Ge–H–Ge). Both 3-Si and 3-Ge exhibit π and σ-3c-2e bonding involving a planar tetracoordinated carbon (ptC) center. Despite their highly electron delocalized nature, all of the Si₂CH₂ and Ge₂CH₂ isomers considered display only modest two π electron aromatic character (NICS(0) _{π z z} =--6.2 to –8.9 ppm, computed at the heavy atom ring center) compared to the cyclic-C ₃H₂ (–13.3 ppm).

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Graphical Abstract The three lowest Si2CH2 and Ge2CH2 isomers.

     

Biocatalysts for fuel cells: efficient hydrogenase orientation for H<Subscript>2</Subscript> oxidation at electrodes modified with carbon nanotubes

We report the modification of gold and graphite electrodes with commercially available carbon nanotubes for immobilization of Desulfovibrio fructosovorans [NiFe] hydrogenase, for hydrogen evolution or consumption. Multiwalled carbon nanotubes, single-walled carbon nanotubes (SWCNs), and amine-modified and carboxyl-functionalized SWCNs were used and compared throughout. Two separate methods were performed: covalent attachment of oriented hydrogenase by controlled architecture of carbon nanotubes at gold electrodes, and adsorption of hydrogenase at carbon-nanotube-coated pyrolytic graphite electrodes. In the case of self-assembled carbon nanotubes at gold electrodes, hydrogenase orientation based on electrostatic interaction with the electrode surface was found to control the electrocatalytic process for H₂ oxidation. In the case of carbon nanotube coatings on pyrolytic graphite electrodes, catalysis was controlled more by the geometry of the nanotubes than by the orientation of the enzyme. Noticeably, shortened SWCNs were demonstrated to allow direct electron transfer and generate high and quite stable current densities for H₂ oxidation via adsorbed hydrogenase, despite having many carboxylic surface functions that could yield unfavorable hydrogenase orientation for direct electron transfer. This result is attributable to the high degree of oxygenated surface functions in addition to the length of shortened SWCNs that yields highly divided materials. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Room Temperature NO<Subscript>2</Subscript> Gas Sensor Based on SnO<Subscript>2</Subscript>-WO<Subscript>3</Subscript> Thin Films

Metal oxide semiconductors (MOS) are important and promising materials in optoelectronics, and it has been widely used in various catalytic applications such as gas sensing due to its high reactivity with many gases. In current work, mixtures of SnO₂-WO₃ (1:1) were prepared to synthesize nanostructured thin films by pulsed laser deposition as gas sensors. The sensitivity of sensors was measured for a relatively low concentration (200 ppm) of NO₂ gas at room temperature; sensors prepared with target exposed to (200) laser shots have higher sensitivity with a maximum value of 96.49 % at time 65 s as compared with the sensors prepared with (150) laser shots where the sensitivity has a maximum value 71.82 % at time 110 s; XRD pattern shows a better crystalline and high intensity with increasing laser shots up to 200; scanning electron microscopy (SEM) micrographs show approximate homogeneity of grains that cover the substrate without cracks and pinholes with nanoparticles fall in micro and nanometer range 50–200 nm. The values of the direct band gap were found to be 2.07143 eV for films prepared with 150 laser shots and 2.02899 eV for films prepared with 200 laser shots which have higher absorbance than the former films due to the increment in thickness and particle size. Empirical equations between sensitivity and gas exposure time have been formulated with great coincidence with the experimental data.     

Effect of fullerenes C<Subscript>60</Subscript> on X-protein amyloids

The inhibitory effect of hydrated fullerene C₆₀ and the sodium salt of the fullerene polycarboxylic derivative C₆₀Cl(C₆H₄CH₂COONa)₅ on the formation of amyloid fibrils by X-protein in vitro has been studied by electron microscopy. It is shown that these compounds not only destroy mature amyloid fibrils but also prevent the formation of new fibrils. This property of fullerenes, which are nanoparticles, can be used to develop a novel medical nanotechnology in the therapy for amyloidoses.     

Comparison of <Superscript>13</Superscript>CH<Subscript>3</Subscript>, <Superscript>13</Superscript>CH<Subscript>2</Subscript>D,and <Superscript>13</Superscript>CHD<Subscript>2</Subscript> methyl labeling strategies in proteins

A comparison of three labeling strategies for studies involving side chain methyl groups in high molecular weight proteins, using ¹³CH₃,¹³CH₂D, and ¹³CHD₂ methyl isotopomers, is presented. For each labeling scheme, ¹H–¹³C pulse sequences that give optimal resolution and sensitivity are identified. Three highly deuterated samples of a 723 residue enzyme, malate synthase G, with ¹³CH₃,¹³CH₂D, and ¹³CHD₂ labeling in Ile δ1 positions, are used to test the pulse sequences experimentally, and a rationalization of each sequence’s performance based on a product operator formalism that focuses on individual transitions is presented. The HMQC pulse sequence has previously been identified as a transverse relaxation optimized experiment for ¹³CH₃-labeled methyl groups attached to macromolecules, and a zero-quantum correlation pulse scheme (¹³CH₃ HZQC) has been developed to further improve resolution in the indirectly detected dimension. We present a modified version of the ¹³CH₃ HZQC sequence that provides improved sensitivity by using the steady-state magnetization of both ¹³C and ¹H spins. The HSQC and HMQC spectra of ¹³CH₂D-labeled methyl groups in malate synthase G are very poorly resolved, but we present a new pulse sequence, ¹³CH₂D TROSY, that exploits cross-correlation effects to record ¹H–¹³C correlation maps with dramatically reduced linewidths in both dimensions. Well-resolved spectra of ¹³CHD₂-labeled methyl groups can be recorded with HSQC or HMQC; a new ¹³CHD₂ HZQC sequence is described that provides improved resolution with no loss in sensitivity in the applications considered here. When spectra recorded on samples prepared with the three isotopomers are compared, it is clear that the ¹³CH₃ labeling strategy is the most beneficial from the perspective of sensitivity (gains ≥2.4 relative to either ¹³CH₂D or ¹³CHD₂ labeling), although excellent resolution can be obtained with any of the isotopomers using the pulse sequences presented here.     

The addition reactions of germylenoid H<Subscript>2</Subscript>GeAlCl<Subscript>3</Subscript> with ethylene: a theoretical investigation

Theoretical calculations using the M062X and QCISD methods were performed on the addition reactions of the aluminum germylenoid H₂GeAlCl₃ with ethylene. The most two stable structures of germylenoid H₂GeAlCl₃, i.e., the p-complex and three-membered ring structures, respectively, were employed as reactants. The calculated results indicate that, for the p-complex, H₂GeAlCl₃ there are two pathways, I and II, of which path I involves just one transition state, while path II involves two transition states between reactants and products. Comparing the reaction barrier heights of path I (44.6 kJ mol^?1) and II (37.6 kJ mol^?1), the two pathways are competitive, with similar barriers under the same conditions, while for the three-membered ring structure, another two pathways, III and IV, also exist. Path III has one transition state; however, in path IV, two transition states exist. By comparing their barrier heights, path III (barrier height 39.2 kJ mol^?1) could occur more easily than path IV (barrier height 92.8 kJ mol^?1). Considering solvent effects on these addition reactions, the PCM model and CH₂Cl₂ solvent were used in calculations, and the calculated results demonstrate that CH₂Cl₂ solvent is unfavorable for the reactions, except for path II. In CH₂Cl₂ solvent, paths II and III are more favorable than paths I and IV.     

Density functional theory study of C<Subscript>2</Subscript>F<Subscript>5</Subscript>I synthesis over activated carbon catalyst

Quantum chemistry calculations based on the density functional theory (DFT) are carried out to investigate the reaction mechanism of C₂F₅I synthesis catalyzed by activated carbon. The possible adsorption configurations of fluorocarbon intermediates are analyzed carefully. Also, the related transition states and reaction pathway are analyzed. According to calculation, firstly, the dehydrofluorination of C₂HF₅, as the rate-determining step, is catalyzed by the carboxyl acid groups. Secondly, the tetrafluoroethylidene radicals disproportionate on graphite (001) surface instead of rearrangement or dimerization. Next, the fluorine abstractions between fluorocarbon intermediates over graphite (001) surfaces proceed successfully. Finally, the desorbed pentafluoroethyl abstracts iodine atom from molecular iodine spontaneously to afford C₂F₅I. In adition, our calculations reveal that the carbon deposit in experiment is caused by the fluorine abstraction from fluoroethinyl. The suggested mechanism corresponds with our calculations and available experiments.     

Long-term effects of elevated atmospheric CO<Subscript>2</Subscript> on species composition and productivity of a southern African C<Subscript>4</Subscript> dominated grassland in the vicinity of a CO<Subscript>2</Subscript> exhalation

We describe the long-term effects of a CO₂ exhalation, created more than 70 years ago, on a natural C₄ dominated sub-tropical grassland in terms of ecosystem structure and functioning. We tested whether long-term CO₂ enrichment changes the competitive balance between plants with C₃ and C₄ photosynthetic pathways and how CO₂ enrichment has affected species composition, plant growth responses, leaf properties and soil nutrient, carbon and water dynamics. Long-term effects of elevated CO₂ on plant community composition and system processes in this sub-tropical grassland indicate very subtle changes in ecosystem functioning and no changes in species composition and dominance which could be ascribed to elevated CO₂ alone. Species compositional data and soil δ¹³C isotopic evidence suggest no detectable effect of CO₂ enrichment on C₃:C₄ plant mixtures and individual species dominance. Contrary to many general predictions C₃ grasses did not become more abundant and C₃ shrubs and trees did not invade the site. No season length stimulation of plant growth was found even after 5 years of exposure to CO₂ concentrations averaging 610 μmol mol⁻¹. Leaf properties such as total N decreased in the C₃ but not C₄ grass under elevated CO₂ while total non-structural carbohydrate accumulation was not affected. Elevated CO₂ possibly lead to increased end-of-season soil water contents and this result agrees with earlier studies despite the topographic water gradient being a confounding problem at our research site. Long-term CO₂ enrichment also had little effect on soil carbon storage with no detectable changes in soil organic matter found. There were indications that potential soil respiration and N mineralization rates could be higher in soils close to the CO₂ source. The conservative response of this grassland suggests that many of the reported effects of elevated CO₂ on similar ecosystems could be short duration experimental artefacts that disappear under long-term elevated CO₂ conditions.     

Insight into the gas phase dissociation of CF<Subscript>3</Subscript>CH<Subscript>2</Subscript>I and its reactions with H and OH by first principles

The Arrhenius kinetic parameters of dissociation reactions and reactions of CF₃CH₂I with radicals like H, O, and OH are determined using highly accurate first principles calculations. Thermophysical properties like molar heat capacity (C_p), thermal stability index, and the bond dissociation energies are also determined for the CF₃CH₂I molecule under the PBE/DNP formalism. Since, there are no theoretical study or experimental investigation reports available regarding the dissociation reactions of CF₃CH₂I and reactions of this molecule with the H and OH radical, a parallel comparative analysis is done with similar iodoalkanes to ascertain the precision of the results obtained. The atmospheric lifetime of 0.54 years is obtained for this molecule.     

Selective effects of H<Subscript>2</Subscript>O<Subscript>2</Subscript> on cyanobacterial photosynthesis

The sensitivity of phytoplankton species for hydrogen peroxide (H₂O₂) was analyzed by pulse amplitude modulated (PAM) fluorometry. The inhibition of photosynthesis was more severe in five tested cyanobacterial species than in three green algal species and one diatom species. Hence the inhibitory effect of H₂O₂ is especially pronounced for cyanobacteria. A specific damage of the photosynthetic apparatus was demonstrated by changes in 77 K fluorescence emission spectra. Different handling of oxidative stress and different cell structure are responsible for the different susceptibility to H₂O₂ between cyanobacteria and other phytoplankton species. This principle may be potentially employed in the development of new agents to combat cyanobacterial bloom formation in water reservoirs.     

Calculations of the C<Subscript>2</Subscript> fragmentation energies of higher fullerenes C<Subscript>80</Subscript> and C<Subscript>82</Subscript>

The C₂ fragmentation energies of the most stable isolated-pentagon-rule (IPR) isomers of the C₈₀ and C₈₂ fullerenes were evaluated with second-order Møller-Plesset (MP2) theory, density-functional theory (DFT) and the semiempirical self-consistent charge density-functional tight-binding (SCC-DFTB) method. Zero-point energy, ionization energy and empirical C₂ corrections were included in the calculation of fragmentation energies for comparison with experimental C₂ fragmentation energies of the fullerene cations. In the case of the most probable Stone-Wales pathway of C₂ fragmentation of C₈₀, the calculated \(D_{0} {\left( {{\text{C}}_{{{\text{80}}}} ^{ + } } \right)}\) agree well with experimental data, whereas in the case of C₈₂ fragmentation, the calculated \(D_{0} {\left( {{\text{C}}_{{{\text{82}}}} ^{ + } } \right)}\) exceed by up to 1.2 eV the experimental ones, which suggests that other IPR isomers may be present in sufficient amounts in experimental samples. Computer-intensive MP2 calculations and DFT calculations with larger basis sets do not yield much improved C₂ fragmentation energies, compared to those reported earlier with B3LYP/3-21G. On the other hand, semiempirical approaches such as SCC-DFTB, which are orders of magnitude less intensive, yield satisfactory fragmentation energies for higher fullerenes and may become a method of choice for routine calculations of fullerenes and carbon nanotubes.

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Figure C₂ fragmentation energies of C₈₀ and C₈₂ fullerenes have been calculated with B3LYP/6-31G* model chemistry, with semiempirical self-consistent-charge density-functional tight-binding (SCC-DFTB) method and with the more rigorous MP2 method. The influence of basis set extension and level of theory on the resulting fragmentation energies is discussed

     

The encapsulated lithium effect on the first hyperpolarizability of C<Subscript>60</Subscript>Cl<Subscript>2</Subscript> and C<Subscript>60</Subscript>F<Subscript>2</Subscript>

In this paper, we report a study on the structure and first hyperpolarizability of C₆₀Cl₂ and C₆₀F₂. The calculation results show that the first hyperpolarizabilities of C₆₀Cl₂ and C₆₀F₂ were 172 au and 249 au, respectively. Compared with the fullerenes, the first hyperpolarizability of C₆₀Cl₂ increased from 0 au to 172 au, while the first hyperpolarizability of C₆₀F₂ increased from 0 au to 249 au. In order to further increase the first hyperpolarizability of C₆₀Cl₂ and C₆₀F₂, Li@C₆₀Cl₂ and Li@C₆₀F₂ were obtained by introducing a lithium atom to C₆₀Cl₂ and C₆₀F₂. The first hyperpolarizabilities of Li@C₆₀Cl₂ and Li@C₆₀F₂ were 2589 au and 985 au, representing a 15-fold and 3.9-fold increase, respectively, over those of C₆₀Cl₂ and C₆₀F₂. The transition energies of four molecules (C₆₀Cl₂, Li@C₆₀Cl₂, C₆₀F₂, Li@C₆₀F₂) were calculated, and were found to be 0.17866 au, 0.05229 au, 0.18385 au, and 0.05212 au, respectively. A two-level model explains why the first hyperpolarizability increases for Li@C₆₀Cl₂ and Li@C₆₀F₂.     

Kinetics of CO conversion into H<Subscript>2</Subscript> by <Emphasis Type="Italic">Carboxydothermus hydrogenoformans</Emphasis>

The objective of this study was to improve the biological water–gas shift reaction for producing hydrogen (H₂) by conversion of carbon monoxide (CO) using an anaerobic thermophilic pure strain, Carboxydothermus hydrogenoformans. Specific hydrogen production rates and yields were investigated at initial biomass densities varying from 5 to 20 mg volatile suspended solid (VSS) L⁻¹. Results showed that the gas–liquid mass transfer limits the CO conversion rate at high biomass concentrations. At 100-rpm agitation and at CO partial pressure of 1 atm, the optimal substrate/biomass ratio must exceed 5 mol CO g⁻¹ biomass VSS in order to avoid gas–liquid substrate transfer limitation. An average H₂ yield of 94 ± 3% and a specific hydrogen production rate of ca. 3 mol g⁻¹ VSS day⁻¹ were obtained at initial biomass densities between 5 and 8 mg VSS⁻¹. In addition, CO bioconversion kinetics was assessed at CO partial pressure from 0.16 to 2 atm, corresponding to a dissolved CO concentration at 70°C from 0.09 to 1.1 mM. Specific bioactivity was maximal at 3.5 mol CO g⁻¹ VSS day⁻¹ for a dissolved CO concentration of 0.55 mM in the culture. This optimal concentration is higher than with most other hydrogenogenic carboxydotrophic species.     

Photosynthetic response of <Emphasis Type="Italic">Quercus ilex</Emphasis> L. plants grown on compost and exposed to increasing photon flux densities and elevated CO<Subscript>2</Subscript>

Quercus ilex plants grown on two different substrates, sand soil (C) and compost (CG), were exposed to photosynthetic photon flux densities (PPFD) at 390 and 800 μmol(CO₂) mol⁻¹ (C₃₉₀ and C₈₀₀). At C₈₀₀ both C and CG plants showed a significant increase of net photosynthetic rate (P _N) and electron transport rate (ETR) in response to PPFD increase as compared to C₃₉₀. In addition, at C₈₀₀ lower non-photochemical quenching (NPQ) values were observed. The differences between C₃₉₀ and C₈₀₀ were related to PPFD. The higher P _N and ETR and the lower dissipative processes found in CG plants at both CO₂ concentrations as compared to C plants suggest that substrate influences significantly photosynthetic response of Q. ilex plants. Moreover, short-term exposures at elevated CO₂ decreased nitrate photo-assimilation in leaves independently from substrate of growth.     

The contribution of C<Subscript>3</Subscript> and C<Subscript>4</Subscript> plants to the carbon cycle of a tallgrass prairie: an isotopic approach

The photosynthetic pathway composition (C₃:C₄ mixture) of an ecosystem is an important controller of carbon exchanges and surface energy flux partitioning, and therefore represents a fundamental ecophysiological distinction. To assess photosynthetic mixtures at a tallgrass prairie pasture in Oklahoma, we collected nighttime above-canopy air samples along concentration and isotopic gradients throughout the 1999 and 2000 growing seasons. We analyzed these samples for their CO₂ concentration and carbon isotopic composition and calculated C₃:C₄ proportions with a two-source mixing model. In 1999, the C₄ percentage increased from 38% in spring (late April) to 86% in early fall (mid-September). The C₄ percentages inferred from ecosystem respiration measurements in 2000 indicate a smaller shift, from 67% in spring (early May) to 77% in mid-summer (late July). We also sampled daytime CO₂ concentration and carbon isotope gradients above the canopy to determine ecosystem discrimination against ¹³CO₂ during net uptake. These discrimination values were always lower than corresponding nighttime ecosystem respiration isotopic signatures would suggest. After accounting for the isotopic disequilibria between respiration and photosynthesis resulting from seasonal variations in the C₃:C₄ mixture, we estimated canopy photosynthetic discrimination. The C₄ percentage calculated from this approach agrees with the percentage determined from nighttime respiration for sampling periods in both growing seasons. Isotopic imbalances between photosynthesis and respiration are likely to be common in mixed C₃:C₄ ecosystems and must be considered when using daytime isotopic measurements to constrain ecosystem physiology. Given the global extent of such ecosystems, isotopic imbalances likely contribute to global variations in the carbon isotopic composition of atmospheric CO₂.     

Effects of salinity on chlorophyll fluorescence and CO<Subscript>2</Subscript> fixation in C<Subscript>4</Subscript> estuarine grasses

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O₂ evolution (J _O2) and net rates of CO₂ uptake (P _N) were measured in the halotolerant estuarine C₄ grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO₂ concentration ratio (C _i/C _a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO₂ fixation decreased under salinity across species, suggesting there is increased leakage of the CO₂ delivered to the bundle sheath cells by the C₄ pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (F_V/F_M) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P _N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P _N in S. densiflora or D. spicata. This inhibition of P _N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C _i/C _a did not decrease under saline conditions. In growth with and without salt, P _N was saturated at ∼500 μmol(quantum) m⁻² s⁻¹ while J _O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy.     

Regulation of geochemical activity of microorganisms in a petroleum reservoir by injection of H<Subscript>2</Subscript>O<Subscript>2</Subscript> or water-air mixture

In the course of pilot trials of biotechnologies for the enhancement of oil recovery in formation waters of the Gangxi bed of the Dagang oil field (China), microbiological processes were investigated. The biotechnologies are based on injection into the petroleum reservoir of different oxygen sources (H₂O₂ solution or a water-air mixture) with nitrogen and phosphorus salts. The injection of water-air mixture with nitrogen and phosphorus salts resulted in an increase in the number of aerobic and anaerobic organotrophic bacteria, rates of sulfate reduction and methanogenesis in formation water and also the content of CO₂ (from 4.8–12 to 15–23.2%) and methane (from 86–88 to 91.8%) in the gas. The preferential consumption of isotopically light bicarbonate by methanogens resulted in a higher content of the light ¹²C in methane; the δ¹³C/CH₄ value changed from ?45.1…?48.3 to ?50.7…?59.3‰. At the same time, mineral carbonates of the formation water became isotopically heavier; the δ¹³C/Σcarbonates value increased from 3.4…4.0 to 5.4…9.6‰. Growth of hydrocarbon-oxidizing bacteria was accompanied by production of biosurfactants and decreased interfacial tension of formation water. Injection of H₂O₂ solution resulted in the activation of aerobic processes and in suppression of both sulfate reduction and methanogenesis. Methane content in the gas decreased from 86–88 to 75.7–79.8%, probably due to its consumption by methanotrophs. Due to consumption of isotopically light methane, the residual methane carbon became heavier, with the δ¹³C/CH₄ values from ?39.0 to ?44.3‰. At the same time, mineral carbonates of the formation water became isotopically considerably lighter; the δ¹³C/Σcarbonates value decreased from 5.4…9.6 to ?1.4…2.7‰. The additional amount of oil recovered during the trial of both variants of biotechnological treatment was 3819 t.     

Transient-state biodegradation behavior of a horizontal biotrickling filter in co-treating gaseous H<Subscript>2</Subscript>S and NH<Subscript>3</Subscript>

A horizontal biotrickling filter (HBTF) was used to inoculate autotrophic sulfide-oxidizing and ammonia-oxidizing microbial consortiums over H₂S-exhausted carbon for co-treating H₂S and NH₃ waste gas in a long-term operation. In this study, several aspects (i.e., pH change, shock loading and starvation) of the dynamic behavior of the HBTF were investigated. The metabolic products of N and S bearing species in recycling liquid and biological activities of the biofilm were analyzed to explain the observed phenomena and further explore the fundamentals behind. In the pH range of 4–8.5, although the removal efficiencies of H₂S and NH₃ remained 96–98% and 100%, respectively, the metabolic products demonstrated different removal mechanisms and pathways. NH₄-N and NO₂/NO₃-N were dominated at pH ≤6 and ≥7, respectively, indicating the differentiated contributions from physical/chemical adsorption and bio-oxidation. Moreover, the HBTF demonstrated a good dynamic stability to withstand shock loadings by recovering immediately to the original. During shock loading, only 15.4% and 17.9% of captured H₂S and NH₃ was biodegraded, respectively. After 2, 11, and 48 days of starvation, the HBTF system reached a full performance within reasonable re-startup times (2–80 h), possibly due to the consumption of reduced S and N species in biomass or activated carbon thus converted into SO₄-S and NO₃-N during starvation period. The results helped to understand the fundamental knowledge by revealing the effects of pH and transient loadings linked with individual removal mechanism for H₂S and NH₃ co-treatment in different conditions.     

Pressure dependence of side chain <Superscript>13</Superscript>C chemical shifts in model peptides Ac-Gly-Gly-Xxx-Ala-NH<Subscript>2</Subscript>

For evaluating the pressure responses of folded as well as intrinsically unfolded proteins detectable by NMR spectroscopy the availability of data from well-defined model systems is indispensable. In this work we report the pressure dependence of ¹³C chemical shifts of the side chain atoms in the protected tetrapeptides Ac-Gly-Gly-Xxx-Ala-NH₂ (Xxx, one of the 20 canonical amino acids). Contrary to expectation the chemical shifts of a number of nuclei have a nonlinear dependence on pressure in the range from 0.1 to 200 MPa. The size of the polynomial pressure coefficients B ₁ and B ₂ is dependent on the type of atom and amino acid studied. For H^N, N and C^α the first order pressure coefficient B ₁ is also correlated to the chemical shift at atmospheric pressure. The first and second order pressure coefficients of a given type of carbon atom show significant linear correlations suggesting that the NMR observable pressure effects in the different amino acids have at least partly the same physical cause. In line with this observation the magnitude of the second order coefficients of nuclei being direct neighbors in the chemical structure also are weakly correlated. The downfield shifts of the methyl resonances suggest that gauche conformers of the side chains are not preferred with pressure. The valine and leucine methyl groups in the model peptides were assigned using stereospecifically ¹³C enriched amino acids with the pro-R carbons downfield shifted relative to the pro-S carbons.