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Recent studies have recognised highly oxygenated organic molecules (HOMs) in the atmosphere as important in the formation of secondary organic aerosol (SOA). A large number of studies have focused on HOM formation from oxidation of biogenically emitted monoterpenes. However, HOM formation from anthropogenic vapours has so far received much less attention. Previous studies have identified the importance of aromatic volatile organic compounds (VOCs) for SOA formation. In this study, we investigated several aromatic compounds, benzene (C6H6), toluene (C7H8), and naphthalene (C10H8), for their potential to form HOMs upon reaction with hydroxyl radicals (OH). We performed flow tube experiments with all three VOCs and focused in detail on benzene HOM formation in the Jülich Plant Atmosphere Chamber (JPAC). In JPAC, we also investigated the response of HOMs to NOx and seed aerosol. Using a nitrate-based chemical ionisation mass spectrometer (CI-APi-TOF), we observed the formation of HOMs in the flow reactor oxidation of benzene from the first OH attack. However, in the oxidation of toluene and naphthalene, which were injected at lower concentrations, multi-generation OH oxidation seemed to impact the HOM composition. We tested this in more detail for the benzene system in the JPAC, which allowed for studying longer residence times. The results showed that the apparent molar benzene HOM yield under our experimental conditions varied from 4.1 % to 14.0 %, with a strong dependence on the OH concentration, indicating that the majority of observed HOMs formed through multiple OH-oxidation steps. The composition of the identified HOMs in the mass spectrum also supported this hypothesis. By injecting only phenol into the chamber, we found that phenol oxidation cannot be solely responsible for the observed HOMs in benzene experiments. When NOx was added to the chamber, HOM composition changed and many oxygenated nitrogen-containing products were observed in CI-APi-TOF. Upon seed aerosol injection, the HOM loss rate was higher than predicted by irreversible condensation, suggesting that some undetected oxygenated intermediates also condensed onto seed aerosol, which is in line with the hypothesis that some of the HOMs were formed in multi-generation OH oxidation. Based on our results, we conclude that HOM yield and composition in aromatic systems strongly depend on OH and VOC concentration and more studies are needed to fully understand this effect on the formation of HOMs and, consequently, SOA. We also suggest that the dependence of HOM yield on chamber conditions may explain part of the variability in SOA yields reported in the literature and strongly advise monitoring HOMs in future SOA studies.

Corrigendum: The legend in Fig. 6e has been mislabeled. The gray colorcorresponds to “Missing” and the other colors should havecorresponded to the same species as in Fig. 6f. The figure,which is also the key figure of the article, can be found belowwith the correct legend. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiala, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with a gas chromatographer coupled to a mass spectrometer (GC-MS) and offline liquid chromatography. Up to 104 compounds, mostly volatile organic compounds (VOCs) and oxidized VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The monthly averaged experimental total OH reactivity was found to be higher in April and May (ca. 20 s(-1)) than in June and July (7.6 and 15.4 s(-1), respectively). The measured values varied much more in spring with high reactivity peaks in late afternoon, with values higher than in the summer, in particular when the soil was thawing. Total OH reactivity values generally followed the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the difference between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, and (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (

12 pags, 4 figs, 3 tabs. -- The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.1c03160., We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII-organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds., This work was funded by the USEPA Science to Achieve Results (STAR) Program. This work was also supported by the Slovak Grant Agency VEGA (grant 1/0777/19), the highperformance computing facility of the Centre for Information Technology (https://uniba.sk/en/HPC-Clara) at Comenius University, and the U.S. National Science Foundation under awards 1609848 and 2004100. We thank Helene Angot (CU Boulder) for the Hg measurement data.

Uncertainty was evaluated in four chemical mechanisms pertaining to O3 concentrations predicted over Japan by the Community Multiscale Air Quality Model (CMAQ) to investigate factors contributing to model overestimation of O3 concentration. The model setting and meteorological and emissions input data were obtained from a Japanese model inter-comparison project, Japan's Study for Reference Air Quality Modeling (J-STREAM). The compared gas-phase chemical mechanisms included the Carbon Bond Mechanism (CB05TUCL), Regional Atmospheric Chemical Mechanism (RACM2), and two mechanisms developed by the State Air Pollution Research Center (SAPRC), namely SAPRC07TC and SAPRC99. The O3 concentrations produced by CB05TUCL were low compared to those from SAPRC07TC. The RACM2 concentrations were similar to those from SAPRC07TC over inland Japan and lower over the sea. The concentrations from SAPRC99 were higher than those from SAPRC07TC in urban areas and lower in other areas. At most of the monitoring sites in Japan, the modeled O3 concentrations were higher than those from observations. Module overestimation can be ranked in the order of SAPRC99 > SAPRC07TC > RACM2 > CB05TUCL for urban sites and SAPRC07TC > SAPRC99 > RACM2 > CB05TUCL for rural sites. The concentration differences between the chemical mechanisms were within 10 ppb, whereas those between the observed and simulated O3 concentrations reached 40 ppb. Differences in O3 concentrations between the chemical mechanisms accounted for only a part of the model overestimation, while the rest remained unexplained. To investigate factors influencing the differences in O3 concentration between the chemical mechanisms, domain- and 10-vertical-layer-average hourly integrated process rates (IPRs) and integrated reaction rates (IRRs) were calculated using process analysis in CMAQ. The O3 chemical IPRs from SAPRC07TC were higher than those from CB05TUCL and RACM2. The SAPRC99 IPRs were higher than those from SAPRC07TC in urban areas and lower in other areas. The IRR differences in the chemical mechanisms showed that IRRs for the O3 and NO reactions were responsible for the differences in the O3 chemical IPR. The coefficients of determination between the O3 chemical process IPR and IRR differences in the chemical mechanisms were highest for the HO2-NO reaction in CB05TUCL and SAPRC99 and the RO2-NO reaction in RACM2. Differences in reaction rate constants and lumped volatile organic compounds may have caused some of the differences in O3 production between the chemical mechanisms.

The Community Earth System Model version 2 (CESM2) includes a detailed representation of chemistry throughout the atmosphere in the Community Atmosphere Model with chemistry and Whole Atmosphere Community Climate Model configurations. These model configurations use the Model for Ozone and Related chemical Tracers (MOZART) family of chemical mechanisms, covering the troposphere, stratosphere, mesosphere, and lower thermosphere. The new MOZART tropospheric chemistry scheme (T1) has a number of updates over the previous version (MOZART‐4) in CESM, including improvements to the oxidation of isoprene and terpenes, organic nitrate speciation, and aromatic speciation and oxidation and thus improved representation of ozone and secondary organic aerosol precursors. An evaluation of the present‐day simulations of CESM2 being provided for Climate Model Intercomparison Project round 6 (CMIP6) is presented. These simulations, using the anthropogenic and biomass burning emissions from the inventories specified for CMIP6, as well as online calculation of emissions of biogenic compounds, lightning NO, dust, and sea salt, indicate an underestimate of anthropogenic emissions of a variety of compounds, including carbon monoxide and hydrocarbons. The simulation of surface ozone in the southeast United States is improved over previous model versions, largely due to the improved representation of reactive nitrogen and organic nitrate compounds resulting in a lower ozone production rate than in CESM1 but still overestimates observations in summer. The simulation of tropospheric ozone agrees well with ozonesonde observations in many parts of the globe. The comparison of NOx and PAN to aircraft observations indicates the model simulates the nitrogen budget well.

International audience; 3-Methyl-4-aza[6]helicene (A6H) enantiomers were synthesized and proposed as high-performance visible light photoinitiators and/or photosensitizers for the free radical polymerization (FRP) of (meth)acrylates upon visible light exposure using inexpensive and safe light-emitting diodes (LEDs) at 405 nm. Excellent polymerization initiating abilities were found, and high final conversions were obtained. Remarkably, an exceptional strongly enhanced chiral property of the polymer films was observed in the presence of the enantiopure azahelicenes. This is the first example of chiral properties introduced in polymer films by the photoinitiator through photopolymerization processes. It is of high interest as these latter are very fast and inexpensive polymerization techniques. Remarkably, contrary to chiral properties introduced by using a combination of commercial photoinitiator and chiral dopant, a dual behavior is found here for A6H. A full picture of the involved chemical mechanisms is given. © 2018 American Chemical Society.

NO X is one of the main compositions in the modern engine emissions and the reduction requirements of NO X have turned to be more stringent. To control NO X emissions better, the technologies of NO X sensors are forced to achieve much faster response and higher accuracy. In this paper, the correlation between ion current signals and NO X emissions has been investigated by both experiments and simulations based on a direct-injection controlled auto-ignition (CAI) engine, which could provide a novel method to detect NO X emissions within combustion cycle. The simulation results presented this positive correlation from the chemical kinetics point of view, which could also directly reflect the formation order of the chemical products and the influence of temperature on the rates of main ionization and NO X generated reactions. Furthermore, the distributions of both ions and NO products have been shown with the CFD results, to illustrate their in-cylinder space correlation. Combined with the simulation results, the experimental results not only validated the positive correlation with two different fuels, but also provided the evidences fitting linear function. Based on the results, the cycle-based NO X emissions were estimated.

[EN] The ability of a computational fluid dynamics (CFD) simulation to reproduce the diesel-like reacting spray ignition process and its corresponding flame structure strongly depends on both the fidelity of the chemical mechanism for reproducing the oxidation of the fuel and also on how the turbulence-chemistry interaction (TCI) is modeled. Therefore, investigating the performance of different chemical mechanisms not only in perfect stirred reactors but directly in the diesel-like spray itself is of great interest in order to evaluate their suitability for being further applied to CFD engine simulations. This research work focuses on applying a presumed probability density function (PDF) unsteady flamelet combustion model to the well-known spray A from the Engine Combustion Network (ECN), using three chemical mechanisms widely accepted by the scientific community as a way to figure out the influence of chemistry in the key characteristics of the combustion process in the frame of diesel-like spray simulations. Results confirm that in spite of providing all of them correct trends for ignition delays (ID) and lift-off lengths (LOL), when comparing with experimental results, the structure of the flame presents noticeable differences, especially in the low and intermediate temperatures and high equivalence ratio regions. Consequently, the selection of the chemical mechanism has an impact on the zones of influence of key species as observed in both spatial coordinates and also in the equivalence ratio-temperature maps. These differences are expected to be relevant considering the implications when coupling pollutant emissions models. The analysis of temperature and oxygen concentration parametric studies evidences how the observed differences are consistent and moderately dependent on the ambient conditions., Authors acknowledge that this work was possible thanks to the Ayuda para la Formación de Profesorado Universitario (FPU 14/03278) belonging to the Subprogramas de Formación y de Movilidad del Ministerio de Educación, Cultura y Deporte from Spain. Also this study was partially funded by the Ministerio de Economía y Competitividad from Spain in the frame of the COMEFF (TRA2014-59483-R) national project.

The Leuven Isoprene Mechanism, proposed earlier to aid in rationalizing the unexpectedly high hydroxyl radical (OH) concentrations in isoprene-rich, low-nitric-oxide (NO) regions (Peeters et al. Phys. Chem. Chem. Phys. 2009, 11, 5935), is presented in an upgraded and extended version, LIM1. The kinetics of the crucial reactions in the proposed isoprene-peroxy radical interconversion and isomerisation pathways are re-evaluated theoretically, based on energy barriers computed at the much higher CCSD(T)/aug-cc-pVTZ//QCISD/6-311G(d,p) level of theory, and using multi-conformer partition functions obtained at the M06-2X/6-311++G(3df,2p) level that, different from the B3LYP level used in our earlier work, accounts for the crucial London dispersion effects in the H-bonded systems involved. The steady-state fraction of the specific Z-δ-OH-peroxy radical isomers/conformers that can isomerise by 1,6-H shift is shown to be largely governed by hydrogen-bond strengths, while their isomerisation itself is found to occur quasi-exclusively by hydrogen atom tunneling. The isomer-specific Z-δ-OH-peroxy 1,6-H shift rate coefficients are predicted to be of the order of 1 s-1 at 298 K, but the experimentally accessible bulk rate coefficients, which have to be clearly distinguished from the former, are two orders of magnitude lower due to the very low Z-δ-OH-peroxy steady-state fractions that are only around or below 0.01 at low to moderate NO and depend on the peroxy lifetime. Two pathways subsequent to the peroxy radical 1,6-H shift are identified, the earlier predicted route yielding the photolabile hydroperoxy-methyl-butenals (HPALDs), and a second, about equally important path, to di-hydroperoxy-carbonyl peroxy radicals (di-HPCARP). Taking this into account, our predicted bulk peroxy isomerisation rate coefficients are about a factor 1.8 higher than the available experimental results for HPALD production (Crounse et al. Phys. Chem. Chem. Phys. 2011, 13, 13607), which is within the respective uncertainty margins. We also show that the experimental temperature dependence of the HPALD production rates as well as the observed kinetic isotope effect for per-deuterated isoprene support quantitatively our theoretical peroxy interconversion rates. Global modeling implementing LIM1 indicates that on average about 28% of the isoprene peroxys react via the 1,6-H shift isomerisation route, representing 100 - 150 Tg carbon per year. The fast photolysis of HPALDs we proposed earlier as primary OH regeneration mechanism (Peeters and Muller, Phys. Chem. Chem. Phys. 2010, 12, 14227) found already experimental confirmation (Wolfe et al. Phys. Chem. Chem. Phys. 2012, 14, 7276); based on further theoretical work in progress, reaction schemes are presented of the oxy co-product radicals from HPALD photolysis and of the di-HPCARP radicals from the second pathway following peroxy isomerisation that are both expected to initiate considerable additional OH recycling. Invited Feature Article, with journal cover ispartof: Journal of Physical Chemistry A vol:118 issue:38 pages:8625-8643 ispartof: location:United States status: published

In this study, the mechanism of the xanthine oxidase (XO) inhibitory activity of pyrogallol, the main inhibitor found in roasted coffee, was investigated. Pyrogallol was unstable and readily converted to purpurogallin in a pH 7.4 solution, a physiological model of human body fluids. The XO inhibitory activity of the produced purpurogallin was higher than that of pyrogallol, as evidenced by comparing their IC50 values (0.2µmolL-1 for purpurogallin, 1.6µmolL-1 for pyrogallol). The XO activity of pyrogallol was enhanced by pre-incubation in pH 7.4 solution. Although the initial XO inhibitory activity of 4-methylpyrogallol was weak (IC50 33.3µmolL-1), its XO inhibitory activity was also enhanced by pre-incubation in the pH 7.4 solution. In contrast, 5-methylpyrogallol, which could not be transformed into corresponding purpurogallin derivatives, did not show XO inhibitory activity before or after incubation in pH 7.4 solution. Molecular docking simulations clarified that purpurogallins have stronger affinities for XO than corresponding pyrogallols. These results revealed that the potent XO inhibitory activity seemingly observed in pyrogallol is actually derived from its chemical conversion, under alkaline conditions, into purpurogallin.

Members of the aspartase/fumarase superfamily share a common tertiary and quaternary fold, as well as a similar active site architecture; the superfamily includes aspartase, fumarase, argininosuccinate lyase, adenylosuccinate lyase, delta-crystallin, and 3-carboxy-cis,cis-muconate lactonizing enzyme (CMLE). These enzymes all process succinyl-containing substrates, leading to the formation of fumarate as the common product (except for the CMLE-catalyzed reaction, which results in the formation of a lactone). In the past few years, X-ray crystallographic analysis of several superfamily members in complex with substrate, product, or substrate analogues has provided detailed insights into their substrate binding modes and catalytic mechanisms. This structural work, combined with earlier mechanistic studies, revealed that members of the aspartase/fumarase superfamily use a common catalytic strategy, which involves general base-catalyzed formation of a stabilized aci-carboxylate (or enediolate) intermediate and the participation of a highly flexible loop, containing the signature sequence GSSxxPxKxN (named the SS loop), in substrate binding and catalysis.

Numerical investigation of laminar counterflow premixed methane-hydrogen-air flames at a low strain rate is presented. Simulations are carried out with a numerical model incorporated with C2 chemical mechanism having 25 species and 121 reaction steps, and with an optically thin radiation submodel. The numerical model is validated using the experimental data reported in the literature in terms of temperature and species concentrations in flames from opposed flow premixed methane-air and hydrogen-air streams. Parametric studies are performed for opposed flowing methanehydrogen and air mixtures. A premixed methane-hydrogen and air with a rich mixture equivalence ratio (1.75) flows from the top duct, and one having a lean mixture equivalence ratio (0.25) flows from the bottom duct. The volumetric fraction of hydrogen in the fuel mixture has been varied from 20 to 80%. The strain rate used in the present study is kept constant at 50 s -1. Variation of velocity, temperature, species concentrations, and net reaction rates of oxygen, methane and hydrogen along the axis for various cases are presented and discussed in detail. Double flame zones are observed for all the cases. The addition of hydrogen to the rich side stream is seen to be effective in modifying the extents of both reaction zones. The reaction rate of methane is seen to be enhanced with the addition of hydrogen. � 2011 by Begell House, Inc.

Numerical investigations of burning and emission characteristics of laminar methane diffusion flames in a coflowing preheated oxidizer are presented. The preheated oxidizer stream contains hot products of combustion, such as carbon dioxide and water vapor, along with oxygen and nitrogen, and is equivalent to that obtained in an exhaust gas recirculation process. Numerical simulations are carried out using a numerical model incorporated with a C2 chemical mechanism having 25 species and 121 reactions and an optically thin radiation submodel. The numerical results are validated against the experimental results for methane-air coflow flames reported in the literature. Parametric studies have been carried out by changing the temperature and composition of the coflowing oxidizer stream. The effects of these parameters on flame height, flame stability, and emission characteristics are analyzed. It is observed that the increase in the temperature of the oxidizer has a stabilizing effect on the flame, even with a lesser concentration of oxygen in the oxidizer. By investigating the stable flame cases in which a maximum temperature of around 2000 K is achieved, it can be concluded that the preheated oxidizer stream (higher temperature and reduced oxygen) helps in significant reduction of emissions such as nitric oxide and carbon monoxide. � 2010 by Begell House, Inc.

HIV-1 protease is most active under weakly acidic conditions (pH 3.5-6.5), when the catalytic Asp25 and Asp25' residues share 1 proton. At neutral pH, this proton is lost and the stability of the structure is reduced. Here we present an investigation of the effect of pH on the dynamics of HIV-1 protease using MD simulation techniques. MD simulations of the solvated HIV-1 protease with the Asp25/25' residues monoprotonated and deprotonated have been performed. In addition we investigated the effect of the inclusion of Na+ and Cl- ions to mimic physiological salt conditions. The simulations of the monoprotonated form and deprotonated form including Na+ show very similar behavior. In both cases the protein remained stable in the compact, "self-blocked" conformation in which the active site is blocked by the tips of the flaps. In the deprotonated system a Na+ ion binds tightly to the catalytic dyad shielding the repulsion between the COO- groups. Ab initio calculations also suggest the geometry of the active site with the Na+ bound closely resembles that of the monoprotonated case. In the simulations of the deprotonated form (without Na+ ions), a water molecule bound between the Asp25 Asp25' side-chains. This disrupted the dimerization interface and eventually led to a fully open conformation. (C) 2004 Wiley-Liss, Inc.

In the framework of the AQMEII initiative WRF-Chem has been applied over Europe adopting two chemical configurations for the calendar year 2010. The first one employed the RADM2 gas-phase chemistry and MADE/SORGAM aerosol module, while the second one implemented the CBM-Z gaseous parameterization and MOSAIC aerosol chemistry. Configurations shared the same domain, meteorological setups and input data. The Comparison demonstrated that CBM-Z has a more efficient ozone-NO titration than RADM2 in regions with sufficiently high levels of NOx and VOCs. At the same time, CBM-Z is found to have a more effective NO2 + OH reaction. The parameterization of the relative humidity of deliquescence point has a strong impact on HNO3 and NO3 concentrations over Europe, particularly over the sea. The MADE approach showed to be more efficient than MOSAIC. Differently, particulate sulfate and SO2 ground concentrations proved to be more influenced by the heterogeneous SO2 cloud oxidation. PM10 and PM2.5 have shown similar results for MOSAIC and MADE/SORGAM, even though some differences were found in the dust and sea salt size partitioning between modes and bins. Indeed, in MADE the sea salt was distributed only in the coarse fraction, while the dust emissions were distributed mainly in the fine fraction. Finally, different chemical mechanisms give different Aerosol Optical Depths (AOD). WRF-Chem is found to under predict the AODs in both configurations because of the misrepresentation of the dust coarse particle, as shown by the analysis of the relationship between the Angstrom exponent and the AOD bias. Differently, when the AOD is dominated by fine particles, the differences in model performance are more evident, with MADE/SORGAM generally performing better than MOSAIC. Indeed the higher availability of both sulfate and nitrate has a significant influence on reconstruction of the AOD estimations. This paper shows the great importance of chemical mechanisms in both gaseous and aerosols predictions, as well as in the calculation of aerosol optical properties.

Ozone concentrations in the Po Valley of northern Italy often exceed international regulations. As both a source of radicals and an intermediate in the oxidation of most volatile organic compounds (VOCs), formaldehyde (HCHO) is a useful tracer for the oxidative processing of hydrocarbons that leads to ozone production. We investigate the sources of HCHO in the Po Valley using vertical profile measurements acquired from the airship Zeppelin NT over an agricultural region during the PEGASOS 2012 campaign. Using a 1-D model, the total VOC oxidation rate is examined and discussed in the context of formaldehyde and ozone production in the early morning. While model and measurement discrepancies in OH reactivity are small (on average 3.4 ± 13%), HCHO concentrations are underestimated by as much as 1.5 ppb (45%) in the convective mixed layer. A similar underestimate in HCHO was seen in the 2002–2003 FORMAT Po Valley measurements, though the additional source of HCHO was not identified. Oxidation of unmeasured VOC precursors cannot explain the missing HCHO source, as measured OH reactivity is explained by measured VOCs and their calculated oxidation products. We conclude that local direct emissions from agricultural land are the most likely source of missing HCHO. Model calculations demonstrate that radicals from degradation of this non-photochemical HCHO source increase model ozone production rates by as much as 0.6 ppb h−1 (12%) before noon.

Fourier transform Raman (FT-Raman) spectra of neat pyridine (Py) and surface enhanced Raman scattering (SERS) spectra of Py with silver nanoparticles (AgNPs) solution at different molar concentrations (X = 1.5 M, 1.0 M, 0.50 M, 0.25 M, and 0.125 M) were recorded using 1064 nm excitation wavelength. The intensity of Raman bands at similar to 1003 (nu(11)) and similar to 1035 (nu(21)) cm(-1) of Py is enhanced in the SERS spectra. Two new Raman bands were observed at similar to 1009 (nu(12)) and similar to 1038 (nu(22)) cm(-1) in the SERS spectra. These bands correspond to the ring breathing vibrations of Py molecules adsorbed at the AgNPs surface. The value of intensity ratios (I-12/I-11) and (I-21/I-22) is increased with dilution and attains a maximum value at X = 0.5 M and upon further dilution (0.25 and 0.125 M) it drops gradually. The theoretically calculated Raman spectra were found to be in good agreement with experimentally observed Raman spectra. Both, experimental and theoretical investigations have confirmed that the Py interacts with AgNPs via the end-on geometry. (C) 2015 Elsevier B.V. All rights reserved.

Numerical and experimental investigations of unconfined methane-oxygen laminar premixed flames are presented. In a lab-scale burner, premixed flame experiments have been conducted using pure methane and pure oxygen mixtures having different equivalence ratios. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations have been measured using a thermocouple. Numerical simulations have been carried out with a C2 chemical mechanism having 25 species and 121 reactions and with an optically thin radiation sub-model. The numerical results are validated against the experimental and numerical results for methane-air premixed flames reported in literature. Further, the numerical results are validated against the results from the present methane-oxygen flame experiments. Visible regions in digital flame photographs have been compared with OH isopleths predicted by the numerical model. Parametric studies have been carried out for a range of equivalence ratios, varying from 0.24 to 1.55. The contours of OH, temperature and mass fractions of product species such as CO, CO 2 and H 2O, are presented and discussed for various cases. By using the net methane consumption rate, an estimate of the laminar flame speed has been obtained as a function of equivalence ratio. � 2012 Taylor and Francis Group, LLC.

A method is presented for extending the range of conditions for which the performance of chemical mechanisms used to predict hydrogen burning velocities can be evaluated. Specifically, by comparing the computed variation of flame temperature with mass flux in burner-stabilized flat flames with those obtained experimentally the predictive power of a chemical mechanism can be tested at constant equivalence ratio over a range of more than 700 K. Sensitivity analyses for the flame temperature shows changing importance of various reactions with mass flux. High-quality flame temperature data obtained by CARS is capable of discerning differences in rate coefficients smaller than their current uncertainty. The method is general, and can be used for other fuels as well. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Aspartate ammonia lyases (also referred to as aspartases) catalyze the reversible deamination of l-aspartate to yield fumarate and ammonia. In the proposed mechanism for these enzymes, an active site base abstracts a proton from C3 of l-aspartate to form an enzyme-stabilized enediolate intermediate. Ketonization of this intermediate eliminates ammonia and yields the product, fumarate. Although two crystal structures of aspartases have been determined, details of the catalytic mechanism have not yet been elucidated. In the present study, eight active site residues (Thr101, Ser140, Thr141, Asn142, Thr187, His188, Lys324 and Asn326) were mutated in the structurally characterized aspartase (AspB) from Bacillus sp. YM55-1. On the basis of a model of the complex in which l-aspartate was docked manually into the active site of AspB, the residues responsible for binding the amino group of l-aspartate were predicted to be Thr101, Asn142 and His188. This postulate is supported by the mutagenesis studies: mutations at these positions resulted in mutant enzymes with reduced activity and significant increases in the K(m) for l-aspartate. Studies of the pH dependence of the kinetic parameters of AspB revealed that a basic group with a pK(a) of approximately 7 and an acidic group with a pK(a) of approximately 10 are essential for catalysis. His188 does not play the typical role of active site base or acid because the H188A mutant retained significant activity and displayed an unchanged pH-rate profile compared to that of wild-type AspB. Mutation of Ser140 and Thr141 and kinetic analysis of the mutant enzymes revealed that these residues are most likely involved in substrate binding and in stabilizing the enediolate intermediate. Mutagenesis studies corroborate the essential role of Lys324 because all mutations at this position resulted in mutant enzymes that were completely inactive. The substrate-binding model and kinetic analysis of mutant enzymes suggest that Thr187 and Asn326 assist Lys324 in binding the C1 carboxylate group of the substrate. A catalytic mechanism for AspB is presented that accounts for the observed properties of the mutant enzymes. Several features of the mechanism that are also found in related enzymes are discussed in detail and may help to define a common substrate binding mode for the lyases in the aspartase/fumarase superfamily.

A new thermo-diffusive analysis of one-dimensional laminar lean or rich off-stoichiometric premixed spray flames has been performed using a chain branching/chain breaking chemical kinetic scheme and under the assumption that the fuel droplets evaporate in a sharp front. The sensitivity of the flame structure, speed and the location of the evaporation front to the initial droplet load have been demonstrated. A linear stability analysis reveals the way in which the spray's presence modifies the neutral stability curves.

Acetohydroxy acid synthase (AHAS) is one of several enzymes that require thiamine diphosphate and a divalent cation as essential cofactors. Recently, the three-dimensional structure of the enzyme from yeast has been determined [Pang et al., J. Mol. Biol. 317 (2002) 249–262]. While this structure sheds light on the binding of the cofactors and the reaction mechanism, the interactions between the substrates and the enzyme remain unclear. We have studied the pH dependence of kinetic parameters in order to obtain information about the chemical mechanism in the active site. Data are consistent with a mechanism in which substrate selectively catalyzed to the enzyme with an unprotonated base having a p K of 6.48, and a protonated group having a p K of 8.25 for catalysis. The temperature dependence of kinetic parameters was pH-dependent, and the enthalpies of ionization, Δ H ion , calculated from the slope of p K 1 and p K 2 are both pH-independent. The solvent perturbation of kinetic parameters was pH-dependent, and the p K 1 from the acidic side and the p K 2 from the basic side were shifted down 0.4 pH units and shifted up 0.6 units as water was replaced by 15% ethanol, respectively. The data are discussed in terms of the acid–base chemical mechanism.

The Photochemistry Experiment in BERLIOZ (PHOEBE) was conducted in July/August 1998 at a rural site located near the small village Pabstthum, about 50 km NW of downtown Berlin. More than 60 nonmethane hydrocarbons (NMHC) in the range of C-2-C-10 were measured using two in situ gas chromatography (GC) systems. The first (GC1) was capable of measuring C-2-C-10 hydrocarbons with a relatively high separation efficiency but low time resolution (80-90 min), while GC2 provided quasi-continuous measurements of C-5-C-10 hydrocarbons with a time resolution of 20 min but with a poorer separation efficiency than GC1. The advantages of both systems were joined by interpolation between two data points of GC1 with the pattern given by GC2. For compounds that could not be reliably measured with GC2, patterns of compounds with similar reactivity were used. Air masses with the lowest photochemical age as estimated from the toluene/benzene ratio and the highest hydrocarbon mixing ratios were observed on 20 and 21 July when air was advected from the direction of Berlin. Alkanes were the most abundant hydrocarbons (similar to60%) on a molecular basis, followed by alkenes and aromatics. The reactivity of the hydrocarbons toward OH was dominated by the alkenes (>60%), with isoprene and a-pinene constituting the major part. The hydrocarbon data were used together with the other trace gases measured at Pabstthum to simulate OH, HO2, and RO2 concentrations with the condensed chemical box model RACM. Relatively good agreement of the simulated radical concentrations with the spectroscopic measurements made at Pabstthum is observed for NOx mixing ratios >5 ppb, whereas the model overestimates OH and HO2 by 100% and 40%, respectively, at low NOx. The discrepancy between measured and modeled OH does not correlate with the concentration of particles. The RO2 concentrations are in good agreement with the measurements over the entire range of NOx. Sensitivity studies show that peroxyacetyl nitrate ( PAN) is an important radical source and that missing volatile organic compound (VOC) reactivity is an unlikely explanation for the overestimation of HOx: By doubling of the VOC reactivity, OH and HO2 can be brought into agreement. However, the model then overestimates the organic RO2 concentrations by almost a factor of 2. Another important finding is that RACM overestimates the measured NO/NO2 ratio by 25%. This and the overestimation of HO2 lead to an overprediction of the local ozone formation rate by about 40% at low NOx mixing ratios.