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71.
Agnieszka Mikolajczuk Benny Geypens Michael Berglund Philip Taylor 《Rapid communications in mass spectrometry : RCM》2009,23(16):2421-2427
Compound‐specific isotopic analysis (CSIA) can provide information about the origin of analysed compounds; for instance, polycyclic aromatic hydrocarbons (PAHs) in aerosols. This could be a valuable tool in source apportionment of particulate matter (PM) air pollution. Because gas chromatography–combustion–isotope ratio mass spectrometry (GC‐C‐IRMS) analysis requires an amount of at least 10 ng of an individual PAH, a high concentration of PAHs in the injected extract is needed. When the concentration is low a large volume injector creates the possibility of introducing a satisfactory amount of individual PAHs. In this study a temperature‐programmable injector was coupled to GC‐C‐IRMS and injection parameters (solvent level, transfer column flow, transfers time) were optimised using six solid aromatic compounds (anthracene, fluoranthene, pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene) dissolved in n‐pentane and EPA 610 reference mixture. CSIA results for solid PAHs were compared with results obtained for the single components analysed by elemental analysis–isotope ratio mass spectrometry. The injection method was validated for two sample injection volumes, 50 and 100 µL. This method was also compared with commonly used splitless injection. To be included in the study, measurements had to have an uncertainty lower than 0.5‰ for and a minimum peak height of 200 mV. The lower concentration limits at which these criteria were fulfilled for PAHs were 30 mg/L for 1 µL in splitless injection and 0.3 and 0.2 mg/L for 50 and 100 µL, respectively, in large volume injection. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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74.
Stenbj?rn Styring Licheng Sun Leif Hammarstr?m Roman Davydov Mats Almgren Mikael Andersson Helena Berglund Anna B?rje Peter Korall Thomas Norrby Christian Philouze Bj?rn ?kermark 《Journal of Chemical Sciences》1997,109(6):389-396
One attractive way to harvest solar energy is to use the concepts of natural photosynthesis in an artificial system. In green
plant photosynthesis, the solar energy is transformed into usable energy in the form of reduced compounds. The electrons come
from water which is oxidized to molecular oxygen, thereby providing the plants with a never ending supply of reducing equivalents.
In photosystem II, the photosensitizer is a chlorophyll species, P680, which is coupled to a cluster composed of four manganese
ions that catalyses the water oxidation. We have tried to mimic this by the synthesis of a binuclear compound [Ru(bpy)2(Mebpy-Mebpy)MnCl2(H2O)2]Cl2 built on bipyridine ligands containing a Ru(II) moiety (the photosensitizer) and a Mn(II) ion (the donor) linked via a bridging
ligand. In the complex, which is structurally defined by NMR, elemental analysis and electrospray mass spectroscopy, we have
observed, (1) the Mn is sufficiently close (about 13?) to interact with the Ru(II) ion, and (2) intramolecular, photochemically
induced electron transfer from Mn(II) to the photogenerated Ru(III) moiety after a light flash in the presence of an electron
acceptor. We suggest that the synthesis, characterization and observation of intramolecular electron transfer in this novel
Ru-Mn compound is an important step towards artificial photosynthesis. 相似文献
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76.
Tungsten trioxide reduced at about 1270 K by means of controlled atmospheres (P02 = 3.7 · 10?8 to 1.7 · 10?13 atm) was studied by high resolution transmission electron microscopy, electron diffraction, and X-ray powder diffraction. The accommodation of oxygen loss in the parent WO3 lattice in the range WO3 to WO2.72 was clarified. The results indicate a solid state mechanism. Intergrowth has been found to take place between several of the structural types that occur in this composition range. The intergrowth features include directional changes in shear plane arrays (“swinging shear planes”). Details of the structural variation with the oxygen content are reported. Ordered shear planes on {102} directions were found to stabilize the orthorhombic WO3 parent lattice at room temperature. W24O68 has been prepared in a fairly well-defined state. 相似文献
77.
Lipase B from Candida antarctica was redesigned to catalyze the promiscuous reaction of carbon-carbon bond formation. Mutation of the catalytic serine to alanine afforded a mutant that catalyzed Michael additions of 1,3-dicarbonyls to alpha,beta-unsaturated carbonyl compounds at high specific rates, such as 4000 s-1. The enzyme-catalyzed Michael addition reaction followed saturation kinetics and showed substrate inhibition. The designed enzyme showed high rate enhancements with a catalytic proficiency higher than 108, which is on the same level as that observed for enzymes with native substrates. 相似文献
78.
Zhang WJ Berglund A Kao JL Couty JP Gershengorn MC Marshall GR 《Journal of the American Chemical Society》2003,125(5):1221-1235
The beta-turn is a well-studied motif in both proteins and peptides. Four residues, making almost a complete 180 degree-turn in the direction of the peptide chain, define the beta-turn. Several types of the beta-turn are defined according to Phi and Psi torsional angles of the backbone for residues i + 1 and i + 2. One special type of beta-turn, the type VI-turn, usually contains a proline with a cis-amide bond at residue i + 2. In an aza-amino acid, the alpha-carbon of the amino acid is changed to nitrogen. Peptides containing azaproline (azPro) have been shown to prefer the type VI beta-turn both in crystals and in organic solvents by NMR studies. MC/MD simulations using the GB/SA solvation model for water explored the conformational preferences of azPro-containing peptides in aqueous systems. An increase in the conformational preference for the cis-amide conformer of azPro was clearly seen, but the increased stability was relatively minor with respect to the trans-conformer as compared to previous suggestions. To test the validity of the calculations in view of the experimental data from crystal structures and NMR in organic solvents, [azPro(3)]-TRH and [Phe(2), azPro(3)]-TRH were synthesized, and their conformational preferences were determined by NMR in polar solvents as well as the impact of the azPro substitution on their biological activities. 相似文献
79.
A. Berglund T. Buran P.J. Carlson C.J.S. Damerell I. Endo A.R. Gillman V. Gracco R.J. Homer M.J. Hotchkiss A. Lundby M. Macri B.N. Ratcliff A. Santroni T. Tso F. Wickens J.A. Wilson 《Physics letters. [Part B]》1975,57(1):100-104
For the first time, the reactions π+p→K+∑+ and K?p→π?∑+ have been studied in the same apparatus. This has been done at an adequately high momentum (10.1 GeV/c) to allow a check of the prediction of exchange degeneracy, that the differential cross sections should be converging at high energy. We have measured the cross section for momentum transfers t between tmin and t = ?0.3 (GeV/c)2. We find that for both reactions the differential cross section shows an exponential fall, with no deviations right in to t =tmin (where some other experiments have shown a dip in the cross section). Furthermore, we find the magnitude of the differential cross sections to be closely similar at t = 0, with a ratio However, the slope for the positive reaction is about 19% steeper than that for the negative reaction. 相似文献
80.
Cyrus Djahedi Malin Bergenstråhle-Wohlert Lars A. Berglund Jakob Wohlert 《Cellulose (London, England)》2016,23(4):2315-2323
The axial modulus of the cellulose Iβ crystal is as high as 120–160 GPa. The importance of hydrogen bonds is often emphasized in this context, although intrinsic stiffness of the hydrogen bonds is relatively low. Here, hydrogen bond–covalent bond synergies are investigated quantitatively using molecular mechanics and molecular dynamics simulations for the so-called leverage effect, a model introduced recently in which strains for intra-molecular hydrogen bonds are higher than for the cellulose chain as a whole, thereby amplifying their contribution to the total stiffness. The present work also includes simulation of the hydrogen bonding band shifts in vibrational spectra during cellulose deformation, which are compared with FT-IR data. The leverage effect hypothesis was supported by the results, although the total contribution to cellulose stiffness is only 12 %. Hydrogen bonding is still critically important and would lower the modulus much more than 12 %, if “artificially” removed in the model. The reason is that intra-molecular hydrogen bonding preserves the crystal structure and directs axial deformation mechanisms towards higher energy deformation and high stiffness. 相似文献