Histidine is an aromatic amino acid crucial for the biological functioning of proteins and enzymes. When biological matter is exposed to ionising radiation, highly energetic particles interact with the surrounding tissue which leads to efficient formation of low‐energy electrons. In the present study, the interaction of low‐energy electrons with gas‐phase histidine is studied at a molecular level in order to extend the knowledge of electron‐induced reactions with amino acids. We report both on the formation of positive ions formed by electron ionisation and negative ions induced by electron attachment. The experimental data were complemented by quantum chemical calculations. Specifically, the free energies for possible fragmentation reactions were derived for the τ and the π tautomer of histidine to get insight into the structures of the formed ions and the corresponding neutrals. We report the experimental ionisation energy of (8.48 ± 0.03) eV for histidine which is in good agreement with the calculated vertical ionisation energy. In the case of negative ions, the dehydrogenated parent anion is the anion with the highest mass observed upon dissociative electron attachment. The comparison of experimental and computational results was also performed in view of a possible thermal decomposition of histidine during the experiments, since the sample was sublimated in the experiment by resistive heating of an oven. Overall, the present study demonstrates the effects of electrons as secondary particles in the chemical degradation of histidine. The reactions induced by those electrons differ when comparing positive and negative ion formation. While for negative ions, simple bond cleav ages prevail, the observed fragment cations exhibit partly restructuring of the molecule during the dissociation process. 相似文献
Journal of Thermal Analysis and Calorimetry - The aim of the present study was to prepare and characterize composites of ultra-high-molar mass polyethylene reinforced with varied amounts of jute... 相似文献
This study evaluated the production of lignocellulose-degrading enzymes by solid-state fermentation (SSF) using a microbial consortium of Aspergillus fumigatus SCBM6 and A. niger SCBM1 (AFN extract). The fungal strains were cultivated in sugarcane bagasse (SCB) and wheat bran (WB) as lignocellulosic substrates for 7 days at 30 °C. After SSF, the highest peaks of enzyme production were 150 and 80 U g−1 for β-xylosidase and β-glucosidase at 48 h, 375 U g−1 for xylanase at 96 h, and 80 U g−1 for endoglucanase and 4 U g−1 for cellulase activity on filter paper (FPase) at 144 h. The efficiency of the produced AFN extract was investigated in the enzymatic hydrolysis of crude biomass sorghum (BS) and after the removal of extractives (ES). After saccharification, the glucose and xylose concentrations were 10× superior in ES than in BS hydrolysate (2.5 g L−1 after 12 h). The presence of inhibitors of alcoholic fermentation, such as formic acid, was also reduced in ES hydrolysates, indicating that the removal of extractives positively contributed to the effectiveness of enzymatic hydrolysis of biomass sorghum using AFN extract.
The horizontal stirring properties of the flow in a region of the East Australian Current are calculated. A surface velocity field derived from remotely sensed data, using the maximum cross correlation method, is integrated to derive the distribution of the finite-time Lyapunov exponents. For the region studied (between latitudes 36 degrees S and 41 degrees S and longitudes 150 degrees E and 156 degrees E) the mean Lyapunov exponent during 1997 is estimated to be lambda( infinity )=4x10(-7) s(-1). This is in close agreement with the few other measurements of stirring rates in the surface ocean which are available. Recent theoretical results on the multifractal spectra of advected reactive tracers are applied to an analysis of a sea-surface temperature image of the study region. The spatial pattern seen in the image compares well with the pattern seen in an advected tracer with a first-order response to changes in surface forcing. The response timescale is estimated to be 20 days. (c) 2002 American Institute of Physics. 相似文献
Sílica-dithizone (Sil-dtz) was synthesized and used to adsorb Hg(II) in solution at pH 6.0. Increasing the temperature accelerates the mass transfer of Hg(II) to the silica surface. The kinetic data were evaluated using the traditional pseudo-first-order Lagergren equation and an alternative Avrami kinetic equation. From the latter equation, two regions presenting distinct kinetic parameters were found, at 25 and 35 degrees C, and the use of the parameter n was also related to the determination of distinct kinetic orders. Variations of the adsorption kinetic rate in relation to the time and the temperature were also calculated and are discussed. The adsorption isotherms data were well fitted to the Freundlich model. Interestingly, good adsorption data correlation of the Langmuir model and experimental values was observed only at 45 and 50 degrees C, suggesting, for this temperature range, the formation of complexes with the proportion Hg:dithizone 1:1 on the silica surface. 相似文献
Addition of fullerenes (C60 or buckyballs) to a linear polymer has been found to eliminate dewetting when a thin (∼50 nm) film is exposed to solvent
vapor. Based on neutron reflectivity measurements, it is found that the fullerenes form a coherent layer approximately 2 nm
thick at the substrate – polymer film interface during the spin-coating process. The thickness and relative fullerene concentration
(∼29 vol%) is not altered during solvent vapor annealing and it is thought this layer forms a solid-like buffer shielding
the adverse van der Waals forces promoted by the underlying substrate. Several polymer films produced by spin- or spray-coating
were tested on both silicon wafers and live surface acoustic wave sensors demonstrating fullerenes stabilize many different
polymer types, prepared by different procedures and on various surfaces. Further, the fullerenes drastically improve sensor
performance since dewetted films produce a sensor that is effectively inoperable. 相似文献
Disordered and crystalline Mn-doped BaTiO3 (BTO:Mn) powders were synthesized by the polymeric precursor method. After heat treatment, the nature of visible photoluminescence (PL) at room temperature in amorphous BTO:Mn was discussed, considering results of experimental and theoretical studies. X-ray diffraction (XRD), PL, and UV-vis were used to characterize this material. Rietveld refinement of the BTO:Mn from XRD data was used to built two models, which represent the crystalline BTO:Mn (BTO:Mnc) and disordered BTO:Mn (BTO:Mnd) structures. Theses models were analyzed by the periodic ab initio quantum mechanical calculations using the CRYSTAL98 package within the framework of density functional theory at the B3LYP level. The experimental and theoretical results indicated that PL is related with the degree of disorder in the BTO:Mn powders and also suggests the presence of localized states in the disordered structure. 相似文献
This letter presents a method for imaging the palate and extracting the palate contour from ultrasound images. Ultrasound does not usually capture the palate because the air at the tongue surface reflects the ultrasound beam back to the transducer. However, when the tongue touches the palate during a swallow, the ultrasound beam is transmitted through the soft tissue until it reaches and is reflected by the palate. In combination with tongue contours, the palate contour has the potential for disambiguation of the tongue surface, registration of images within and across subjects, and calculation of phonetically important measures. 相似文献
[reaction: see text] The synthesis and characterization of a series of symmetric bis-dialkyldiamine-based diazeniumdiolates, RN[N(O)NO(-)Na(+)](CH(2))(x)()N[N(O)NO(-)Na(+)]R', are reported. Preparation of corresponding intramolecular diazeniumdiolates of the form RN[N(O)NO](-)(CH(2))(x)()NH(2)(+)R' with alkyl groups > (CH(2))(4)CH(3) have been shown previously to lack stability. In contrast, sodium-stabilized bis-diazeniumdiolates of such lipophilic species can be readily formed when these same diamines are reacted with NO in basic media. The resulting compounds release 4 mol of NO per mole of original diamine. This approach enables the synthesis of more lipophilic NO donors than previously possible. 相似文献
Abstract. Given an m × n rectangular mesh, its adjacency matrix A , having only integer entries, may be interpreted as a map between vector spaces over an arbitrary field K . We describe the kernel of A : it is a direct sum of two natural subspaces whose dimensions are equal to
and
, where c = gcd (m+1,n+1) - 1 . We show that there are bases to both vector spaces, with entries equal to 0,1 and -1 . When K = Z/(2), the kernel elements of these subspaces are described by rectangular tilings of a special kind. As a corollary, we count
the number of tilings of a rectangle of integer sides with a specified set of tiles. 相似文献