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991.
We have studied the hydrogenase-catalyzed production of H, when either its natural electron mediator cytochrome c3 (c3 ) or the artificial mediator methyl viologen (MV) was reduced by illumination of proflavin (PF) in the presence of ethylenediaminetetraacetate (EDTA). The reduction rates of MV and c3 were comparable at equivalent concentrations of PF, taking into account the four redox sites of c3 . However, when the concentration of c3 exceeded that of PF, the reduction rate decreased. We explain this by light quenching. In the H2 -producing system, MV was more efficient than c3 as electron mediator when the initial reaction rates were compared. However, under certain conditions with MV, the rate of H2 production decreased rapidly with time of illumination, whereas with c3 it consistently remained stable. Possible reasons for this difference are discussed.
Severe inactivation of hydrogenase was observed in the absence of the primary electron donor EDTA. It is concluded that this inactivation is caused by the excited state of PF 相似文献
Severe inactivation of hydrogenase was observed in the absence of the primary electron donor EDTA. It is concluded that this inactivation is caused by the excited state of PF 相似文献
992.
Weber L Pumpenmeier L Stammler HG Neumann B 《Dalton transactions (Cambridge, England : 2003)》2004,(4):659-667
Reaction of one equivalent of the complexes [FeCp*(CO)2PnCl2] (Pn = P, As, Sb) with tetramethylcyclopentadienyllithium afforded compounds [FeCp*(CO)2[Pn(Cl)(C5Me4H)]]. Dehydrochlorination by means of tert-butyllithium led to decomposition. Only in the case of the phosphorus compound was evidence for the initial formation of a phosphaalkene given by 31P NMR spectroscopy. Similarly treatment of equimolar amounts of [FeCp*(CO)2PnCl2] with 2,7-di-tert-butyl-9-H-fluorenyllithium or 2,7-di-tert-butyl-9-trimethylsilylfluorenyllithium yielded the asymmetrically substituted ferriopnicogenanes [FeCp*(CO)2[Pn(Cl)-9-R-Fl*]] (Pn = P, As, Sb; R = H, Me3Si; Fl* = 2,7-di-tert-butylfluorenylidene). Dehydrohalogenation of [FeCp*(CO)2[Pn(Cl)-9-H-Fl*]] with lithium diisopropylamide resulted in the formation of the anticipated phosphaalkene [FeCp*(CO)2[P = Fl*]], whereas in the case of the arsenic and antimony derivatives the novel ferriopnicogenanes [FeCp*(CO)2[Pn(9-H-Fl*)2]] (Pn = As, Sb) were obtained as products. The new compounds were characterized by elemental analyses and spectra (IR, 1H, 13C, 29Si, 31P NMR). The molecular structures of [FeCp*(CO)2[Pn(Cl)(C5Me4H]]] (Pn = As, Sb), [FeCp*(CO)2[As(Cl)(9-Me3Si-Fl*)]] and [FeCp*(CO)2[Sb(9-H-Fl*]2] were elucidated by single X-ray diffraction analyses. 相似文献
993.
Buffoni Hall RS Bornman JF Björn LO 《Journal of photochemistry and photobiology. B, Biology》2002,66(1):13-20
The response of the lichen, Cladonia arbuscula (Wallr.) Flot. ssp. mitis (Sandst.) Ruoss to enhanced UV-B (280-315 nm) radiation was investigated with respect to: (a) changes in phenolic content; (b) differential pigment accumulation under visible and UV radiation with increasing distance from thallus apices; and (c) the internal distribution of UV-B radiation within the thallus measured with quartz optical fibres. In a short-term experiment, lichens were exposed for 7 days in a growth chamber to visible light with or without additional UV-B radiation. For a longer term experiment, lichens were grown outdoors under both natural UV radiation, and supplemental UV-A (315-400 nm)+UV-B provided by lamps. Controls were placed under filters that removed the radiation below 290 nm from the natural sunlight. The concentration of total phenolic compounds was measured spectrophotometrically at the termination of the experiments, in different parts of the lichen podetia. UV-exposed lichens showed increased accumulation of phenolics compared to those not grown under UV. At the termination of the long-term experiment, fibre optic measurements of the penetration of radiation into lichen thallus reflected the influence of growth under UV radiation, whereby UV was more strongly attenuated as compared to that in lichens not exposed to enhanced levels of UV-B radiation. Results indicated that in Cladonia, UV-B radiation induces the accumulation of phenolic compounds that may have a protective role. In addition, the morphological distribution of phenolic compounds was different under visible and supplemental UV-B radiation. Internal radiation measurements served to visualise the attenuation of radiation with thallus depth for different wavelengths in the UV-B waveband. 相似文献
994.
Lachenmeier DW Kroener L Musshoff F Madea B 《Rapid communications in mass spectrometry : RCM》2003,17(5):472-478
A new method combination, headspace solid-phase dynamic extraction coupled with gas chromatography/tandem mass spectrometry (HS-SPDE/GC/MS/MS), is introduced to determine drugs of abuse in hair samples. This highly automated procedure utilizes SPDE for pre-concentration and on-coating derivatization as well as GC and triple quadrupole MS/MS for selective and sensitive detection. All these steps, apart from washing and cutting of the hair samples, are performed without manual intervention on a robot-like autosampler.SPDE is a solventless extraction technique related to solid-phase microextraction (SPME). The analytes are absorbed from the sample headspace directly into a hollow needle with an internal coating of polydimethylsiloxane by repeated aspirate/dispense cycles.The HS-SPDE/GC/MS/MS procedure was applied to the analysis of methadone, the trimethylsilyl derivatives of cannabinoids and the trifluoroacetyl derivatives of amphetamines and designer drugs. The method was shown to be sensitive with detection limits between 6 and 52 pg/mg hair matrix and precision between 0.4 and 7.8% by the use of an internal standard technique. Linearity was obtained from 0.1-20 ng/mg with coefficients of correlation between 0.995 and 0.999.Compared with conventional methods of hair analysis, HS-SPDE/GC/MS/MS is easier to use, substantially faster, with the degree of sensitivity and reproducibility demanded in clinical and forensic toxicology. The main advantage of the SPDE technique in relation to SPME is the robustness of the capillary. 相似文献
995.
Lars Eriksson Roland Stenutz Gran Widmalm 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(6):o328-o329
The overall conformation of the title compound, C13H24O10, is described by the glycosidic torsion angles ?H (H1g—C1g—O2r—C2r) and ψH (C1g—O2r—C2r—H2r), which have values of 13.6 and 16.1°, respectively. The former is significantly different from the value predicted by consideration of the exo‐anomeric effect (?H~ 60°) and from that in solution (?H~ 50°), as determined previously by NMR spectroscopy. An intramolecular O3r—H?O2g hydrogen bond may help to stabilize the conformation in the solid state. The orientation of the hydroxymethyl group of the glucose residue is gauche–gauche, with a torsion angle ω (O5g—C5g—C6g—O6g) of ?70.4 (4)°. Both pyranose rings are in their expected chair conformations, i.e.4C1 for d ‐glucose and 1C4 for l ‐rhamnose. 相似文献
996.
997.
The porous structure of pulp fibres with different yields and its influence on paper strength 总被引:4,自引:0,他引:4
The porous structure of the interior of papermaking fibres is a well-known important property of the fibres. Changes of this structure will influence tensile and burst strength of paper formed from the fibres and a change in pore size of the pores within the fibre wall is also important for the ability of molecules to diffuse in and out of the fibre wall. Relevant examples of this latter effect are the removal of lignin during cooking and the addition of performance chemicals during papermaking. In this paper, pore sizes and the pore size distribution of unbleached softwood fibres have been studied. A well-characterised fibre material consisting of laboratory cooked spruce and pine pulp of various lignin contents was used. Pore size and pore size distribution were measured by studies of the relaxation behaviour of 2H in fibres saturated with 2H2O. Beside this the total and surface charge of the fibres were also measured together with strength properties of papers from unbeaten fibres. For both pulps, there is a maximum in pore radius at a yield around 46%. Calculations of fibre wall volume from water retention values and yield levels show that there is a discontinuity in pore radius as a function of the fibre wall volume around a yield of 51%. It is suggested that this discontinuity is caused by the breakdown of the hemicellulose/lignin matrix within the fibre wall at this yield level. The strength of the papers formed from the fibres shows a correlation with the surface charge of the fibres. Based on the change in surface charge with yield and the change in total charge with yield, this correlation is suggested to be due to an opening up of the external part of the fibre wall. This stresses the importance of the chemical composition and physical structure of the outer layer of the fibre wall. 相似文献
998.
At concentrations above 50 μg l?1, silver(I) is determined in nitric acid medium by means of potentiostatic deposition onto a platinum-fibre electrode and subsequent constant-current stripping in the sample or potentiometric stripping in a potassium permanganate medium. Interference from copper(II) is reduced by a pulsed potential procedure whereby copper deposited onto the fibre electrode is reoxidized intermittently. At concentrations below 50 μg l?1, silver(I) is determined by using a mercury-coated carbon-fibre electrode and constant-current stripping in acetonitrile containing 0.20 M perchloric acid. Potentiostatic deposition for 30 min yielded a detection limit of 0.24 μg l?1 silver(I) at the 3σ level. 相似文献
999.
Lars Esser Roberto Macchieraldo Roman Elfgen Melanie Sieland Bernd Michael Smarsly Barbara Kirchner 《Molecules (Basel, Switzerland)》2021,26(1)
To gain a deeper understanding of the TiCl solvation effects in multi-component ionic liquids, we performed ab initio molecular dynamics simulations of 1-butyl-3-methylimidazolium [CCIm], tetrafluoroborate [BF], chloride [Cl] both with and without water and titanium tetrachloride TiCl. Complex interactions between cations and anions are observed in all investigated systems. By further addition of water and TiCl this complex interaction network is extended. Observations of the radial distribution functions and number integrals show that water and TiCl not only compete with each other to interact mainly with [Cl], which strongly influences the cation-[BF] interaction, but also interact with each other, which leads to the fact that in certain systems the cation-anion interaction is enhanced. Further investigations of the Voronoi polyhedra analysis have demonstrated that water has a greater impact on the nanosegregated system than TiCl which is also due to the fact of the shear amount of water relative to all other components and its higher mobility compared to TiCl. Overall, the polar network of the IL mixture collapses by including water and TiCl. In the case of [Cl] chloride enters the water continuum, while [BF] remains largely unaffected, which deeply affects the interaction of the ionic liquid (IL) network. 相似文献
1000.
Multiresidue method for the gas-chromatographic analysis of some polychlorinated and polybrominated pollutants in biological samples 总被引:1,自引:0,他引:1
Bo Jansson Renate Andersson Lillemor Asplund ?ke Bergman Kerstin Litzén Kerstin Nylund Lars Reuterg?rdh Ulla Sellstr?m Ulla-Britt Uvemo Cajsa Wahlberg et al. 《Fresenius' Journal of Analytical Chemistry》1991,340(7):439-445
Summary A multiresidue method for the analysis of a number of chlorinated and brominated organic compounds in biological samples has been developed. The method includes gas-chromatographic analysis of hexachlorinated and hexabrominated benzenes (HCB and HBB), polychlorinated and polybrominated biphenyls (PCB and PBB), polychlorinated phenols (PCP), guaiacols (PCG), paraffins (CP) and naphthalenes (PCN), polybrominated diphenyl ethers (PBDE) and the pesticides DDT, polychlorinated terpenes (PCC or toxaphene), chlordanes, hexachlorocyclohexanes (HCH), aldrin and dieldrin. The use of charcoal column in the clean-up procedure makes it also possible to analyze the planar PCB congeners (pPCB). Cod muscle samples were spiked with 10 and 50 times the detection levels of the compounds to be analyzed. The mean recoveries, relative to internal standards, for 40 compounds or compound groups were between 51 and 120%. 相似文献