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131.
132.
Ralf Eiden Ralf Falter Barbara Augustin-Castro Heinz Friedrich Schöler 《Analytical and bioanalytical chemistry》1997,357(4):439-441
Distillation as a way of sample digestion has been combined with on-line RP C18 preconcentration and HPLC-UV-PCO-CVAAS (high
performance liquid chromatography – ultra violet – post column oxidation – cold vapour atomic absorption spectrometry) for
the determination of methylmercury at background levels in sediments, soils and fish tissue. To prove the accuracy of this
method, it was applied to sediment and fish tissue reference materials. The results correspond with the reference values within
their error ranges. Excellent recoveries (92–95%) were obtained for the sediment samples by means of the standard addition
method. The standard deviations of the sediment samples were within an acceptable range (7.2–12.5%), those of the fish samples
were substantially lower (3.4–5.0%). The detection limit is 0.04 ng/g for 1 g sample weight.
Received: 23 November 1995/Revised: 16 April 1996/Accepted: 20 April 1996 相似文献
133.
Molekulare Zusammensetzung von flüssigem Schwefel. Teil: 3 Quantitative Analyse im Bereich 115–350°C
Molecular Composition of Liquid Sulfur. Part 3: Quantitative Analysis in the Temperature Region 115–350°C Relative concentrations of S6, S7, S8, Sx (x > 8) and Sμ (insoluble sulfur) in equilibrium melts of elemental sulfur have been determined from i. r. and Raman spectra. At the freezing point (115°C) the melt consists of 0.6% S6, 2.8% S7, 1.5% Sx, and 95.1% S8. – The solubility of S7 in CS2 has been determined at −77 to −26°C; the solubilities of both S7 and S8 in CS2 are considerably enhanced by the presence of Sx. The thermal decomposition of S8 and Sμ formation from S6, S7, and Sx has been investigated. 相似文献
134.
J rg Enderlein Peter M. Goodwin Alan Van Orden W. Patrick Ambrose Rainer Erdmann Richard A. Keller 《Chemical physics letters》1997,270(5-6):464-470
We have developed a maximum likelihood estimator to distinguish between similar molecules at the single molecule level based upon fluorescence decay measurements. Time resolved fluorescence measurements for single Rhodamine 6G and tetramethylrhodamine isothiocyanate molecules in fluid flow are derived from time-correlated single photon counting. A maximum likelihood estimator is developed and applied to data from a mixture of molecules. Single molecules are identified and distinguished by their fluorescence time decays. Comparison is made between identification error rates and theoretical predictions. To our knowledge, this is the first reported example of single molecule identification by fluorescence decay in a mixture. 相似文献
135.
136.
We used a combination of theoretical and experimental methods to derive the spectroscopic properties of imidazolium-based ionic liquids. Vibrational frequencies, NMR chemical shifts, and quadrupole coupling constants react in comparable manner to changes in the chemical environment. This suggests that both the IR and the NMR spectroscopic properties reflect a similar type of electronic perturbation caused by hydrogen bonding. These relationships of the spectroscopic properties provide detailed information about structural complexes and may thus serve as good indicators of ion-pair formation. They also help to decide which spectroscopic tool is the most sensitive for investigating molecular interactions. The measurement of only one spectroscopic property allows the prediction of other properties that cannot be so easily measured. In some cases, this is the only way to obtain reliable coupling constants for deriving molecular correlation times from macroscopic NMR relaxation times, thus opening a new path for studying structure-dynamics relations in ionic liquids. 相似文献
137.
Solid state (19)F NMR revealed the conformation and alignment of the fusogenic peptide sequence B18 from the sea urchin fertilization protein bindin embedded in flat phospholipid bilayers. Single (19)F labels were introduced into nine distinct positions along the wild-type sequence by substituting each hydrophobic amino acid, one by one, with L-4-fluorophenylglycine. Their anisotropic chemical shifts were measured in uniaxially oriented membrane samples and used as orientational constraints to model the peptide structure in the membrane-bound state. Previous (1)H NMR studies of B18 in 30% TFE and in detergent micelles had shown that the peptide structure consists of two alpha-helical segments that are connected by a flexible hinge. This helix-break-helix motif was confirmed here by the solid-state (19)F NMR data, while no other secondary structure (beta-sheet, 3(10)-helix) was compatible with the set of orientational constraints. For both alpha-helical segments we found that the helical conformation extends all the way to the respective N- and C-termini of the peptide. Analysis of the corresponding tilt and azimuthal rotation angles showed that the N-terminal helix of B18 is immersed obliquely into the bilayer (at a tilt angle tau approximately 54 degrees), whereas the C-terminus is peripherally aligned (tau approximately 91 degrees). The azimuthal orientation of the two segments is consistent with the amphiphilic distribution of side-chains. The observed 'boomerang'-like mode of insertion into the membrane may thus explain how peptide binding leads to lipid dehydration and acyl chain perturbation as a prerequisite for bilayer fusion to occur. 相似文献
138.
Hans‐Dietrich Stachel Barbara Zimmer Eduard Eckl Katharina Semmlinger Wolfgang Weigand Ralf Wünsch Peter Mayer 《Helvetica chimica acta》2005,88(6):1208-1220
The 1,2‐dithiolosultam derivative 14 was obtained from the (α‐bromoalkylidene)propenesultam derivative 9 (Scheme 1). Regioselective cleavage of the two ester groups (→ 1b or 2b ) allowed the preparation of derivatives with different substituents at C(3) in the dithiole ring (see 27 and 28 ) as well as at C(6) in the isothiazole ring (see 17 – 21 ; Scheme 2). Curtius rearrangement of the 6‐carbonyl azide 21 in Ac2O afforded the 6‐acetamide 22 , and saponification and decarboxylation of the latter yielded ‘sulfothiolutin’ ( 30 ). Hydride reductions of two of the bicyclic sultams resulted in ring opening of the sultam ring and loss of the sulfonyl group. Thus the reduction of the dithiolosultam derivative 14 yielded the alkylidenethiotetronic acid derivative 33 (tetronic acid=furan‐2,4(3H,4H)‐dione), and the lactam‐sultam derivative 10 gave the alkylidenetetramic acid derivative 35 (tetramic acid=1,5‐dihydro‐4‐hydroxy‐2H‐pyrrol‐2‐one) (Scheme 3). Some of the new compounds ( 14, 22, 26 , and 30 ) exhibited antimycobacterial activity. The oxidative addition of 1 equiv. of [Pt(η2‐C2H4)L2] ( 36a , L=PPh3; 36b , L=1/2 dppf; 36c , L=1/2 (R,R)‐diop) into the S? S bond of 14 led to the cis‐(dithiolato)platinum(II) complexes 37a – c . (dppf=1,1′‐bis(diphenylphosphino)ferrocene; (R,R)‐diop={[(4R,5R)‐2,2‐demithyl‐1,3‐dioxolane‐4,5‐diyl]bis(methylene)}bis[diphenylphosphine]). 相似文献
139.
Ralf Wischnat Joachim RudolphRoman Hanke Roger KaeseAchim May Heidi TheisUndine Zuther 《Tetrahedron letters》2003,44(23):4393-4394
An efficient scalable synthesis of 2-halothiazolium-type peptide coupling reagents has been developed. The key step is the formation of the 2-bromothiazole scaffold through cyclization of α-thiocyanato ketones with hydrogen bromide. Using this method, the new coupling reagent 2-bromo-N-methylthiazolium bromide (BMTB) was synthesized. BMTB was tested in a difficult model coupling reaction of two sterically hindered N-methylated amino acids and showed higher activity than the well-established peptide coupling reagent HATU. 相似文献
140.
G. Keller I. Barzen R. Erz W. Dötter S. Ulrich K. Jung H. Ehrhardt 《Fresenius' Journal of Analytical Chemistry》1991,341(5-6):349-352
Summary It is shown that small variations of the deposition parameters during magnetron sputtering of tungsten carbide thin films may result in drastic changes of film properties. An increasing working gas pressure for example lowers stress and hardness values. Simultaneously, the texture of the WC1–x cristallites turns from 200 preferential orientation to 111, whereas the composition of the films does not change. In reactive sputtering with a tungsten target there is a narrow range from 2 to 3% C2H2 gas admixture to the working gas where the films are stochiometric (WC) and hard, and grain size and morphology are similar to that of non-reactively sputtered films. The generation of different crystallite structures and orientations in the range of 0–3% C2H2 admixtures are used to produce a multiphase thin film with extremely low crack propagation. 相似文献