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91.
Stefan Berger 《Tetrahedron》1981,37(8):1607-1611
The pH dependence of the 13C NMR spectra of phenolphthalein has been measured and interpreted. The spectrum of the neutral compound 1 is compared with the spectra of the dianion 4, the trianion5 and the carbocation7. At the first time spectroscopic evidence for the carbinol 6 is given. 相似文献
92.
Sabine Hanelt Birgit Braun Stefan Marx Hansjörg A. W. Schneider-Poetsch 《Photochemistry and photobiology》1992,56(5):751-758
We have sequenced cDNA and genomic clones coding for phytochrome of the fern Selaginella. On the amino acid level, this phytochrome shares sequence homologies with phytochromes of higher plants which range between 62 (phytochrome B of Arabidopsis) and 55 (56)% [phytochrome C of Arabidopsis (Avena)]. Introns in the Selaginella gene are short and occupy positions known from phytochrome sequences of higher plants. A rooted phylogenetic tree based on mutation distances puts Selaginella phytochrome closest to the hypothetical ancestor. A similar tree arises if the tree is constructed with partial sequences (about 200 amino acids) around the chromophore attachment site. An extension of this tree by sequences of other cryptogamic plants (Mougeotia, Ceratodon, Psilotum) shows all these sequences including those of the phytochromes B and C of Arabidopsis on a branch, well separated from the branch formed by phytochromes known to accumulate in etiolated plants. The rooted phytochrome phylogenetic tree, however, is difficult to reconcile with the fossil record. 相似文献
93.
Fourier-transform ion-cyclotron-resonance (FTICR) mass spectrometry has been used to uncover the mechanisms by which FeO+ dehydrates heptan-4-one ( 5a ) and nonan-5-one ( 6a ) in the gas phase. The study of isotopomeric ketones provides evidence that H2O loss is not due to a 1,1-elimination, thus ruling out the intermediacy of high-valent iron-carbene species. Rather, H2O is generated in a formal 1,2-elimination involving the ω/ω ? 1 positions of the alkyl chain (‘remote C? H bond activation’). In the consecutive alkene/H2O elimination, the olefins (ethylene from 5a and propene from 6a ) originate from the terminal part of one alkyl chain, and the H-atom is transferred to the FeO+ moiety in the course of this process, builds up together with an H-atom from the ω/ω-1 position of the other alkyl chain the H2O molecule. In either case, the O-atom of H2O is provided by the FeO+ species. 相似文献
94.
95.
Improving the catalytic activity of Candida antarctica lipase B by circular permutation 总被引:3,自引:0,他引:3
Lipases (EC 3.1.1.3) play an important role in asymmetric biocatalysis. Tailoring these enzymes to novel, unnatural substrates is one of the primary challenges of protein engineering. We have used circular permutation, the intramolecular relocation of a protein's N- and C-termini, to explore the effects of altered active site accessibility and protein backbone flexibility on the catalytic performance of lipase B from Candida antarctica (CALB). Our combinatorial approach identified 63 unique functional protein permutants of CALB, and kinetic analysis of selected candidates indicated that a majority of enzyme variants either retained or surpassed wild-type CALB activity on a series of standard substrates. Beyond the potential benefits of these tailor-made lipases as new catalysts for unnatural substrates, our study validates circular permutation as a promising general method for lipase engineering. 相似文献
96.
A high-performance thin-layer chromatographic (HPTLC) method is described for the determination of tributyltin compounds (bis(tri-n-butyltin) oxide, TBTO, and tri-n-butyltin naphthenate, TBTN) and their degradation products (dibutyltin and monobutyltin compounds). The organotin compounds are extracted from wood with ethanol containing 0.5% (v/v) of hydrochloric acid and the separation of the defferent kinds of organotin compounds is achieved by thin-layer chromatography. The sample spots are measured using a scanning densitometer after decomposing the organotin compounds to inorganic tin by ultraviolet irradiation and visualization of the spots with pyrocatechol violet. Applications of the method to detection and quantification of organotin compounds in preservative solutions, in recently impregnated wood, and in wood samples from five-year-old window frames are described. 相似文献
97.
Buckart S Ganteför G Kim YD Jena P 《Journal of the American Chemical Society》2003,125(46):14205-14209
The change in the electronic structure of Au(n)- clusters induced by the exchange of an Au atom by hydrogen is studied using photoelectron spectroscopy. Au anion clusters react with one hydrogen atom but not with molecular hydrogen. The spectra of Au(n)- and Au(n-1)H- clusters show almost identical features for n > 2 suggesting that hydrogen behaves as a protonated species by contributing one electron to the valence pool of the Au(n)- cluster. This behavior is in sharp contrast to that of the commonly understood electronic structure of hydrogen in metals; namely, it attracts an electron from the conduction band of the metal and remains in an "anionic" form or forms covalent bonding. We discuss the influence of the unique electronic structure of H on the unusual catalytic behavior of Au clusters. 相似文献
98.
Englert BC Scholz S Leech PJ Srinivasarao M Bunz UH 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(3):995-1000
We describe the synthesis of two cyclobutadiene(cyclopentadienyl)cobalt-containing poly(p-phenylene ethynylene)s (PPEs) and their use as precursors for stable ceramic surface coatings. Organometallic PPEs were shaped into hexagonally ordered assemblies by using the breath-figure method. Such breath figures can be washed away with an appropriate solvent. Upon pyrolysis at 500 degrees C under either nitrogen or air, the bubble arrays persist as ceramics and are insoluble in organic solvents or water. The formed pyrolyzed bubble arrays were analyzed by optical and scanning electron microscopy, as well as energy dispersive X-ray microanalysis (EDX). The composition of the ceramic materials is discussed based on EDX and IR data. 相似文献
99.
E. Anklam Maria Rosa Bassani Thomas Eiberger Stefan Kriebel Markus Lipp Reinhard Matissek 《Analytical and bioanalytical chemistry》1997,357(7):981-984
Pyrolysis Mass Spectrometry (Py-MS) was used for the discrimination of cocoa butters from other vegetable fats. Mass spectra
ranging from 50 amu to 250 amu were analyzed by principal component analysis (PCA) and with neural nets. The application of
neural nets leads to a good discrimination between the two classes. Detailed analysis of the nets revealed that only the first
60 masses were used within the net. The use of PCA requires a careful selection of the number of masses included in the calculation.
Canonical variance analysis was applied to determine the significant masses. Optimal performance of PCA was observed only
using the first 22 significant masses. Most of these masses were different from the ones used by the neural net. It seems
that the mass spectra obtained by Py-MS contain sufficient information for the discrimination of pure cocoa butter from other
vegetable fats, but none of the methods seems to be able to extract all information available. Neural net provides a very
robust method for this task and no prior data selection was necessary.
Received: 13 May 1996 / Revised: 7 August 1996 / Accepted: 7 August 1996 相似文献
100.
Smith BM Lappi SE Brewer SH Dembowy S Belyea J Franzen S 《Langmuir : the ACS journal of surfaces and colloids》2004,20(4):1184-1188
The surface of a germanium internal reflectance element (IRE) was modified to bind 6X-histidine (his)-tagged biomolecules. The step-by-step surface modification was monitored via single-pass attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR). Initially an adlayer of 7-octenyltrimethoxysilane (7-OTMS) was formed on the Ge crystal through the surface hydroxyl groups, which were produced via ozonolysis of the Ge surface. The vinyl moiety of 7-OTMS was oxidized to a carboxylic acid, which was activated by 1,1'-carbonydiimidazole (CDI) to produce a labile imidazole. The labile imidazole that resulted from the CDI coupling was then displaced by the primary amine of nitrilotriacetic acid (NTA). Nickel sulfate was added to the system, and it coordinated with the three carbonyl groups and the nitrogen on NTA, thus leaving the ability of Ni to coordinate with two adjacent histidine residues. Binding of his-tagged biotin to nickel nitrilotriacetic acid (Ni-NTA) was observed by ATR-FT-IR spectroscopy. The surface modification method presented in this paper had minimal nonspecific binding, the Ni-NTA surface was reusable if stored properly, and complete removal of the organic surface was achievable. 相似文献