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991.
[reaction: see text] Beta,gamma-unsaturated methyl ketones with electron-withdrawing groups at the gamma-position of the ene moiety undergo ODPM rearrangements and Norrish type I reactions on direct irradiation at 254 nm. The results are consistent with the involvement of alkene S(2) (pi,pi*) as reactive excited states in these processes. 相似文献
992.
Maria Carmela Lombardo Marco Sammartino Vincenzo Sciacca 《Comptes Rendus Mathematique》2005,341(11):659-664
In this Note we are concerned with the well-posedness of the Camassa–Holm equation in analytic function spaces. Using the Abstract Cauchy–Kowalewski Theorem we prove that the Camassa–Holm equation admits, locally in time, a unique analytic solution. Moreover, if the initial data is real analytic, belongs to with , and does not change sign, we prove that the solution stays analytic globally in time. To cite this article: M.C. Lombardo et al., C. R. Acad. Sci. Paris, Ser. I 341 (2005). 相似文献
993.
We show theoretically that photonic crystal membranes cause large variations in the spontaneous emission rate of dipole emitters, not only inside but also in the near field above the membranes. Our three-dimensional finite-difference time-domain calculations reveal an inhibition of more than five times and an enhancement of more than ten times for the spontaneous emission rate of emitters with select dipole orientations and frequencies. Furthermore, we demonstrate theoretically the potential of a nanoscopic emitter attached to the end of a glass fiber tip as a local probe for mapping the large spatial variations of the photonic crystal local radiative density of states. This arrangement is promising for on-command modification of the coupling between an emitter and the photonic crystal in quantum optical experiments. 相似文献
994.
Sa?a?NovakEmail author Susana?Maria?Henriques?Olhero José?Maria?Fonte?Ferreira Andreja?Zupan?i? 《Rheologica Acta》2004,43(5):559-566
Due to the hygroscopic nature of fine alumina powders, the presence of water may have a detrimental effect on the rheological properties of a suspension in melted paraffin for low-pressure injection moulding (LPIM). For this reason, a modification of the powder surface from hydrophilic to hydrophobic is essential for the production of high-quality moulded ceramics. In our paper, the efficiency of the application and the chemisorption of protective monomolecular layers of a long-chain carboxylic acid or its salts will be presented. The effect of the powder treatment on the shear viscosity and the viscoelastic properties of the suspensions for LPIM as well as on their stability in terms of the detrimental effect of water will be discussed.This paper was presented at the first Annual European Rheology Conference (AERC) held in Guimarães, Portugal, September 11-13, 2003. 相似文献
995.
Hydrogen-bonded banana liquid crystals 总被引:1,自引:0,他引:1
Gimeno N Ros MB Serrano JL de la Fuente MR 《Angewandte Chemie (International ed. in English)》2004,43(39):5235-5238
996.
Industrial methods for the production of optically active intermediates 总被引:15,自引:0,他引:15
Breuer M Ditrich K Habicher T Hauer B Kesseler M Stürmer R Zelinski T 《Angewandte Chemie (International ed. in English)》2004,43(7):788-824
Enantiomerically pure amino acids, amino alcohols, amines, alcohols, and epoxides play an increasingly important role as intermediates in the pharmaceutical industry and agrochemistry, where both a high degree of purity and large quantities of the compounds are required. The chemical industry has primarily relied upon established chemical methods for the synthesis of these intermediates, but is now turning more and more to enzymatic and biotechnological fermentation processes. For the industrial implementation of many transformations alternative methods are available. The advantages of the individual methods will be discussed herein and exemplified by syntheses of relevant compounds. 相似文献
997.
Maqueda M Gálvez A Bueno MM Sanchez-Barrena MJ González C Albert A Rico M Valdivia E 《Current protein & peptide science》2004,5(5):399-416
After the discovery of bacteriocin AS-48, a 70-residue cyclic peptide produced by Enterococcus faecalis subsp. liquefaciens, some naturally-occurring cyclic proteins from bacteria have been reported. AS-48 is encoded by the 68-kb pheromone-responsive plasmid pMB2, and the gene cluster involved in production and immunity has been identified and sequenced. This peptide exerts a bactericidal action on sensitive cells (most of the Gram-positive and some Gram-negative bacteria). Its target is the cytoplasmic membrane, in which it opens pores, leading to the dissipation of the proton motive force and cell death, a mechanism similar to that proposed for the action of defensins or, most generally, cationic antibacterial peptides. This fact, together with its remarkable stability and solubility over a wide pH range, suggest that this bacteriocin could be a good candidate as a natural food preservative. The amino acid composition of purified AS-48 shows the absence of modified or dehydrated residues, making it clearly different from lantibiotics. Bacteriocin AS-48 also differs from defensins in that it does not contain cysteines and consequently no disulfide bridges, which makes is high stability even more remarkable. Composition analysis of AS-48 shows a high proportion of basic to acidic amino acids, conferring to this peptide a strong basic character, with an isoelectric point close to 10.5. Determination of the AS-48 structural gene DNA sequence, together with the sequences of AS-48 protease digestion fragments and mass spectrometry determinations, allowed us to determine unambiguously the cyclic structure of the molecule, being the first example of a posttranslational modification in which a cyclic structure arises from a "head-to-tail" linkage. We have solved the three-dimensional structure of AS-48 in solution, and it consists of a globular arrangement of five alpha-helices enclosing a compact hydrophobic core. Interestingly, the head-to-tail peptide link between Trp-70 and Met-1 lies in the middle of alpha-helix 5, which is shown to have a pronounced effect on the stability of the three-dimensional structure. Analysis of structure-function relationship allowed us to propose models to understand the aspects of the molecular function of AS-48. The purpose of this work is to review recent developments in our understanding about the biochemical and biological characteristics and structure of this unusual type of bacteriocin. 相似文献
998.
Eva?Chinarro Ana?Maria?Segad?esEmail author José?Ramón?Jurado 《Journal of Solid State Electrochemistry》2004,8(9):606-610
Metals and ceramics can behave as active electrocatalyst materials, particularly in hydrocarbon oxidation in anodic reaction fuel cells. Combustion synthesis is a very reliable, fruitful and rapid synthesis method to produce metals, ceramics and cermets with low particle size and high specific surface area. This work describes the preparation of nanoparticle Pt/Ru alloys, ceramic perovskites such as Sm0.95CoO3–, and Sm0.95CoO3–/Pt cermets, and shows how promising these materials can be in the role of electrochemically active materials.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003 相似文献
999.
1000.