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It is known that the lac operon regulatory pathway is capable of showing bistable behavior. This is an important complex feature, arising from the nonlinearity of the involved mechanisms, which is essential to understand the dynamic behavior of this molecular regulatory system. To find which of the mechanisms involved in the regulation of the lac operon is the origin of bistability, we take a previously published model which accounts for the dynamics of mRNA, lactose, allolactose, permease and beta-galactosidase involvement and simplify it by ignoring permease dynamics (assuming a constant permease concentration). To test the behavior of the reduced model, three existing sets of data on beta-galactosidase levels as a function of time are simulated and we obtain a reasonable agreement between the data and the model predictions. The steady states of the reduced model were numerically and analytically analyzed and it was shown that it may indeed display bistability, depending on the extracellular lactose concentration and growth rate. 相似文献
105.
Two new cytotoxic compounds, nakiterpiosin (1) and nakiterpiosinone (2), were isolated from the Okinawan sponge Terpios hoshinota. Their structures were determined by spectroscopic analysis. The absolute stereostructure of 1 was also determined by a modified Mosher's method. Nakiterpiosin (1) and nakiterpiosinone (2) showed potent cytotoxicity against mouse lymphocytic leukemia cell (P388). 相似文献
106.
Morita Y Nishida S Kobayashi T Fukui K Sato K Shiomi D Takui T Nakasuji K 《Organic letters》2004,6(9):1397-1400
[structure: see text] A bowl-shaped neutral radical with a corannulene system has been designed and synthesized for the first time as a stable solid in air. An unequivocal characterization of the electronic properties of the radical shows that an appreciable amount of spin delocalization extends onto the corannulene unit's curved surface. 相似文献
107.
This article reviews recent discovery and progress of copolymerization of CO with cyclic monomers such as methylenecyclopropanes and propylene oxide promoted by transition metal complexes. 2-Aryl-1-methylenecyclopropanes react with CO in the presence of the Pd catalyst under mild conditions (1 atm, room temperature) to afford the polyketones. The reaction involves alternating insertion of the two monomers and opening of the three-membered ring of the cyclic monomers. Mechanistic study of the polymer growth revealed that 2-aryl-1-methylenecyclopropane undergoes 1,2-insertion into the Pd–acyl bond and subsequent -alkyl elimination of the cyclopropylmethylpalladium group. A bicyclic methylenecyclopropane, 7-methylenebicyclo[4.1.0]heptane, also reacts with CO in the presence of the Pd complexes with diimine ligands to afford the polyketone with six-membered rings in the repeating unit. The NMR spectra of the polyketone indicate the regulated tacticity of the monomer units. Co complexes and a mixture of Co and Ru complexes with carbonyl ligands promote the copolymerization of CO with propylene oxide in the presence of amine additives under CO pressure at 50 atm. Structures of the produced polyesters vary depending on the reaction conditions. 相似文献
108.
We have fabricated a cryogenic atomic force microscope that is designed for structural investigation of freeze-fractured biological specimens. The apparatus is operated in liquid nitrogen gas at atmospheric pressure. Freeze-fracturing, freeze-etching and subsequent imaging are carried out in the same chamber, so that the surface topography of a fractured plane is easily visualized without ice contamination. A controlled superficial sublimation of volatile molecules allows us to obtain three-dimensional views of ultrastructures of biological membranes. 相似文献
109.
N-tert-Butylbenzenesulfenamide (1)-catalyzed oxidation of various primary and secondary alcohols to the corresponding aldehydes and ketones was efficiently carried out by using N-chlorosuccinimide (NCS) in the coexistence of potassium carbonate and molecular sieves 4 Å at easy-to-control temperatures ranging from 0°C to room temperature. The present catalytic oxidation was performed without giving any damage to the functional groups in alcohols, and was particularly effective in the oxidation of alcohols that formed labile aldehydes because of its mild reaction conditions. Further, selective oxidation of primary hydroxy groups took place in 1-catalyzed oxidation of several diols. Mechanistic investigation suggested that the chlorination of the sulfenamide 1 by NCS led to the formation of a key species, N-tert-butylbenzenesulfinimidoyl chloride (2), which in turn oxidized alcohols in the presence of potassium carbonate to afford carbonyl products by accompanying regeneration of the catalyst 1. 相似文献
110.
Noriaki Minakawa Yuka KatoKoichi Uetake Daisuke KagaAkira Matsuda 《Tetrahedron》2003,59(10):1699-1702
An improved large scale synthesis of 1,4-anhydro-4-thio-d-ribitol (4) from d-ribose has been accomplished by combining the O-allyl and O-p-methoxybenzyl protecting groups. Compound 4 was obtained in 31% yield in eight steps with three chromatographic separations. 相似文献