Dynamics and bistability in a reduced model of the lac operon

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.     

Nakiterpiosin and nakiterpiosinone, novel cytotoxic C-nor-D-homosteroids from the Okinawan sponge Terpios hoshinota

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).     

The first bowl-shaped stable neutral radical with a corannulene system: synthesis and characterization of the electronic structure

[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.     

Copolymerization of Carbon Monoxide with Cyclic Compounds Catalyzed by Transition Metal Complexes

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.     

Acoustic characteristics of an electrodynamic planar digital loudspeaker

In this paper, an electrodynamic planar loudspeaker driven by a digital signal is experimentally discussed. The digital loudspeaker consists of 22 voice coils, 11 permanent magnets, a diaphragm with streamlined sections molded in plastic, and a suspension made of handmade Japanese paper between the diaphragm and the frame. First, the acoustic responses are affected by the arrangement of the voice coils, so an asymmetric arrangement is studied. This asymmetric arrangement is designed to obtain as flat a frequency response to an analog signal as possible. This arrangement is compared with a symmetric one and results show that the flatness of the frequency response around 1 kHz and 4 kHz is improved and that the sound reproduction band is from 40 Hz to 10 kHz. Second, to evaluate the acoustic responses to a digital signal, the digital loudspeaker is driven with a pulse code modulation signal. Results show that the digital loudspeaker can reproduce pure sound with a total harmonic distortion of less than 5% from 40 Hz to 10 kHz, exceeding this value only in a narrow frequency band near 4 kHz. This digital loudspeaker was demonstrated to have good linearity over its dynamic range of 84 dB.     

Direct observations of freeze-etching processes of ice-embedded biomembranes by atomic force microscopy

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.     

N-tert-Butylbenzenesulfenamide-catalyzed oxidation of alcohols to the corresponding carbonyl compounds with N-chlorosuccinimide

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.     

An improved large scale synthesis of 1,4-anhydro-4-thio-d-ribitol

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.     

Four guaianolides from Sinodielsia yunnanensis

Four new guaianolides, sinodielides A-D (1-4), were isolated from Sinodielsia yunnanensis WOLFF together with a known polyacetylene, falcarindiol, and two known coumarins, bergapten and scopoletin. Their structures were established by spectral and X-ray analyses.