Identification of 4-amino-4-deoxychorismate synthase as the molecular target for the antimicrobial action of (6s)-6-fluoroshikimate

(6S)-6-Fluoroshikimate has antimicrobial activity. The molecular basis of this effect had not been identified, but there was speculation that (6S)-6-fluoroshikimate is first converted in vivo into 2-fluorochorismate, which then could inhibit 4-amino-4-deoxychorismate synthase (ADCS). 2-Fluorochorismate was prepared from E-fluorophosphoenolpyruvate and erythose-4-phosphate by the sequential reactions of DAHP synthase, dehydroquinate synthase, dehydroquinase, shikimate dehydrogenase, EPSP synthase, and chorismate synthase. Inhibition studies on ADCS showed that it was inhibited rapidly and irreversibly by 2-fluorochorismate. Electrospray mass spectrometry of the inactivated enzyme showed an additional mass of 198 +/- 10 Da. A novel peptide of 1087.6 Da was identified in the HPLC trace for the tryptic digest of 2-fluorochorismate-inactivated ADCS. Sequencing of this peptide by MS/MS showed that the peptide corresponded to residues 272-279 with a modification of 206.1 Da on Lys-274. This observation is particularly exciting in the context of a recent proposal for the catalytic mechanism of ADCS.     

Spongistatin synthetic studies. An efficient, second-generation construction of an advanced ABCD intermediate

[reaction: see text] A short, efficient, and stereocontrolled synthesis of (-)-4, an advanced ABCD subunit of the spongistatins, has been achieved. Central to the synthetic strategy is the multicomponent linchpin union of silyl dithianes with epoxides to access both the AB and CD fragments. Fragment coupling was then achieved via an efficient stereoselective aldol reaction. The linear sequence required 22 steps and proceeded in 4.0% overall yield.     

Molecular recognition and conductance in crown ethers

Crown ethers have the remarkable property of recognizing and binding specific metal cations in complex mixtures. We propose to combine molecular recognition with molecular electric conductance. The question we address is: can the event of binding a cation be sensed by a change in conductance? Specifically, we study a short molecular wire (MW) containing a crown-6 molecule connected via sulfur atoms to two gold atomic wires acting as metallic leads. Upon binding a cation, the density of states of the system is only slightly affected. This reflects the fact that the cation binding is largely electrostatic in nature and is accompanied by little electronic reorganization. Yet, the cationic binding does significantly lower conductance. We also identify strong interference affecting the conductance. A striking feature is the insensitivity of conductance to the type of ligand with the exception of the proton.     

Measuring intracellular calcium fluxes in high throughput mode

The measurement of intracellular calcium fluxes in real time is widely applied within the pharmaceutical industry to measure the activation of G-protein coupled receptors (GPCRhyp;s), either for pharmacological characterisation or to screen for new surrogate ligands. Initially restricted to G(q) coupled GPCRs, the introduction of promiscuous and chimeric G-proteins has further widened the application of these assays. The development of new calcium sensitive dyes and assays has provided sensitive, homogeneous assays which can be readily applied to high throughput screening (HTS). In this paper we describe the full automation of this assay type using a fluorometric imaging plate reader (FLIPR ) integrated into a Beckman/Sagian system to establish a simple robotic system that is well suited for the current medium throughput screening in this area of lead discovery. Using a recently completed HTS we discuss important determinants for FLIPR based screening, highlight some limitations of the current approach, and look at the requirements for future automated systems capable of keeping up with expanding compound files.     

A small angle X-ray scattering study of polyethylene fibres,using the two-dimensional correlation function

Summary By Fourier transformation of the intensity distribution in the small angle X-ray scattering diagram of a polymer fibre one obtains the corresponding two-dimensional correlation function. This is compared with correlation functions calculated on the basis of different models of the semi-crystalline morphology. Here, two steps may be distinguished: in the first, the correctness of a model may be checked with the aid of qualitative features, such as the relative positions and sizes of positive and negative areas; in the second a quatitative agreement is pursued by fixing the various parameters involved. This method was applied to cold drawn low density polyethylene before and after annealing at different temperatures. In the case of the non-annealed sample, the microparacrystalline model, discussed byHosemann andLoboda-Cackovic (J. Appl. Cryst.11, 540 (1978)) was found to give the best fit. Annealing at 80 °C or higher temperatures seems to lead to what has been indicated as a three-dimensional chess-board structure; it consists of fibrils in which crystalline and amorphous regions alternate in such a way that the crystalline regions in a fibril are adjacent to amorphous regions in neighbouring fibrils.

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Zusammenfassung Durch Fourier-Transformation der Intensitätsverteilung im Kleinwinkelstreudiagramm von Polymerfasern bekommt man die zugehörige zweidimensionale Korrelationsfunktion. Diese wird verglichen mit Korrelationsfunktionen, die für verschiedene Modelle der teilkristallinen Morphologie berechnet wurden.Hier werden zwei Schritte unterschieden. Im ersten Schritt wird die Richtigkeit eines Modells überprüft mit Hilfe von qualitativen Merkmalen, wie z. B. die relativen Lagen und Größen der positiven und negativen Bereiche. Im zweiten Schritt wird eine quantitative Übereinstimmung dadurch angestrebt, daß die unterschiedlich bezüglichen Parameter festgestellt werden. Diese Methode wurde angewandt bei kaltverstrecktem Polyäthylen niedriger Dichte, sowohl vor wie nach Temperung bei verschiedenen Temperaturen. Im Falle einer nicht getemperten Probe zeigte sich, daß das mikrokristalline Modell, vorgeschlagen von Hosemann und Loboda-Cackovic (J. Appl. Cryst.11, 540 (1978)), am besten zutrifft. Temperung bei 80°C oder bei höheren Temperaturen scheint zu einer Struktur zu führen, die als dreidimensionale Schachbrett-Struktur bezeichnet wurde. Sie besteht aus Fibrillen, in denen kristalline und amorphe Bereiche abwechseln, und zwar so, daß die kristallinen Bereiche in einer Fibrille nur von amorphen Bereichen in nächstliegenden Fibrillen umgeben sind.

     

Enzymatic hydrogen atom abstraction from polyunsaturated fatty acids

The oxidation of polyunsaturated fatty acids such as arachidonic and linoleic acid initiates a plethora of cell signaling pathways in animals and plants. The chemistry of the enzymatic oxidation has been investigated for several enzymes, most notably prostaglandin synthase and the lipoxygenases, revealing many surprises and impressive examples of enzymatic control of hydrogen atom abstraction and subsequent oxygenation.     

Olefin oxygenation by the hydroperoxide adduct of a nonheme manganese(IV) complex: epoxidations by a metallo-peracid produces gentle selective oxidations

The reactive intermediates and mechanisms of oxygenation of olefins by manganese complexes were investigated by treating olefins with newly synthesized [MnIV(Me2EBC)(OH)2](PF6)2 in the presence and absence of peroxide and by studying its catalytic epoxidation reaction in normal aqueous solution and, individually, with isotopically labeled H218O, 18O2, and H218O2. The manganese oxo species is not the reactive intermediate for the oxygen transfer process mediated by this manganese complex. A novel manganese(IV) peroxide intermediate, MnIV(Me2EBC)(O)(OOH)+, was captured by mass spectrometry and is proposed as the intermediate that oxygenates olefins in this catalytic system.     

Analysis of 19-norandrosterone in human urine by gas chromatography–isotope-dilution mass spectrometry: method adopted by LGC for participation in the Comité Consultatif pour la Quantité de Matière (CCQM) Pilot Study P68

The method used at LGC for analysis of “total” 19-norandrosterone (19-norandrosterone glucuronide plus “free” 19-norandrosterone) in urine for the Comité Consultatif pour la Quantité de Matière Pilot Study (CCQM-P68) is described. The analytical method used was a modified version of the method developed at the National Measurement Institute of Australia, which used a hydrolysis and derivatisation procedure first described by the German Sports University. This method is routinely used by World Anti-Doping Agency-accredited laboratories for sports drug testing. The main modifications made to the method were the use of 19-norandrosterone glucuronide as a calibration standard and 19-norandrosterone glucuronide-d4 as an isotopically labelled internal standard, and the use of a bench-top quadrupole gas chromatograph–mass spectrometer. The results produced by LGC (2.14 ± 0.15 ng g⁻¹ expanded uncertainty, coverage factor k = 2) were in excellent agreement with those from other participating national metrology institutes and thus further validates the exact-matching isotope-dilution mass spectrometric procedures used at LGC for a wide range of reference measurement applications, including measurement of ng g⁻¹ levels of steroids in a biological matrix.     

Catalyst economy. Part 2: Sequential metathesis—Kharasch sequences using the Grubbs metathesis catalysts

Sequential ring-closing metathesis (RCM)-Kharasch cyclizations are promoted by the Grubbs metathesis catalysts and provide rapid access to bicyclic lactones and lactams.     

Addition of grignard reagents to aryl acid chlorides: an efficient synthesis of aryl ketones

[chemical reaction: see text]. Direct addition of Grignard reagents to acid chlorides in the presence of bis[2-(N,N-dimethylamino)ethyl] ether proceeds selectively to provide aryl ketones in high yields. A possible tridentate interaction between Grignard reagents and bis[2-(N,N-dimethylamino)ethyl] ether moderates the reactivity of Grignard reagents, preventing the newly formed ketones from nucleophilic addition by Grignard reagents.