Crystallographic studies on the reaction of isopenicillin N synthase with an unsaturated substrate analogue

Isopenicillin N synthase (IPNS) catalyses conversion of the linear tripeptide delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-valine (ACV) to isopenicillin N (IPN), the central step in biosynthesis of the beta-lactam antibiotics. The unsaturated substrate analogue delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-vinylglycine (ACvG) has previously been incubated with IPNS and single product was isolated, a 2-alpha-hydroxymethyl isopenicillin N (HMPen), formed via a monooxygenase mode of reactivity. ACvG has now been crystallised with IPNS and the structure of the anaerobic IPNS:Fe(II):ACvG complex determined to 1.15 A resolution. Furthermore, by exposing the anaerobically grown crystals to high-pressure oxygen gas, a structure corresponding to the bicyclic product HMPen has been obtained at 1.60 A resolution. In light of these and other IPNS structures, and recent developments with related dioxygenases, the [2 + 2] cycloaddition mechanism for HMPen formation from ACvG has been revised, and a stepwise radical mechanism is proposed. This revised mechanism remains consistent with the observed stereospecificity of the transformation, but fits better with apparent constraints on the coordination geometry around the active site iron atom.     

Novel reversible ionic-to-covalent transition in a highly conducting TTF derivative

TTF produces two completely different phenomena with the different isomers of chloranil; with the para isomer it produces an insulating, alternating stack, ambient temperature neutral solid. While with the ortho isomer, it produces a more interesting system consisting of interconvertible conducting ionic and covalent components.     

Synthesis and diastereoselective functionalisation of tricarbonyl[(3a,7a)-octahydro-3-substituted-2-(η-phenyl)benzoxazole]chromium(0) complexes

Two novel tricarbonyl[(3a,7a)-octahydro-3-substituted-2-(η⁶-phenyl)benzoxazole]chromium(0) complexes have been synthesised in excellent yield and characterised by X-ray crystallography. Treatment of complex 1 with butyllithium followed by electrophile quench (MeI, MeSSMe, Me₃SiCl, Me₃SnCl, Ph₂PCl, PhCH₂Br, PhCHO) gave 1,2-disubstituted (η⁶-arene)Cr(CO)₃ complexes in excellent yield, by completely diastereoselective ortho-deprotonation. The structures of the products 5 and 10 have been established by X-ray crystallographic analyses.     

Ultrasound-promoted coupling of aryl and heteroaryl halides in the presence of lithium wire

Summary Although sonochemical reactions of bromobenzene and of 2-, 3- and 4-halogenopyridines with lithium wire inTHF solution yield the expected dimers, considerable dehalogenation also occurs, as monitored by¹³C-NMR spectroscopy. A pathway for the unusual formation of 4,4-bipyridyl from the 2-halogenopyridines is proposed. No dimers were detected from the reactions with 2-bromo-4-methylquinoline and 7-bromo-2,4-dimethylquinoline. The synthetic potential of their alternative facile sonochemical dehalogenation is propounded.

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Ultraschallunterstützte Kupplung von Aryl- und Hetarylhalogeniden in Gegenwart von Lithiumdraht. Die Bildung von dimeren und dehalogenierten Produkten

Zusammenfassung Obwohl die Ultraschall-Reaktionen von Brombenzol und 2-, 3- und 4-Halogenpyridinen mit Lithiumdraht inTHF die erwarteten Dimeren ergeben, tritt auch ein beträchtliches Ausmaß an Dehalogenierung ein. Es wird ein möglicher Mechanismus für die ungewöhnliche Bildung von 4,4-Bipyridyl aus 2-Halogenpyridin vorgeschlagen. Aus den Reaktionen von 2-Brom-4-methylchinolin und 7-Brom-2,4-dimethylchinolin resultierten keine dimeren Produkte. Das synthetische Potential der sonochemischen Dehalogenierung wird diskutiert.

     

A second look at a recent algebraic proof of nonlocality

We reply to M. Jones' criticisms of our proof that factorable stochastic hidden-variable theories are inconsistent with GHZ-style strict correlations predicted by quantum mechanics. We endorse Jones' clarifications of our definitions of the setsM and _TF. And we reply to his main criticism by invoking one additional locality-motivated assumption. Further, we show how this additional assumption can be weakened while preserving our proof's validity.1. Ironically, a draft version of our paper (March, 1990) used this assumption, and discussed its motivation and weakening. But we were then misled into thinking it unnecessary: So all credit to Jones for seeing the wood for the trees!2. This new assumption, AI (see below), alsorestores the link between TF, the strict correlations, and Shimony'sTF, which is severed by Jones' criticism (as he notes—cf. Ref. 1, pp. 162, 165 for our original claim of a link). Thus the correct result is that if TF, the strict correlations,and AI hold in any state, then so doesTF.     

Elimination of the concentration dependence in mass isotopomer abundance mass spectrometry of methyl palmitate using metastable atom bombardment

An important problem in mass isotopomer abundance mass spectrometry (MIAMS) is the dependence of measured mass isotopomer abundances on sample concentration. We have evaluated the role of ionization energy on mass isotopomer abundance ratios of methyl palmitate as a function of sample concentration. Ionization energy was varied using electron impact ionization (EI) and metastable atom bombardment (MAB). The latter generates a beam of metastable species capable of ionizing analyte molecules by Penning ionization. We observed that ionization of methyl palmitate by EI (70 eV) showed the greatest molecular ion fragmentation and also showed the greatest dependence of relative isotopomer abundance ratios on sample concentration. Ionization using the 3P2 and 3P0 states of metastable krypton (9.92 and 10.56 eV, respectively) resulted in almost no molecular ion fragmentation, and the isotopomer abundances quantified were essentially independent of sample concentration. Ionization using the 3P2 and 3P0 states of metastable argon (11.55 and 11.72 eV, respectively) showed molecular ion fragmentation intermediate between that of EI and MAB(Kr) and showed an isotopomer concentration dependence which was less severe than that observed with EI but more severe than that observed with MAB(Kr). The observed decrease in the dependence of isotopomer abundance on sample concentration with a decrease in molecular ion fragmentation is consistent with the hypothesis that proton transfer from a fragment cation to a neutral molecule is the gas phase reaction mechanism responsible for the concentration dependence. Alternative explanations, e.g., hydrogen abstraction from a neutral molecule to a molecular cation, is not supported by these results. Moreover, the MAB ionization technique shows potential for eliminating one source of error in MIAMS measurements of methyl palmitate, in particular, and of fatty acids methyl esters, in general.     

Bis(1‐ferrocenylbutane‐1,3‐dionato‐κ2O,O′)copper(II) and bis­(1,3‐diferrocenylpropane‐1,3‐dionato‐κ2O,O′)­copper(II)

The title compounds, [CuFe₂(C₅H₅)₂(C₉H₈O₂)₂], (I), and [CuFe₄(C₅H₅)₄(C₁₃H₉O₂)₂], (II), are four‐coordinate square‐planar copper(II) complexes with two bidentate 1‐ferrocenylbutane‐1,3‐dionate or 1,3‐diferrocenylpropane‐1,3‐dionate ligands, respectively. The copper ion in (I) lies on an inversion centre, with one‐half of the mol­ecule in the asymmetric unit, while in (II), there are two independent half mol­ecules in the asymmetric unit, with the copper ions also situated on inversion centres. The ferrocene substituents in (I) are in an anti arrangement. The mol­ecules assemble in the crystal structure in layers with ferrocene groups at the surface. The pairs of ferrocene substituents on each ligand in complex (II) are syn and these adopt an anti arrangement with respect to the pair on the other diketonate ligand. As found in (I), complexes assemble in a layered structure with ferrocene‐coated surfaces.     

Ion-specific repulsive interactions in colloidal silica dispersions evidenced through osmotic compression measurements and implication in frontal ultrafiltration experiments

Comparative osmotic compression experiments were performed on colloidal silica dispersions in the presence of various chloride salts at the same 0.01 M concentration with different counterions and highlighted the influence of ionic specificity on the resistance to water removal. These results were complemented with frontal ultrafiltration measurements that demonstrate modulation of the permeate flux according to the salt used.