Biochemical characterization of malate synthase G of P. aeruginosa

Background  

Malate synthase catalyzes the second step of the glyoxylate bypass, the condensation of acetyl coenzyme A and glyoxylate to form malate and coenzyme A (CoA). In several microorganisms, the glyoxylate bypass is of general importance to microbial pathogenesis. The predicted malate synthase G of Pseudomonas aeruginosa has also been implicated in virulence of this opportunistic pathogen.     

A biosensing model system: selective interaction of biotinylated PPEs with streptavidin-coated polystyrene microspheres

Formation of a highly fluorescent composite formed from the biotinylated PPE 3 and streptavidin covered polystyrene microspheres is reported.     

Reactions of a Transient Carbonyl(chloro)(hydrido)ruthenium(II) Complex with Ethylene,Alkynes, and CO; Chemistry of the New Anion [Ru2(CO)4Cl5]−

A 97% yield of [Ru ₃ (CO) ₁₂ ] (2) in 20 minutes is obtained simply by treating [Ru(CO)₃Cl₂(thf)] with KOH under CO (1 atm). The reduction to Ru⁰ can be explained in terms of the facile reductive elimination of HCl from the transient hydrido complex 1 . Though elusive, the latter can be intercepted by olefins or alkynes to produce trappable alkyl or alkenyl complexes such as 3 and 4 .█ denotes a vacant coordination site.     

Rapid assembly of the bicyclo[5.3.1]undecenone core of penostatin F: a successive Diels-Alder/Claisen reaction strategy with an efficient stereochemical relay

The first synthesis of the tricyclic core of Penostatin F (1) using a stereocontrolled Diels-Alder reaction and a Claisen rearrangement in succession has been achieved in nine steps from commercially available methyl acetoacetate and (E)-2-decenal. Penostatin F is a metabolite isolated from a fungal strain of Penicillium sp., OUPS-79, separated from the marine alga Enteromorphia intestinalis and exhibits significant cytotoxicity against cultured P388 Leukemia cells (ED(50) = 1.4 micromol/mL). [reaction: see text]     

Development of multianalyte sensor arrays composed of chemically derivatized polymeric microspheres localized in micromachined cavities.

The development of a chip-based sensor array composed of individually addressable polystyrene-poly(ethylene glycol) and agarose microspheres has been demonstrated. The microspheres are selectively arranged in micromachined cavities localized on silicon wafers. These cavities are created with an anisotropic etch and serve as miniaturized reaction vessels and analysis chambers. A single drop of fluid provides sufficient analysis media to complete approximately 100 assays in these microetch pits. The cavities possess pyramidal pit shapes with trans-wafer openings that allows for both fluid flow through the microreactors/analysis chambers and optical access to the chemically sensitive microspheres. Identification and quantitation of analytes occurs via colorimetric and fluorescence changes to receptor and indicator molecules that are covalently attached to termination sites on the polymeric microspheres. Spectral data are extracted from the array efficiently using a charge-coupled device allowing for the near-real-time digital analysis of complex fluids. The power and utility of this new microbead array detection methodology is demonstrated here for the analysis of complex fluids containing a variety of important classes of analytes including acids, bases, metal cations, metabolic cofactors, and antibody reagents.     

The Ambivalent Chemistry of a Free Anionic N‐Heterocyclic Carbene Decorated with a Malonate Backbone: The Plus of a Negative Charge

The anionic heterocycle “[maloNHC]^?”, ([ 1 ]^?), is the archetype of a growing family of N‐heterocyclic carbenes incorporating an anionic backbone; here, a malonate group. A comprehensive experimental exploration of its chemistry as a free entity (in the form of its lithium salt [ 1 ] ? Li) is presented, and rationalized using DFT calculations at the B3LYP/6‐31+G** level of theory. For the sake of comparison, similar computations were performed on other representative carbene types. Reactions of [ 1 ]?Li with a broad series of electrophilic reagents were used to ascertain its intrinsic nature as a nucleophilic carbene. Unexpectedly, [ 1 ]?Li was also seen to react with the nucleophilic tert‐butylisocyanide, to give an anionic ketenimine, which could be subsequently derivatized, either into an imine by protonation of the ketenimine moiety, or into a neutral ketenimine by alkylation of the intracyclic malonate moiety. Further experiments on the electrophilic behavior of [ 1 ]?Li revealed its unexpected reactivity toward p‐chlorobenzaldehyde, resulting in a formal C? H activation and the first structurally characterized keto‐tautomer of the Breslow intermediate. Finally, [ 1 ]?Li remarkably activates polar E? H bonds, including N? H bonds from ammonia and amines, Si? H bonds, and B? H bonds. Importantly, DFT calculations indicate the importance of counterion effects. In particular, the key to the observed reactivity appears to be a modulation of energy levels associated with a dynamic variability of the Li? O distance between the remote malonate group and the counterion.     

Catalytic Wittig Reactions of Semi‐ and Nonstabilized Ylides Enabled by Ylide Tuning

The first examples of catalytic Wittig reactions with semistabilized and nonstabilized ylides are reported. These reactions were enabled by utilization of a masked base, sodium tert‐butyl carbonate, and/or ylide tuning. The acidity of the ylide‐forming proton was tuned by varying the electron density at the phosphorus center in the precatalyst, thus facilitating the use of relatively mild bases. Steric modification of the precatalyst structure resulted in significant enhancement of E selectivity up to >95:5, E/Z.     

A mechanophysical phase transition provides a dramatic example of colour polymorphism: the tribochromism of a substituted tri(methylene)tetrahydrofuran-2-one

Background

Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed “tribochromism”. We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies.

Results

A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb – which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb.

Conclusions

Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice.

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Graphical abstract Graphical representation of the structural and colour change in the tribochromic compound (III).