Pyochelin enantiomers and their outer-membrane siderophore transporters in fluorescent pseudomonads: structural bases for unique enantiospecific recognition

Pyochelin (Pch) and enantiopyochelin (EPch) are enantiomeric siderophores, with three chiral centers, produced under iron limitation conditions by Pseudomonas aeruginosa and Pseudomonas fluorescens , respectively. After iron chelation in the extracellular medium, Pch-Fe and EPch-Fe are recognized and transported by their specific outer-membrane transporters: FptA in P. aeruginosa and FetA in P. fluorescens . Structural analysis of FetA-EPch-Fe and FptA-Pch-Fe, combined with mutagenesis and docking studies revealed the structural basis of the stereospecific recognition of these enantiomers by their respective transporters. Whereas FetA and FptA have a low sequence identity but high structural homology, the Pch and EPch binding pockets do not share any structural homology, but display similar physicochemical properties. The stereospecific recognition of both enantiomers by their corresponding transporters is imposed by the configuration of the siderophore's C4' and C2' chiral centers. This recognition involves specific hydrogen bonds between the Arg91 guanidinium group and EPch-Fe for FetA and between the Leu117-Leu116 main chain and Pch-Fe for FptA. FetA and FptA are the first membrane receptors to be structurally described with opposite binding enantioselectivities for their ligands, giving insights into the structural basis of their enantiospecificity.     

Dynamical reweighting: improved estimates of dynamical properties from simulations at multiple temperatures

Dynamical averages based on functionals of dynamical trajectories, such as time-correlation functions, play an important role in determining kinetic or transport properties of matter. At temperatures of interest, the expectations of these quantities are often dominated by contributions from rare events, making the precise calculation of these quantities by molecular dynamics simulation difficult. Here, we present a reweighting method for combining simulations from multiple temperatures (or from simulated or parallel tempering simulations) to compute an optimal estimate of the dynamical properties at the temperature of interest without the need to invoke an approximate kinetic model (such as the Arrhenius law). Continuous and differentiable estimates of these expectations at any temperature in the sampled range can also be computed, along with an assessment of the associated statistical uncertainty. For rare events, aggregating data from multiple temperatures can produce an estimate with the desired precision at greatly reduced computational cost compared with simulations conducted at a single temperature. Here, we describe use of the method for the canonical (NVT) ensemble using four common models of dynamics (canonical distribution of Hamiltonian trajectories, Andersen thermostatting, Langevin, and overdamped Langevin or Brownian dynamics), but it can be applied to any thermodynamic ensemble provided the ratio of path probabilities at different temperatures can be computed. To illustrate the method, we compute a time-correlation function for solvated terminally-blocked alanine peptide across a range of temperatures using trajectories harvested using a modified parallel tempering protocol.     

On the spontaneous carboxylation of 1-butyl-3-methylimidazolium acetate by carbon dioxide

The formation of 1-butyl-3-methylimidazolium-2-carboxylate in the mixture of CO(2) with 1-butyl-3-methylimidazolium acetate under mild conditions (298 K, 0.1 MPa) has been put in evidence in the liquid phase using Raman and infrared spectroscopy complemented by DFT calculations and NMR ((1)H, (13)C, (15)N) spectroscopy.     

Interplay between an elusive 4-(isopropylamino)imidazol-2-ylidene and its isolable mesoionic tautomer, and associated reactivities

An elusive free 4-(isopropylamino)imidazol-2-ylidene is engaged in a tautomeric equilibrium with its mesoionic tautomer, 4-(isopropylamido)imidazolium, which displays the typical reactivity of a cyclic diaminocarbene; once coordinated to a Rh(I) centre, it undergoes a smooth 4e(-) oxidation of its backbone to yield an amido-amidino-carbene, a weak electron donor viable only in its complexed form.     

Benzimidazole-based anion receptors exhibiting selectivity for lactate over pyruvate

The influence of anions on tautomerism in benzimidazole containing anion receptors has been studied via a variety of techniques in both solution and the solid state. The results show that hydrogen bonding interactions between the receptors and guests have a significant effect of the nature of the tautomer present. The compounds show a preference for complexation of lactate over pyruvate.     

Internal quality controls applied in inductively coupled plasma mass spectrometry multi-elemental analysis in the second French Total Diet Study

In 2006, the French Food Safety Agency (AFSSA) started the second French Total Diet Study to assess exposure to essential and non-essential elements through dietary intake by analysing 1352 food samples. A total of 21 elements were analysed to date by Inductively Coupled Plasma Mass Spectrometry after closed vessel microwave digestion, in 660 samples corresponding to at half of the study. This work presents and discusses the results of the various internal quality controls (IQC) applied to ensure that the analytical procedure is correctly carried out, from digestion to analysis, and to enable analytical chemists to validate the results obtained. The whole IQC allows to estimate uncertainties according to elements and to select those that should be discarded from the study or only given as indicative values for elements that were not within defined quality limits.     

Functionalisation of fluorescent BODIPY dyes by nucleophilic substitution

The BODIPY chromophore can be easily modified by nucleophilic mono- or disubstitution of 3,5-dichloroBODIPY with O-, N-, S- and C-nucleophiles. Absorption and fluorescence spectral data of the new BODIPY derivatives are also reported.     

Synthesis and reactivity of uranium(IV) amide complexes supported by a triamidotriazacyclononane ligand

Reaction of [U{(SiMe2NPh)3-tacn}Cl] with LiNEt2 or LiNPh2 affords the corresponding amide compounds, [U{(SiMe2NPh)3-tacn}(NR2)] (R = Et (1), R = Ph (2)). The complexes have been fully characterized by spectroscopic methods and the solid-state structure of 1 was determined by single-crystal X-ray diffraction analysis. The six nitrogen atoms of the tris(dimethylsilylanilide)triazacyclononane ligand are in a trigonal prismatic configuration with the nitrogen atom of the diethylamide ligand capping one of the trigonal faces of the trigonal prism. Crystallization of 2 from CH3CN solution gave crystals of the six-membered heterocycle [U{(SiMe2NPh)3-tacn}{kappa2-(HNC(Me))2CC[triple bond]N}] (3). The reactivity of the amides was investigated. Both compounds undergo acid-base reactions with protic substrates such as HOC6H2-2,4,6-Me3, 3,5-Me2pzH (pz = pyrazolyl) and HSC5H4N to give the corresponding [U{(SiMe2NPh)3-tacn}X] (X = OC6H2-2,4,6-Me3 (4), 3,5-Me2pzH (5), kappa2-SC5H4N (6)) complexes. The solid-state structures of and were determined by single-crystal X-ray diffraction and revealed that the compounds are eight-coordinate with dodecahedral geometry.     

Détonabilité de mélanges stoechiométriques méthane–hydrogène–oxygène–azoteDetonation characteristics of stoichiometric mixtures of methane–hydrogen–oxygen–nitrogen

The goal of this Note is to determine the influence of H₂ addition on the detonation and detonability of stoichiometric CH₄/O₂/N₂ mixtures for three values of the nitrogen dilution $\beta ={\mathrm{N}}_2/{\mathrm{O}}_2$ ($\beta =0$ (oxygen); 2; 3.76 (air)) and also of the influence of the initial temperature. It is based on the measurement of the mean cell size of the steady self-sustained detonation in these mixtures, this characteristic length being representative of the mixture detonability. Results indicate that the detonability of the (CH₄H₂) mixture is mainly controlled by the heavier fuel, i.e., CH₄ and for instance the detonability of the mixture where 20% of CH₄ volume has been replaced by H₂ is nearly the same as that of the mixture where CH₄ is the only fuel. The influence of the initial temperature on the detonability depends on N₂ concentration. To cite this article: C. Matignon et al., C. R. Mecanique 334 (2006).