A self-consistent reaction field model of solvation using distributed multipoles. I. Energy and energy derivatives

The self-consistent reaction field model developed previously by the authors in the case of single center multipole expansion of the electronic structure of the solute has been extended to the case of a distributed multipole expansion. Three different expansions have been tested and two of them have proved to be rapidly convergent. The performances of the code are illustrated by the density functional theory treatment of few test systems: guanine, cytosine, and cytosine hydrated with one and three water molecules. A robust fast computer code has been tested to get the electronic structure, the electrostatic contribution to the solute-solvent free energy of interaction, and the optimized molecular geometry in solution.     

Biosensors based on highly sensitive acetylcholinesterases for enhanced carbamate insecticides detection

This paper presents the construction of amperometric biosensors for the highly sensitive detection of carbamate insecticides based on the inhibition of acetylcholinesterase (AChE). This enzyme was immobilised by entrapment in an optimised sol-gel matrix on TCNQ-modified screen-printed electrodes. The enzyme activity was estimated by measuring the thiocholine produced by the enzymatic hydrolysis of the acetylthiocholine using TCNQ as mediator. Wild and genetically engineered AChEs from Drosophila melanogaster (Dm) were chosen for their high sensitivity towards insecticides, which substantially improves the LOD compared with cholinesterases from other sources. The wild type and three mutant enzymes were tested against three carbamate insecticides: carbaryl, carbofuran and pirimicard. The best LOD were obtained with the Y370A mutant for carbaryl (1 × 10⁻⁸ M), the E69W mutant for pirimicarb (2 × 10⁻⁸ M) and the I161V mutant for carbofuran (8 × 10⁻¹⁰ M). The biosensors were applied to the analysis of two potable water samples.     

Measurement of PCDDs, PCDFs, and non-ortho-PCBs by comprehensive two-dimensional gas chromatography-isotope dilution time-of-flight mass spectrometry (GC x GC-IDTOFMS)

Comprehensive two-dimensional gas chromatography with isotope-dilution time-of-flight mass spectrometry (GC × GC-IDTOFMS) was used to measure polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated dibenzofuran (PCDF), and coplanar polychlorinated biphenyl (cPCB) concentrations in ash, sediment, vegetation, and fish samples. The GC × GC capability was achieved by using a quad jet, dual stage, thermal modulator. Zone compression of the GC peaks from modulation resulted in a significant increase of the signal intensity over classical GC-IDTOFMS. The GC × GC column set used an Rtx-Dioxin 2 phase as the first dimension (¹D) and an Rtx-500 as the second dimension (²D). The chromatographic separation of the 17 PCDD/Fs and the 4 cPCBs was attained in ¹D except for 2,3,7,8-TCDD and CB126 for which deconvoluted ion currents (DIC) were required to be reported separately. The Rtx-500 phase separated the bulk matrix interfering compounds from the target analytes in ²D. The instrumental limit of detection (iLODs) was 0.5 pg for 2,3,7,8-TCDD. The calibration curves showed good correlation coefficients for all the compounds investigated in the concentration range of 0.5–200 pg. GC × GC-IDTOFMS results compared favorably to those from conventional isotope-dilution one-dimensional gas chromatography-high resolution mass spectrometry (GC-IDHRMS). The comprehensive mass analysis of the TOFMS further permitted the identification of other contaminants of concern in the samples.     

Selective catalysis with peptide dendrimers

Peptide dendrimers incorporating 3,5-diaminobenzoic acid 1 as a branching unit (B) were prepared by solid-phase synthesis of ((Ac-A(3))(2)B-A(2))(2)B-Cys-A(1)-NH(2) followed by disulfide bridge formation. Twenty-one homo- and heterodimeric dendrimers were obtained by permutations of aspartate, histidine, and serine at positions A(1), A(2), and A(3). Two dendrimers catalyzed the hydrolysis of 7-hydroxy-N-methyl-quinolinium esters (2-5), and two other dendrimers catalyzed the hydrolysis of 8-hydroxy-pyrene-1,3,6-trisulfonate esters (10-12). Enzyme-like kinetics was observed in aqueous buffer pH 6.0 with multiple turnover, substrate binding (K(M) = 0.1-0.5 mM), rate acceleration (k(cat)/k(uncat) > 10(3)), and chiral discrimination (E = 2.8 for 2-phenylpropionate ester 5). The role of individual amino acids in catalysis was investigated by amino acid exchanges, highlighting the key role of histidine as a catalytic residue, and the importance of electrostatic and hydrophobic interactions in modulating substrate binding. These experiments demonstrate for the first time selective catalysis in peptide dendrimers.     

Analyse de population dans les calculs LCAO: charges et moments atomiques

Résumé Les auteurs montrent que le moment dipolaire d'une molécule peut, avec une bonne approximation, se réduire à la somme vectorielle des moments atomiques et d'un moment de charges ponctuelles centrées sur les atomes. Les charges ainsi définies sont indépendantes du système d'axes de coordonnées.

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Population analysis by LCAO calculations: Charges and atomic moments

The authors show that the dipole moment of a molecule can be reduced to the vector sum of the atomic moments and of a moment due to point charges centred on atoms with a good accuracy. The so defined charges are independent of the coordinate system.

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Zusammenfassung Die Autoren zeigen, daß das Dipolmoment von Molekülen als Vektorsumme der Atommomente und eines Moments, das von Punktladungen an den Atomen herrührt, dargestellt werden kann. Die dabei definierten Ladungen sind unabhängig vom Koordinatensystem.

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Equipe de Recherche Associée au C.N.R.S. n 22.

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Remerciements. Ce travail a été en partie présenté au 2ème Colloque International des Chimistes Théoriciens d'expression latine (Paris, septembre 1970).Les auteurs remercient les participants à ce Colloque et en particulier Madame A. Pullman et Monsieur P. Claverie pour leurs remarques et leurs suggestions. Leur gratitude va également au Centre National de la Recherche Scientifique pour l'aide matérielle qu'il apporte à ces recherches.     

Spectrométrie de Masse d'Hétérocycles Azotés. X–Contribution à l'Etude du Comportement de ll'Ion Aniline et des Ions Aminopyridines avant Fragmentation par Perte de HCN

The study of the mass analysed ion kinetic energy spectra of deuterated derivatives of aniline, aminopyridines and 2-chloro-5-aminopyridine shows that prior to HCN loss, hydrogen scrambling does not occur for aminopyridines and is limited but noticeable for aniline. In the case of this last compound the extent of scrambling varies markedly for small variations in the energy of the ions studied, these variations being within the energy window corresponding to metastable ions. Furthermore, an examination of the mass analysed ion kinetic energy spectra of monodeuterated derivatives of aminopyridines leads to the rejection of the generally accepted mechanism for HCN loss from the molecular ions of these compounds.     

Galactose oxidase models: solution chemistry, and phenoxyl radical generation mediated by the copper status

Galactose oxidase (GO) is an enzyme that catalyzes two-electron oxidations. Its active site contains a copper atom coordinated to a tyrosyl radical, the biogenesis of which requires copper and dioxygen. We have recently studied the properties of electrochemically generated mononuclear Cu(II)-phenoxyl radical systems as model compounds of GO. We present here the solution chemistry of these ligands under various copper and dioxygen statuses: N(3)O ligands first chelate Cu(II), leading, in the presence of base, to [Cu(II)(ligand)(CH(3)CN)](+) complexes (ortho-tert-butylated ligands) or [(Cu(II))(2)(ligand)(2)](2+) complexes (ortho-methoxylated ligands). Excess copper(II) then oxidizes the complex to the corresponding mononuclear Cu(II)-phenoxyl radical species. N(2)O(2) tripodal ligands, in the presence of copper(II), afford directly a copper(II)-phenoxyl radical species. Addition of more than two molar equivalents of copper(II) affords a Cu(II)-bis(phenoxyl) diradical species. The donor set of the ligand directs the reaction towards comproportionation for ligands possessing an N(3)O donor set, while disproportionation is observed for ligands possessing an N(2)O(2) donor set. These results are discussed in the light of recent results concerning the self-processing of GO. A path involving copper(II) disproportionation is proposed for oxidation of the cross-linked tyrosinate of GO, supporting the fact that both copper(I) and copper(II) activate the enzyme.     

Aldehyde detection by chromogenic/fluorogenic oxime bond fragmentation

Amination of 4-nitrophenol, umbelliferone and 4-methylumbelliferone gave the corresponding oxyamines 1-3. These oxyamines react with aldehydes and ketones to form oximes. In the case of aliphatic aldehydes and electron-poor aromatic aldehydes, the oximes undergo base-catalyzed fragmentation in aqueous buffer in the presence of bovine serum albumin to give the parent phenols, which is the acyclic analog of Kemp's elimination reaction of 5-nitrobenzisoxazole 28. The process can be used as a spectrophotometric assay for formaldehyde under aqueous neutral conditions.     

DNA G-四链体的结构完整性对催化性质的影响

DNA酶中的G-四链体-血红素(G4-hemin)DNA酶结构具有较高的设计性和化学稳定性,因此格外受研究者关注.G-平面作为辅酶因子hemin的结合位点,不仅提供大π平面与hemin结合,而且其平面上的G碱基还可以充当近端配位基团与hemin进行配位.因此,研究G-平面完整性在G4-DNA酶体系中的作用具有重要意义....