Localized bond orbitals and the correlation problem

The stability of the results of the Perturbative Configuration Interaction using Localized Orbitals (PCILO) with respects to the choice of the hybrids and to the bond polarities is studied for several small strained molecules. The stability of 2^nd and 3^rd orders is quite satisfactory. The pertinence of the maximum overlap criterion to build hybrids is discussed.

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Zusammenfassung An verschiedenen kleinen Ringen wird die StabilitÄt der Ergebnisse einer Störungs-CI-Rechnung studiert, wobei bezüglich der Wahl der Hybride und der BindungspolaritÄten lokalisierte Orbitale benutzt werden. Die StabilitÄt der Ergebnisse für Störungen 2. und 3. Ordnung ist zufriedenstellend. Das Kriterium der maximalen überlappung zum Aufbau von Hybriden wird diskutiert.

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Résumé On étudie la stabilité des résultats de la méthode: Développement Perturbatif de la matrice d'Interaction de Configuration en base d'Orbitales de Liaison (PICOL) par rapport aux paramétres qui déterminent les orbitales de liaison hybridation et polarité pour plusieurs petites molécules contraintes. La stabilité aux 2é et 3é ordres est assez bonne. On discute les rapports du critére de Recouvrement Maximum avec les problémes énergétiques.

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NATO postdoctoral fellow

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Associé au CNRS.     

Determination of cobalt in plant digests by adsorption stripping voltammetry

The procedure involves adsorption of cobalt onto a static mercury drop as its dimethylgloximate complex (pH 9.3, adsorption potential ?0.70 V, adsorption time 2 min), followed by a d.c. cathodic scan, effecting reduction at ?1.15 V (SCE). Of the dominant electroactive trace elements in plants (Mn, Fe, Zn), only zinc interfered; it was masked by nitrolotriacetic acid (2 × 10^?4 M). The detection limit is 0.01 μg l^?1 cobalt in the digest; the relative standard deviation is 2.5% at 0.75 μg l^?1. Calibration is linear in the range 0–6.0 μg l^?1 cobalt. Results obtained by the voltammetric method, by electrothermal atomic absorption spectrometry and neutron activation analysis are compared for seven pasture samples containing 0.1–0.2 mg kg ^?1 cobalt. The activation method provides validation for the same preparation and voltammetric results.     

Réactivité d'analogues bihétérocycliques du phénanthrène,vis-à-vis des agents d'acétylation et de lithiation

The influence of annelaction and of the heteroatom upon the reactity of some haterocyclic analogus of phenanthrene with a pyrrole nucleos was tested toward acetylation and lithiation. If methy-l[1]benzofuro-[3,2-b]pyrrole was acetylated only in the 2-position, the sulfer and selenious analogues with [3,2-b] and [2,3-b] annelarion were acetylated in the -2 and -3 position. No reaction at all or only with poor yield (in the oxygenated series) was observed with butyllithium except on the case of l-methyl[1]benzoselenono[2,3-b]pyrrole, where the opening of the selenophene nucleos gives, after carbonation and action of diazomethane, l-methyl-2-methoxycarbony 1-3-(o-methylselenophenyl) pyrrole.     

Comparison of M[bond]S, M[bond]O, and M[bond](eta 2-SO) structures and bond dissociation energies in d6 (CO)5M(SO2)nq complexes using density functional theory

Density functional theory studies of the series of isomeric d(6) (pentacarbonyl)metal complexes (CO)(5)M(eta(1)-SO(2))(nq), (CO)(5)M(eta(1)-OSO)(nq)(), and (CO)(5)M(eta(2)-SO(2))(nq) (M = Ti-Hf, nq = 2-; M = V-Ta, nq = 1-; M = Cr -W, nq = 0; M = Mn-Re, nq = 1+; M = Fe-Os, nq = 2+) provide accurate structural modeling and quantitative prediction of the relative stabilities of the isomers. The eta(1)-S-bound complexes display planar SO(2) moieties that adopt staggered orientations with respect to the carbonyl ligands, in keeping with experimental observations. The OSO chain in the eta(1)-O-bound complexes generally adopts the u-shape with a staggered orientation. The dianions (CO)(5)(Ti-Hf)(eta(1)-OSO)(2-) differ in that the OSO chain adopts the eclipsed z-shape orientation. The eta(2)-SO(2) complexes exhibit a facial interaction and are stable only for anionic and neutral complexes, supporting the view that this motif involves substantial M --> SO(2) pi-back-bonding. The relative stabilities of the isomers generally follow u-shaped trends both across a row and down a family. This fits with qualitative ideas that the bond dissociation energies (BDEs) for the (CO)(5)M(SO(2))(nq) complexes track competition between relative hardness/softness of the metal fragment and its capacity for pi-back-bonding. Quantitatively, examination of BDEs by bond energy decomposition approaches suggests that electrostatic considerations dominate bonding for the eta(1)-SO(2) complexes and covalent effects dominate for the eta(2)-SO(2) species, while both are important for eta(1)-OSO complexes.     

Characterization of micros-ms dynamics of proteins using a combined analysis of 15N NMR relaxation and chemical shift: conformational exchange in plastocyanin induced by histidine protonations

An approach is presented that allows a detailed, quantitative characterization of conformational exchange processes in proteins on the micros-ms time scale. The approach relies on a combined analysis of NMR relaxation rates and chemical shift changes and requires that the chemical shift of the exchanging species can be determined independently of the relaxation rates. The applicability of the approach is demonstrated by a detailed analysis of the conformational exchange processes previously observed in the reduced form of the blue copper protein, plastocyanin from the cyanobacteria Anabaena variabilis (A.v. PCu) (Ma, L.; Hass, M. A. S.; Vierick, N.; Kristensen, S. M.; Ulstrup, J.; Led, J. J. Biochemistry 2003, 42, 320-330). The R1 and R2 relaxation rates of the backbone 15N nuclei were measured at a series of pH and temperatures on an 15N labeled sample of A.v. PCu, and the 15N chemical shifts were obtained from a series of HSQC spectra recorded in the pH range from 4 to 8. From the R1 and R2 relaxation rates, the contribution, Rex, to the transverse relaxation caused by the exchanges between the different allo-states of the protein were determined. Specifically, it is demonstrated that accurate Rex terms can be obtained from the R1 and R2 rates alone in the case of relatively rigid proteins with a small rotational anisotropy. The Rex terms belonging to the same exchange process were identified on the basis of their pH dependences. Subsequently the identifications were confirmed quantitatively by the correlation between the Rex terms and the corresponding chemical shift differences of the exchanging species. By this approach, the Rex terms of 15N nuclei belonging to contiguous regions in the protein could be assigned to the same exchange process. Furthermore, the analysis of the exchange terms shows that the observed micros-ms dynamics in A.v. PCu are caused primarily by the protonation/deprotonation of two histidine residues, His92 and His61, His92 being ligated to the Cu(I) ion. Also the exchange rate of the protonation/deprotonation process of His92 and its pH and temperature dependences were determined, revealing a reaction pathway that is more complex than a simple specific-acid/base catalysis. Finally, the approach allows a differentiation between two-site and multiple-site exchange processes, thus revealing that the protonation/deprotonation of His61 is at least a three-site exchange process. Overall, the approach makes it feasible to obtain exchange rates that are sufficiently accurate and versatile for studies of the kinetics and the mechanisms of local protein dynamics on the sub-millisecond time scale.     

Spectrophotometric determination of copper, nickel, cobalt, iron and manganese as their pyridine thiocyanates Simultaneous determination in mixtures

Spectrophotometric determinations of copper, nickel, cobalt, iron, and manganese, based on the chloroform extraction of the metal pyridine thiocyanates, have been investigated. Optimum conditions require the pH of the aqueous solution to be in the range about 5–8; tartaric acid is used to prevent precipitation of hydrous oxides. An excess of pyridine must be used because chloroform readily extracts pyridine from the aqueous solution. Results are improved by making the extraction from a solution of high ionic strength (2 or above), which is provided by magnesium nitrate. Perchlorate decreases the absorbance, but the effect is essentially constant over a perchlorate concentration range of 0.8 to 2M. Although the metal pyridine thiocyanates are extracted by benzene, substituted benzenes, and halogenated hydrocarbons, chloroform is superior to other solvents in extraction efficiency and in colour stability of the extracted species. An example is given of the simultaneous determination of copper, nickel, cobalt, and iron in the same solution. Anions that also form metal pyridine compounds must be absent.     

Dinuclear vanadium and tetranuclear iron complexes obtained with the open Wells–Dawson [Si2W18O66] tungstosilicate

Reaction of two transition metal cations M (M = V^V, Fe^III) on the open Wells–Dawson anion α-[{K(H₂O)₂}Si₂W₁₈O₆₆]^15– leads to dinuclear and tetranuclear complexes, respectively. The molecular anions [{KV₂O₃(H₂O)₂}(Si₂W₁₈O₆₆)]^11– and [{Fe₄(OH)₆}(Si₂W₁₈O₆₆)]^10– have been structurally characterized by single crystal X-ray diffraction. The oxo/hydroxometallic clusters [KV₂O₃(H₂O)₂]⁵⁺ and [Fe₄(OH)₆]⁶⁺ are included in the pocket between the two subunits of [Si₂W₁₈O₆₆]^16–. The Fe^III atoms of the iron complex can be reduced to Fe^II by a single four-electron step. To cite this article: N. Leclerc-Laronze et al., C. R. Chimie 9 (2006).     

Ready access to sialylated oligosaccharide donors

[formula: see text] Numerous glycoconjugates contain the disaccharide Neu5Ac alpha (2-->3)DGalp. An efficient way to incorporate this disaccharide into synthetic glycoconjugates is to develop a disaccharide building block. This communication reports a chemoenzymatic route to such a building block which requires as few as four steps. Some examples using more chemical steps are also presented, which increase the flexibility. These disaccharide donors were used to prepare synthetic trisaccharides.     

Column selection and optimization for sulfur compound analyses by gas chromatography

The analysis of sulfur-containing compounds using fused silica capillary columns and the Sulfur Chemiluminescence Detector has been investigated. This combination of an inert chromatographic system and a high sensitivity, selective detector provides significant advantages for the analysis of low levels of sulfur compounds in complex matrices over existing techniques. Capillary columns coated with thick films (1–4 μm) of methyl silicone stationary phase permit separation of most sulfur containing compounds and, when used with sub-ambient column temperatures, these columns can be used for the separation of sulfur gases. The effects of stationary film thickness, column length, and internal diameter for the measurement of sulfur compounds in hydrocarbon matrices has been determined.     

Prediction of selectivity for enantiomeric separations of uncharged compounds by capillary electrophoresis involving dual cyclodextrin systems

The single-isomer polyanionic cyclodextrin (CD) derivative heptakis-6-sulfato-beta-cyclodextrin (HSbetaCD) has been tested as chiral additive for the enantioseparation of non-steroidal anti-inflammatory drugs, such as fenoprofen, flurbiprofen, ibuprofen and ketoprofen, in capillary electrophoresis, using a pH 2.5 phosphoric acid-triethanolamine buffer in the reversed polarity mode. In most cases, the enantiomers of these acidic compounds, present in uncharged form at that pH, were only poorly resolved with HSbetaCD alone. However, the use of HSbetaCD in combination with the neutral CD derivative, heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMbetaCD), which has a particularly high enantioselectivity towards these compounds, has led to complete enantioresolution in reasonably low migration times in most cases. Affinity constants for the enantiomers with the two cyclodextrins were determined, using linear regression in a two-step approach. Affinity constants with the charged HSbetaCD were first calculated in single systems while those with the neutral TMbetaCD were determined in dual systems. Selectivity for the enantiomeric separation of these compounds in dual CD systems could be predicted using recently developed mathematical models.