PubMed Health⌕ Search

Biomedical subjects

Jacques Weber

Publications and source records attributed to Jacques Weber.

9 recordsLinked to original sources
PubMed2003

Connecting terminal carboxylate groups in nine-coordinate lanthanide podates: consequences on the thermodynamic, structural, electronic, and photophysical properties.

The hydrolysis of terminal (t)butyl-ester groups provides the novel nonadentate podand tris[2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine] (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 - 3H](3-) reacts with Ln(ClO(4))(3) to give the poorly soluble and stable podates [Ln(L13 - 3H)] (log(beta(110)) = 6.7-7.0, Ln = La-Lu). The isolated complexes [Ln(L13 - 3H)](H(2)O)(7) (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln-O(carboxamide) bonds are only marginally longer than the Ln-O(carboxylate) bonds in [Ln(L13 - 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the covalent Me-TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 - 3H)] (Phi(Eu)(tot)= 1.8 x 10(-3)) and [Tb(L13 - 3H)] (Phi(Eu)(tot)= 8.9 x 10(-3)) in water remain modest and strongly contrast with that obtained for the lanthanide luminescence step (Phi(Eu) = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet ((1)pi pi*) and triplet ((3)pi pi*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the "carboxylate effect" in lanthanide complexes are evaluated for programming predetermined properties in functional devices.

Journal Article↗
PubMed2003

The change of aromaticity along a Diels-Alder reaction path.

[reaction: see text] Quinodimethanes are highly reactive toward dienophiles since Diels-Alder cycloaddition results in an aromatic product. Density functional-based (13)C, (1)H NMR, NICS, and MO-NICS calculations indicate that the increase of aromatic character of the developing benzenoid ring along the reaction path is especially pronounced after the transition state is reached, even though the number of pi orbitals decreases. The forming aliphatic ring exhibits large ring current effects during the reaction.

Journal Article↗
PubMed2002

Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited: a new assignment of structural changes and crystal-field effects 25 years later.

Variable-temperature (1)H and (13)C NMR measurements of the D(3)-symmetrical triple-helical complexes [Ln(L1-2H)(3)](3)(-) (L1 = pyridine-2,6-dicarboxylic acid; Ln = La-Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)(3)](3)(-) (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P <==> M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La-Eu) and one for small lanthanides (Ln = Tb-Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A(0)2 parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb-Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La-Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.

Journal Article↗
PubMed2002

Communication between consultants and referring physicians: a qualitative study to define learning and assessment objectives in a specialty residency program.

BACKGROUND: Outpatient consultation constitutes a major part of medical practice. However, little is known about the skills which should be taught to residents in order for them to improve their consultant-referring physician relationships. PURPOSES: To specify the consultant skills which are required to ensure an effective communication between specialists and referring physicians. METHODS: A qualitative study based on (a) a literature search and (b) focus group interviews. RESULTS: Skills thus identified and described are classified in two groups: observable skills and principles/attitudes. CONCLUSIONS: The consensual specification of these abilities permits a greater efficacy in the teaching of consultant skills.

Ambulatory Care↗
PubMed2002

[Patient-doctor relationship in the irritable bowel syndrome. Results of a French prospective study on the influence of the functional origin of the complaints].

UNLABELLED: The attention given by the physician and the quality of the patient-physician relationship mainly determine the outcome of a consultation. Care seeking is a main characteristic of patients with functional bowel disorders, including irritable bowel syndrome (IBS), while patients with suspected organic disease (Org) would rather expect a precise answer about their condition. The aim of this study was thus to evaluate the outcome of the consultation with a gastroenterologist in IBS patients, as compared to a group of patients with suspected organic disease. PATIENTS AND METHODS: A prospective multicenter cross-sectional study "one given week" included 158 patients consulting for the first time 18 gastroenterologists. Patients were consulting for abdominal pain and were classified as IBS or Org on the basis of the physician's clinical impression. Questionnaires including 27 common questions were distributed to the patients and the physicians at the end of the consultation and filled separately. RESULTS: The diagnosis of IBS was done in 110 patients and that of Org in 48. Groups were comparable for the characteristics, the intensity and the frequency of attacks of abdominal pain. The index of satisfaction of the patients was not different between groups (IBS: 8.7 +/- 1.4; Org: 9.1 +/- 1.4; P=0.16). The intensity of abdominal pain was reported in the same range by the patient (IBS: 5.1 +/- 2.9; Org: 4.5 +/- 2.8) and the physician (IBS: 4.6 +/- 2.3; Org: 4.8 +/- 2.6) in both groups. Digestive and extra-digestive symptoms were quoted equally by physicians, whatever the group the patient belonged to. Information given by the physician on diagnosis and therapy were equally well perceived by patients of both groups. However, the physician tended to evaluate the efficacy of the prescribed therapy to be lower in IBS (7.1 +/- 2.0) than in Org patients (8.0 +/- 1.7; P<0.01). CONCLUSION: In this study, the consultation with a gastroenterologist is equally well perceived by IBS and Org patients and it seems to meet the expectancy of the patient in most cases. However, the physician appeared less confident in the efficacy of the therapy proposed to IBS patients.

Adult↗
PubMed2001

Introducing the objective structured clinical examination to a general practice residency programme: results of a French pilot study.

OSCE for evaluating clinical competence still remains limited in France. This study presents the results of the first experimental use of an OSCE as a formative assessment of French general practice trainees. Fifty trainees rotated through a circuit of 15 standardized patient-based OSCE cases. Differences in scores were determined by analysis of variance. Reliability was calculated with the coefficient alpha. Pearson correlations were used to determine the relationship between station scores and overall OSCE score. Written questionnaires based on Likert-type scales were used to measure OSCE feasibility. No difference was found between total session scores. Significant item-total score correlations were found for 12 of the 15 clinical problems. The reliability of the examination was 0.58. Most participants agreed that the clinical situations were realistic, simulated patients were believable and sampling of cases was representative of general practice. These data confirm the feasibility of OSCE in assessing performance of general practice trainees. Optimal training of observers should improve examination reliability. This study warrants further development to confirm its usefulness as a summative assessment tool.

Journal Article↗
PubMed1998

Structure and Reactivity of [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)](4+) Complexes. Quantum Chemical Calculations, X-ray Structural Characterization, and Raman Spectroscopic Measurements.

A series of compounds containing the [Mo(3)-&mgr;(3)S-(&mgr;S(2))(3)-(dtc)(3)](+) complex (dtc = diethyldithiocarbamate) with the anions I(-) (1), I(-) and Br(-) (2), S(2)(-) (3), ClO(4)(-) (4), NO(3)(-) (5), and SO(4)(2)(-) (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH(2)Cl(2) in the crystal having the composition [Mo(3)S(7)(dtc)(3)]I.0.5CH(2)Cl(2) (1a), which was in contradiction to the previous protocol. The corresponding ClO(4)(-) compound (4a) is isotypic. Crystal data: C(15.5)H(31)Cl(2)Mo(3)N(3)O(4)S(13), orthorhombic space group Aba2, a = 25.816(5) Å, b = 17.761(4) Å, c = 16.250(3) Å, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms S(ax) of the disulfido bridges. The Raman data showed a significant decrease of the S(eq)-S(ax) stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the S(eq)-S(ax) bond length and with a significant shortening of the X.S(ax) distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO(3)(-) and ClO(4)(-) the S(ax).X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I(-) and S(2)(-) significant covalency is observed. In addition, the general reaction of [Mo(3)S(7)](4+) complexes with a nucleophile was modeled. With regard to the side-on bonding of the &mgr;-S(2) groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to &mgr;(3)-S, are less pronounced.

Journal Article↗
PubMed1997

Trans- and Cis-Water Reactivities in d(6) Octahedral Ruthenium(II) Pentaaqua Complexes: Experimental and Density Functional Theory Studies(1)(,)(2).

The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H(2)O-ax)(H(2)O-eq)(4)L](2+) general formula. We have studied a series of complexes, where L = H(2)O, MeCN, Me(2)SO, H(2)C=CH(2), CO, and F(2)C=CH(2). We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E degrees '(+2/+3), increases with the pi-accepting capabilities of the ligands. For L = N(2), the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)-N(2) bond. The water exchange reactions have been followed by (17)O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H(2)O)(5)(H(2)C=CH(2))](2+) (DeltaV(ax)() and DeltaV(eq)() = +6.5 +/- 0.5 and +6.1 +/- 0.2 cm(3) mol(-)(1)) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F(2)C=CH(2) congruent with CO < Me(2)SO < N(2) < H(2)C=CH(2) < MeCN < H(2)O. The increase of the lability is accompanied by a decrease of the E degrees ' values, but no change was found in the calculated Ru-H(2)O(eq) bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N(2) << MeCN < H(2)O < CO < Me(2)SO < H(2)C=CH(2) < F(2)C=CH(2). A variation of the Ru-H(2)O(ax) bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru-H(2)O(ax) bond energies. It appears, also, that a decrease of the electronic density along the Ru-O(ax) bond and the increase of the lability can be related to an increase of the pi-accepting capability of the ligand. For L = N(2), the calculations have shown that the Ru(II)-N(2) bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N(2) ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H(2)O)(5)N(2)](2+) complex.

Journal Article↗