Ab-initio SCF potential energy surfaces for the nucleophilic attack of hydride on coordinated carbon monoxide

Ab-initio SCF calculations are reported for the nucleophilic addition of the hydride ion H⁻ on the iron pentacarbonyl Fe(CO)₅ complex. The stereochemistry of the attack has been established from the determination of two potential energy surfaces. The reaction is shown to be highly exothermic and with no activation barrier. This activation of CO towards the nucleophilic addition is rationalized in terms of molecular orbital interactions.     

Structure and properties of a model of deoxyheme,an ab initio SCF calculation

Ab initio SCF calculations have been used to study the structure and the electronic properties of four- and five-coordinate Fe(II) porphyrins. The following systems have been considered: FeP (P = porphine dianion) (S = 1 and S = 2) and Fe(NH₂)₄ (S = 1) as four-coordinate systems. FePNH₃, Fe(NH₂)₄NH₃ and Fe(NH₂)₄1m as models of the deoxyheme (S = 2). Fe(NH₂)₄SH (S = 2) as a model of the reduced cytochrome P450. The basis sets used are either of the split-valence type or of double-zeta quality. The ground-state electronic configurations have been assigned. The potential energy curve of FePNH₃, as a function of the out-of-plane displacement of the iron atom, has a minimum for a displacement of 0.32 Å. a value significantly smaller than the accepted value of 0.6 Å in human deoxyhemoglobin. The out-of-plane displacement is larger, by ≈ 0.2 Å, when a mercaptide ligand replaces the ammonia or imidazole ligand. The electric field gradient tensor, the quadrupole splitting and the asymmetry parameter have been calculated and compared with the experimental values derived from the ⁵⁷Fe Mössbauer spectra.     

An improved semi-empirical Hückel-method for the study of physico-chemical properties of aromatic systems

This work describes an improved semi-empirical Hückel-method for the study of physicochemical properties of aromatic systems. Coherent heteroatomic coulomb- and exchange parameters are calculated according to McWeeny's theory [30]. The variation of the exchange integrals _ijwith bond length is introduced in the calculations by means of an iterative selfconsistent -procedure. A new iteration function for the aromatic C=C link and a general method to obtain similar relations for several heteroatomic bonds are described. A simple computer program calculating various molecular properties is presented.

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Zusammenfassung Eine Hückelmethode für aromatische Systeme mit Heteroatomen wird beschrieben, wobei die Heteroparameter nach McWeeny [30] berechnet werden. Eine neue selbstkonsistente -Methode wird angegeben. Ein entsprechendes Programm zur Berechnung verschiedener Molekülgrößen ist bei den Autoren erhältlich.

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Résumé Ce travail décrit une méthode LCAO semi-empirique améliorée dans l'approximation de Hückel. Les paramètres hétéroatomiques (intégrales coulombiennes et d'échange) sont déterminés de façon cohérente par la théorie de McWeeny [30]. La variation des intégrales d'échange _ijavec la longueur des liaisons est introduite dans les calculs à l'aide d'une méthode itérative. Une nouvelle fonction d'itération est décrite pour le lien C=C aromatique et une méthode générale d'obtention de fonctions similaires pour diverses liaisons hétéroatomiques est donnée. Un programme pour calculatrice électronique permettant l'obtention de diverses propriétés moléculaires est décrit.

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Chargé de Recherches of the Fonds National Belge de la Recherche Scientifique.     

MISTRAL: A new program for precise atomic mass determinations of nuclides far from stability

The MISTRAL project (Mass measurements at ISolde using a Transmission RAdiofrequency spectrometer on-Line) is scheduled to begin experiments towards the end of 1996. With high resolution (10⁵), potentially high accuracy (5×10^–7) and excellent sensitivity (10 s^–1), the MISTRAL spectrometer promises to provide needed mass measurements in regions of very-short lived nuclei. The spectrometer operation principles are described and a discussion concerning the interest in using highly charged ions is presented.CSNSM, Orsay; IAP, Bucharest; PPE Division, CERN; GSI, Darmstadt; University of Mainz; McGill University, Montreal; IPN, Orsay; Chalmers University, Göteborg; University of Giessen and the ISOLDE Collaboration.     

Magnetic resonance imaging measurements of vascular permeability and extracellular volume fraction of breast tumors by dynamic Gd-DTPA-enhanced relaxometry

Vascular permeability (k(ep), min(-1)) and extracellular volume fraction (v(e)) are tissue parameters of great interest to characterize malignant tumor lesions. Indeed, it is well known that tumors with high blood supply better respond to therapy than poorly vascularized tumors, and tumors with large extracellular volume tend to be more malignant than tumors showing lower extracellular volume. Furthermore, the transport of therapeutic agents depends on both extracellular volume fraction and vessel permeability. Thus, before treatment, these tissue parameters may prove useful to evaluate tumor aggressiveness and to predict responsiveness to therapy and variations during cytotoxic therapies could allow to assess treatment efficacy and early modified therapy schedules in case of poor responsiveness. As a consequence, there is a need to develop methods that could be routinely used to determine these tissue parameters. In this work, blood-tissue permeability and extracellular volume fraction information were derived from magnetic resonance imaging dynamic longitudinal relaxation rate (R(1)) mapping obtained after an intravenous bolus injection of Gd-DTPA in a group of 92 female patients with breast lesions, 68 of these being histologically proven to be with carcinoma. For the sake of comparison, 24 benign lesions were studied. The measurement protocol based on two-dimensional gradient echo sequences and a monoexponential plasma kinetic model was that validated in the occasion of previous animal experiments. As a consequence of neoangiogenesis, results showed a higher permeability in malignant than in benign lesions, whereas the extracellular volume fraction value did not allow any discrimination between benign and malignant lesions. The method, which can be easily implemented whatever the imaging system used, could advantageously be used to quantify lesion parameters (k(ep) and v(e)) in routine clinical imaging. Because of its large reproducibility, the method could be useful for intersite comparisons and follow-up studies.     

Algorithms for ordering unorganized points along parametrized curves

Letn points be given on several arcs of curve. The equations of these curves are unknown. We give two algorithms, based on geometrical criteria, that allow to determine an order on these points: the order given by the curvilinear abscissa. The computational complexity of these algorithms is inO(n logn) and the space complexity inO(n).     

Combien existe-t-il d’octaèdres dont les longueurs des arêtes sont données?

Résumé Nous montrons que, en général, il existe au plus huit octaèdres différents dont les longueurs des arêtes sont données. Nous donnons un exemple pour lequel ce nombre huit est atteint.

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We prove that, in general, there are at the most eight distinct octahedrons with given edge-lengths. We also give an example for which this number is reached.

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Logiciels de Calculus formels utilisés:Maple. CoCOA: Alessandro Giovini, Gianfranco Niesi, Department of Mathematics, University of Genova, viale Leon Battista Alberti 4, 16132 Genova, Italy.     

Selective complexation and transport of europium ions at the interface of vesicles

The aim of the present work was to design functionalized lipidic membranes that can selectively interact with lanthanide ions at the interface and to exploit the interaction between membranes induced by this molecular-recognition process with a view to building up self-assembled vesicles or controlling the permeability of the membrane to lanthanide ions. Amphiphilic molecules bearing a beta-diketone unit as head group were synthesized and incorporated into phospholipidic vesicles. Binding of Eu(III) ions to the amphiphilic ligand can lead to formation of a complex involving ligands of the same vesicle membrane (intravesicular complex) or of two different vesicles (intervesicular complex). The effect of Eu(III) ions on vesicle behavior was studied by complementary techniques such as fluorimetry, light scattering, and electron microscopy. The formation of an intravesicular luminescent Eu/beta-diketone ligand (1/2) complex was demonstrated. The linear increase in the binding constant with increasing concentration of ligands in the membrane revealed a cooperative effect of the ligands distributed in the vesicle membrane. The luminescence of this complex can be exploited to monitor the kinetics of complexation at the interface of the vesicles, as well as ion transport across the membrane. By encapsulation of 2,6-dipicolinic acid (DPA) as a competing ligand which forms a luminescent Eu/DPA complex, the kinetics of ion transport across the membrane could be followed. These functional vesicles were shown to be an efficient system for the selective transport of Eu(III) ions across a membrane with assistance by beta-diketone ligands.