Gérard Pèpe

Rubredoxin from Desulfovibrio gigas. A molecular model of the oxidized form at 1.4 A resolution.

The crystal structure of rubredoxin from the sulfate-reducing bacterium Desulfovibrio gigas has been determined at 1.4 A resolution (1 A = 0.1 nm) by X-ray diffraction methods; starting with a model of the isostructural rubredoxin from Desulfovibrio vulgaris. Refinement of the molecular model has been carried out by restrained least-squares techniques and Fourier series calculations. The present model includes a formyl at the N-terminal end and 121 possible sites for solvent molecules with full or partial occupancy, which corresponds to the modeling of nearly all the solvent medium. The crystallographic R factor against the data with 10 A greater than d greater than 1.4 A with F greater than 2 sig(F), is 0.136; and R = 0.140 when all the data are considered. The estimated average root-mean-square (r.m.s.) error on the positional parameters is about 0.12 A. The overall structural features of this molecule are close to those of the two highly refined rubredoxins from Clostridium pasteurianum and D. vulgaris. Superposition of these two molecules on the rubredoxin from D. gigas shows in both cases an overall r.m.s. deviation of 0.5 A for the atoms in the main-chain and of 0.4 A for the atoms in the side-chains that make up the hydrophobic core. The iron atom is co-ordinated to four cysteine sulfur atoms forming an almost regular tetrahedron, with Fe-SG distances ranging from 2.27 A to 2.31 A and angles varying from 103 degrees to 115 degrees. The intramolecular hydrogen-bonding pattern is quite comparable to those found in other proteins refined at high resolution. All the polar groups are involved in hydrogen bonds: intramolecular, intermolecular or with solvent molecules. The main structural differences from the other rubredoxins are in the nature and the distribution of some of the charged residues over the molecular surface. The possible influence of several structural factors on the intramolecular and intermolecular electron transfer properties such as the NH...SG bonds, the solvent exposure of the redox center, and the aromatic core is discussed. The conservation, during evolution, of a ring of acidic residues in the proximity of the FeSG4 center suggests that this ring may be implicated in the recognition processes between rubredoxins and their functional partners.

Synthesis and unexpected photochemical behaviour of biphotochromic systems involving spirooxazines and naphthopyrans linked by an ethylenic bridge

Abstract Using the Wittig reaction, six new biphotochromic compounds have been prepared by the coupling of conveniently functionalised spiro[indoline–naphthoxazines] and naphthopyrans. Compared to molecules taken individually, unusual photochromic behaviour has been detected with compounds for which a double bond of Z -configuration links both entities by the 5 or 5′ positions of the naphthalenic parts. Indeed, in this case the thermally stable uncoloured and coloured forms (bistable system) can be photochemically converted acting as photochemical switches.

The influence of additives on the crystal habit of gibbsite

Abstract Crystallization of gibbsite (Al(OH) 3 ) is an important stage of the Bayer process, production of alumina from bauxite ores. In both pure or industrial supersaturated sodium aluminate solutions, gibbsite crystals are always agglomerated. In the present paper, we present results of a study concerning the influence of different polycarboxylic acids as crystal habit modifier for gibbsite. In pure solution, agglomerated hexagonal plates are observed. Whereas acicular and tabular morphologies are found in the presence of different additives. These results are discussed referring to the crystallographic structure of gibbsite. It is found that only oxygen atoms are present on gibbsite surface. This observation leads us to propose an additive way of acting by formation of a molecular complex between the growth unit and the carboxylic groups of the additive.

The molecular electrostatic potential and drug design

Abstract The molecular electrostatic potential (MEP) is now a classical tool in chemical activity analysis, especially in drug design. In this paper we discuss, from different MEP applications in drug studies, significant results obtained over the last few years. This analysis led us to define how the MEP can be used in drug design, and the limits of its application, whatever the accuracy of the method used to calculate it.

Practical synthesis of 8-acyl-7-alkyl-1,6-naphthyridin-5(6H)-ones

Reaction of a set of enaminones with 2-chloronicotinoyl chloride or 2,6-dichloro-5-fluoronicotinoyl chloride mainly leads to N-acylation products which cyclize directly or reacting with sodium hydride to give 8-acyl-7-alkyl-1,6-naphthyridin-5(6H)-ones. Due to their easy availability these compounds are attractive precursors for synthesis of polycondensed heterocycles like naphtho[2,3-h][1,6]naphthyridin-5-ones and pyrido[3,2-c][1,6]naphthyridin-6-ones.

First permanent opened forms in spiro[indoline-oxazine] series: synthesis and structural elucidation

The geometrical structure (TTC isomer), as well as the electronic distribution (quinoidic form), of the first permanent opened forms of spirooxazines 5a and b have been determined by 1H NMR spectroscopy and dipole moment measurements. The crystal structure of compound 5b has been determined by X-ray diffraction to confirm the conformations. Molar absorption coefficients were found to be 4.8 × 104 and 4.9 × 104 dm3 mol–1 cm–1, respectively, for 5a and b.

Synthesis of (′) cis-substituted cyclohexenyl and cyclohexanyl nucleosides via a double Mitsunobu-type reaction

This letter describes the synthesis of (±) cis-substituted cyclohexenyl and cyclohexanyl nucleosides. The synthesis of cis isomers was successfully achieved by the use of two consecutive Mitsunobu reactions involving an inversion of configuration and a sugar–base condensation.

New and facile synthesis of 3-styrylflavones

Abstract A simple synthetic access to 3-styrylflavones is developed through the reaction of 1-(2-hydroxyphenyl)-3-phenylpropane-1,3-diones with phenylacetic aldehydes. The structure of parent compound is confirmed by X-ray analysis.

Conformational analysis of the amino termini (5 residues) of human glycophorin AM and AN: differentiation of the structural features of the TN and T antigenic determinants in relation to their specificity

The N-terminus of glycophorin A, the main transmembrane erythrocyte glycoprotein responsible for the MN blood-group specificity, has been modelled. As the minimum size of the protein recognised by the antiglycophorin A antibodies is the N-terminal glycopentapeptide, attention was focused on the TN and T antigenic determinants of this size in order to determine wether differences in 3D structure exist and how a specific response with different antibodies is induced.

Spectrokinetic study of a series of photochromic 2-ferrocenyl-2-methyl[2H]-chromenes

The study of the photochromic behaviour of a series of 2-ferrocenyl-2-methyl[2H]-chromenes has shown original spectrokinetic properties: the broadening of the absorption spectrum of the photomerocyanines with the appearance of a second band in the visible area and also important kinetic modifications in polar protic solvents giving to this series of chromenes a special interest. From spectrokinetic results, a structure of photomerocyanines stabilised by polar and protic solvent such as alcohols could be proposed.