Cycloadditions dipolaires-1,3 II. Addition des diarylnitrilimines au N-methyl-indole. Etude expérimentale et essai d'interprétation

The reactivity of N-methylindole towards diarylnitrilimines has been studies. The cycloadducts are 8-methyl-3a,8a-dihydro-pyrazolo[3,4-b] indole derivatives. The structure reported was assigned on the bases of satisfactory analytical, spectroscopic and chemical data. An interpretation based on the second order perturbation method of frontier orbitals is in agreement with the observed regiospecificity.     

Molecular orbital calculations on the conformation of polypeptides and proteins

Quantum mechanical calculations using the PCILO method have been performed on the tripeptide model CH₃CO-X-Y-NHCH₃. Competition between C₅, C₇, C₁₀ rings and open structures has been investigated through mapping of the whole {φ, Ψ} conformational space and energy minimization. From these results, it appears that the C₁₀ ring simulating the folding named U-turn, involving a hydrogen bond between the i...i + 3 residues, is the most probable structure although not the most stable in energy. The results are used for predicting the frequency of U-turns in proteins, α-chymotrypsin is given as an example.     

Remarkable remote chiral recognition in a reaction mediated by a catalytic antibody

The crystal structures of catalytic antibody D2.3 Fab with the two enantiomers, 7D and 7L, which represent transition state analogues for the hydrolysis of the corresponding esters, 6D and 6L, were determined to better understand remarkable reactivity differences: the L-ester displayed significantly tighter binding (K(M)) and increased catalytic activity (k(cat)) with D2.3, even though the chiral center is 7 bonds distant from the reaction center. Surprisingly, the electron densities of the liganded phosphonates, 7D and 7L, within the D2.3 binding/reaction site were essentially identical, highlighting the subtle influences of protein interactions on chemical behavior.     

Rationale for kinetic heterogeneity of ultrafast light-induced electron transfer from Ru(II) complex sensitizers to nanocrystalline TiO2

Because of their successful use in dye-sensitized solar cells, Ru(II) polypyridyl complex dyes adsorbed on nanocrystalline TiO2 films have been regarded as model systems for the experimental study of the ultrafast dynamics of interfacial light-induced electron transfer. Most studies have reported charge injection kinetics from Ru(dcbpyH2)2(NCS)2 (N3) to take place with a fast (sub-100 fs) phase, followed by a slower (0.7-100 ps) multiexponential component. This complex, multiphasic behavior observed for the electron injection process has prevented the development of a satisfying kinetic model and has led to often contradicting conclusions. Here, we show that the observed kinetic heterogeneity can result from the aggregation of sensitizer molecules on the surface. Carefully controlled deposition of Ru(II) complex dye molecules onto nanocrystalline titania consistently yields a monophasic injection dynamics with a time constant shorter than 20 fs. The latter figure suggests the process is beyond the scope of vibration-mediated electron transfer kinetic models and might be controlled by the electron dephasing in the solid.     

Inframolecular protonation process of norbadione A: influence of the ionic environment and stereochemical consequences

The microscopic protonation mechanism, at an inframolecular level, of norbadione A, a pigment extracted from mushrooms and known to complex cesium cations, was determined by using 1H NMR titrations and the cluster expansion method. This study revealed a pH dependent Z to E isomer switch that occurs in both pulvinic moieties. As a consequence, norbadione A can exist in solution in four stereomeric forms (E-E, E-Z, Z-E, and Z-Z), which can be of interest in the development of molecular-level devices. In the presence of 0.15 M NaCl, the calculated microconstants showed an unusual apparent cooperativity between the enol groups, which results from the release of the sodium cations upon protonation of norbadione A.     

Electrospray micro liquid chromatography – Fourier transform infrared micro spectrometry

Microcolumn liquid chromatography has been interfaced with infrared microspectrometry by means of a novel technique involving electrospray nebulization. The eluent from the LC column is subjected to a high positive potential relative to the surroundings and the electric field generated charges the surface of the emerging liquid causing it to disintegrate into charged droplets which are attracted to an earthed, infrared-transparent, zinc selenide deposition plate. Nitrogen sheath gas is used to evaporate the solvent in the droplets, and the separated components are deposited on the zinc selenide plate for subsequent analysis by means of a Fourier transform infrared microscope and spectrometer. Provided low flow rates (～4 μl/min) are used, total elimination of the solvent is achieved and easily identifiable infrared spectra can be measured from less than 500 pg of sample (caffeine).     

Synthesis and NMD A Antagonistic Properties of the Enantiomers of 4-(3-phosphonopropyl)piperazine-2-carboxylic acid (CPP) and of the unsaturated analogue (E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (CPP-ene)

The (R)- and (s)-enantiomers of 4-(3-phosphonopropyl)piperazine-2-carboxylic acid (D - and L -CPP, resp.; 15 and 16 , resp.), and of its unsaturated analogue (E)-4-(3-phosphonoprop-2-enyl)piperazine-2-carboxylic acid (D - and L -CPP-ene, resp.; 13 and 14 , resp.) were prepared. The absolute configuration of the enantiomers was determined by a chemical correlation of the menthyl ester 7 with D -asparagine. The affinity of these derivatives for the NMDA receptor was determined by displacement of [³H]CPP in rat cerebral cortical membranes. In two functional tests (the frog hemisected spinal cord preparation and the sodium efflux test from rat brain slices), D -CPP-ene appears to be the most potent, enantiomerically pure, competitive NMDA antagonist known to date.     

Evidence for linkage position determination in known feruloylated mono- and disaccharides using electrospray ion trap mass spectrometry

Various feruloylated arabinose- and galactose-containing mono- and disaccharides with known linkage configurations (2-O-(trans-feruloyl)-L-arabinopyranose, 5-O-(trans-feruloyl)-L-arabinofuranose, O-[2-O-(trans-feruloyl)-alpha-L-arabinofuranosyl]-(1-->5)-L-arabinofuranose, and O-[6-O-(trans-feruloyl)-beta-D-galactopyranosyl]-(1-->4)-D-galactopyranose) were analyzed by electrospray ionization mass spectrometry using an ion trap or a quadrupole time-of-flight (Q-TOF) mass analyzer. Collision-induced dissociation (CID) experiments using the two mass analyzers generated similar tandem mass spectrometric (MS/MS) fragmentation patterns. However, the ester-bond cleavage ions were more abundant using the Q-TOF mass analyzer. Compared with the positive ion mode, the negative ion mode produces simpler and more useful CID product-ion patterns. For arabinose-containing feruloylated compounds, results obtained with both analyzers show that it is possible to assign the location of the feruloyl group to the O-2 or O-5 of arabinosyl residues. In the characterization of the 2-O-feruloyl and 5-O-feruloyl linkages, the relative abundance of the cross-ring fragment ions at m/z 265 (-60 u or -62 u after 18O-labelling) and at m/z 217 (-108 u or -110 u after 18O-labelling) play a relevant role. For galactose-containing feruloylated compounds, losses of 60, 90 and 120 Da observed in MS3 experiment correspond to the production of 0,2A1, 0,3A1 and (0,2A1-60 Da) cross-ring cleavage ions, respectively, fixing the location of feruloyl group at the O-6 of the galactose residue.     

Evolution of structural features and mechanical properties during the conversion of poly[(methylamino)borazine] fibers into boron nitride fibers

Poly[(methylamino)borazine] (PolyMAB) green fibers of a mean diameter of 15 μm have been pyrolyzed under ammonia up to 1000°C and heat treated under nitrogen up to 2000°C to prepare boron nitride (BN) fibers. During the polymer-to-ceramic conversion, the mechanical properties of the green fibers increase within the 25-400°C temperature range owing to the formation of a preceramic material and remain almost constant up to 1000°C. Both the crystallinity and the mechanical properties slightly increase within the 1000-1400°C range, in association with the consolidation of the fused-B₃N₃ basal planes. A rapid increase in tensile strength (σ^R) and elastic modulus (Young's modulus E) is observed in relation with crystallization of the BN phase for fibers treated between 1400°C and 1800°C. At 2000°C, “meso-hexagonal” BN fibers of 7.5 μm in diameter are finally obtained, displaying values of σ^R=1.480 GPa and E=365 GPa. The obtention of both high mechanical properties and fine diameter for the as-prepared BN fibers is a consequence of the stretching of the green fibers on a spool which is used during their conversion into ceramic.     

Furo[3,2-e]pyrrolo[1,2-α][1,4]diazepine and derivatives

The 5,10-dihydro-4H-furo[3,2-e]pyrrolo[1,2-α][1,4]diazepin-5-one ( 7 ) and furo[3,2-e]pyrrolo[1,2-α][1,4]diaze-pine ( 10 ) are synthesized from suitable isocyanates 3a,b in acetic acid. The reactivity of 10 (C- and N-alkyla-tion) is investigated.