Growth and Dissolution of Organic Crystals with “Tailor-Made” Inhibitors—Implications in Stereochemistry and Materials Science

When crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology. A stereochemical correlation exists between the crystal structure, its modified morphology, and the molecular structure of the inhibitor. This correlation has been successfully exploited for the efficient resolution of conglomerates, the engineering of organic crystals with desired morphologies, the direct and relative assignment of the absolute configurations of chiral molecules and crystals, and for the design of a new model for the spontaneous generation of optical activity. In an analogous way dissolution of organic crystals in the presence of these growth inhibitors induces etch pits at preselected faces. The effect of solvent on crystal growth has been analyzed in some model systems. The experimental results are complemented by atom-atom potential energy calculations.     

Hydration of a synthetic clay with tetrahedral charges: a multidisciplinary experimental and numerical study

The interaction of water with a synthetic saponite clay sample, with a layer charge of 1 per unit cell (0.165 C m(-2)), was investigated by following along water adsorption and desorption in the relative pressure range from 10(-6) to 0.99 (i) the adsorbed amount by gravimetric and near-infrared techniques, (ii) the basal distance and arrangement of water molecules in the interlayer by X-ray and neutron diffraction under controlled water pressure, and (iii) the molecular structure and interaction of adsorbed water molecules by near-infrared (NIR) and Raman spectroscopy under controlled water pressure. The results thus obtained were confronted with Grand Canonical Monte Carlo (GC/MC) simulations. Using such an approach, various well-distinct hydration ranges can be distinguished. In the two first ranges, at low water relative pressure, adsorption occurs on external surfaces only, with no swelling associated. The next range corresponds to the adsorption of water molecules around the interlayer cation without removing it from its position on top of the ditrigonal cavity of the tetrahedral layer and is associated with limited swelling. In the following range, the cation is displaced toward the mid-interlayer region. The interlamellar spacing thus reached, around 12.3 A, corresponds to what is classically referred to as a "one-layer hydrate," whereas no water layer is present in the interlayer region. The next hydration range corresponds to the filling of the interlayer at nearly constant spacing. This leads to the formation of a well-organized network of interlayer water molecules with significant interactions with the clay layer. The structure thus formed leads to a complete extinction of the d001 line in D2O neutron diffraction patterns that are correctly simulated by directly using the molecular configurations derived by GC/MC. The next range (0.50 < P/P0 < 0.80) corresponds to the final swelling of the structure to reach d spacing values of 15.2 A (usually referred to the "two-layer hydrate"). It is associated with the development of a network of liquidlike water molecules more structured than in bulk water. The final hydration range at high relative pressure mainly corresponds to the filling of pores between clay particles.     

Micelle-vesicle transition of fatty acid based ion-pair surfactants: interfacial evidence and influence of the ammonium counterion structure.

We describe the synthesis and the physicochemical study of new ion-pair amphiphiles from a mixture of bicyclic, cyclic, linear or branched amines and fatty acids of three chain lengths. Surface-tension measurements of bicyclic, cyclic and branched structures of ammonium/alkanoate acid ion pairs show a phase transition, with two plateaux in the plot of surface tension versus log(c) (c=concentration). Such behaviour is related to the structure of the counterion, the alkyl chain length and the temperature. Pulsed gradient spin echo NMR spectroscopy experiments were performed to demonstrate the existence of micelles on the first plateau and to confirm the phase transition. The existence of vesicles on the second plateau of the surface tension was proved by CryoTEM observation and dynamic light scattering (DLS) measurements. Mainly, according to the structure of the counterion, there is either a strong association and a positioning along the chain leading to vesicles or a less strong association leading to external positioning and the formation of micelles at low concentrations or vesicles at higher concentrations.     

(BETS)2[Fe(tdas)2]2: a new metal in the molecular conductor family

The structure of bis­[4,5‐ethyl­enedi­thio‐2‐(4,5‐ethyl­enedi­thio‐1,3‐diselena­cyclo­pent‐4‐en‐2‐yl­idene)‐1,3‐diselena­cyclo­pent‐4‐enium] bis(μ‐1,2,5‐thia­diazo­le‐3,4‐di­thiol­ato‐κ³S⁴,S⁵:S⁴)bis[(1,2,5‐thia­diazo­le‐3,4‐di­thiol­ato‐κ²S⁴,S⁵)­iron(III)], (BETS)₂[Fe(tdas)₂]₂ [BETS is alternatively called bis­(ethyl­enedi­thio)­tetraselenafulvalenium] or (C₁₀H₈S₄Se₄)₂[{Fe(C₂N₂S₃)₂}₂], consists of segregated columns of dimers of BETS and columns of dimers of [Fe(tdas)₂]. Each dimer displays inversion symmetry. Numerous chalcogen–chalcogen contacts are observed within and between the columns, producing a network of interactions responsible for the metal‐like behaviour of the compound.     

Silylstannylation of allenes and silylstannylation-cyclization of allenynes. Synthesis of highly functionalized allylstannanes and carbocyclic and heterocyclic compounds

Catalyzed by Pd(0), trialkylsilyltrialkylstannane (R(3)Si-SnR'(3)) reagents undergo highly selective additions to 1,2-dien-7-ynes and 1,2-dien-8-ynes to give 2-vinylalkylidenecyclopentanes with silicon and tin substituents on the double bonds. Similar additions of distannanes and borostannanes show that the reactions with silylstannanes are superior in terms of ease of handling of the bifunctional reagents and the isolation of the products after the reaction. The chemo- and regioselectivities are controlled by the enhanced reactivity of the allene unit, while the (Z)-geometry of the exocyclic stannylvinylidene is a consequence of the syn-carbometalation and subsequent reductive elimination from Pd with retention of configuration at the vinyl carbon. Synthesis of highly functionalized pyrrolidines and indolizidines and the reluctance of certain kinds of allenynes and silicon-tin reagents to undergo the cyclization illustrate the scope and limitations of the reaction. Based on the isolation of intermediates, a mechanism for the formation of the cyclic compounds is proposed. Model transition states to explain the stereoselectivity in cyclization of substituted allenynes are provided. Further elaboration using the vinyltin and vinylsilane moieties should lead to highly functionalized carbocyclic and heterocyclic compounds. Under similar conditions, addition of silylstannanes to highly functionalized allenes gives E-allylstannanes with high stereoselectivity. Functional groups such as THP- and silyl-ethers, lactones, beta- and gamma-lactams, alpha,beta-unsaturated esters, olefins, and substituted acetylenes are tolerated under the reaction conditions.     

Recent Trends in Photoaffinity Labeling

Investigation of receptor—ligand interactions remains an inexhaustible challenge for chemists and biologists. Structural exploration of biological receptors is the starting point for a better understanding of how they function. Photoaffinity labeling is a biochemical approach to identify and characterize receptors targeting further structural investigations. The primary structure of a receptor protein was typically obtained by reverse genetics after exhaustive purification and sequencing of the N-terminal peptide, which allowed the design of the corresponding oligonucleotide probes. Synthesis of these oligonucleotide probes then led to identification of cDNA clones by hybridization. Following this strategy, several membrane neurotransmitter receptors and constituent polypeptides, present in very small quantities in the central nervous system, were identified and their sequence deduced from the cDNA sequence. Since photoaffinity labeling implies the formation of a covalent bond between a radiolabeled ligand analogue and a receptor binding site, it becomes theoretically possible to isolate and sequence radiolabeled peptides and then synthesize the corresponding oligonucleotide probes. Photoaffinity labeling might avoid the critical solubilization and purification steps of the classical approach. To our knowledge, no such example of primary structure determination based on photoaffinity labeling experiments has been reported. However, the extraordinary developments in gene cloning technologies, in particular homology cloning and expression cloning, have made this approach obsolete and raised the question of new perspectives for photoaffinity labeling technology. In this article we present an update on selected original developments, as well as new challenges for this method. Photoaffinity labeling not only gives access to structural elements but is also a potential tool for the investigation of functional aspects of biological receptors, for example their role in signal transduction mechanisms.     

Ruthenium bis(bipyridine) sulfoxide complexes: new catalysts for alkene epoxidation

The ruthenium bis(bipyridine) sulfoxide complexes Ru-1 and Ru-2 exhibit high catalytic activity for epoxidation of unfunctionalized olefins in the presence of [bis(acetoxy)iodo]benzene; with the chiral catalyst, Ru-2, asymmetric induction up to 94% was observed for beta-methylstyrene.     

Streaming potential in open and packed fused-silica capillaries

The surface properties of novel stationary phases in packed and open tubular columns for capillary electrochromatography (CEC) were examined by measuring the streaming potential in a home made apparatus. The surfaces investigated include materials such as porous styrenic sorbents and octadecyl-silica as well as fused-silica tubing, in both raw and surface modified forms. Functionalization of the surface was carried out, for instance, by reductive amination or organosilane grafting on to capillary inner wall. The dependence of the streaming potential on pH was examined with aqueous solutions in the pH range from 2.5 to 9.0. Electrokinetic properties of 50 microm I.D. fused-silica capillaries have been determined by both streaming potential and electrosmotic flow measurements. Both methods gave similar pH profiles of the zeta-potential and the isoelectric points. This confirms the viability of our approach to evaluate the specific charged groups of the packing which is one of the important factors influencing electrosmotic flow (EOF) velocity and protein adsorption during a chromatographic run. In addition to bare silica capillaries, styrenic monolithic columns with different surface functionalities, which have been extensively used in our laboratory for CEC separation of peptides and proteins, were employed for comparison of two methods. Plots of zeta potential as a function of percent ACN show a complex behavior, indicating that zeta potential cannot be predicted simply from binary mixture solvent properties. It is demonstrated that the evaluation of the zeta potential by the streaming potential method is nondestructive, relatively fast, without untoward effects introduced by Joule heating and yet another means for the characterization of the surfaces under conditions employed in CEC.     

Photochromism of spiropyran nanocrystals embedded in sol-gel matrices

Spectroscopic and kinetic properties of a new photochromic medium, consisting of nanocrystals of spyropyran molecules (1,3-dihydro-1,3,3,5',6',pentamethyl-spiro[2H-indole-2,2'-[2H]pyrano [3,2-b]pyridinium] iodide) embedded in an organo-silicate sol-gel film, are presented and compared to microcrystals obtained by slow evaporation of a solvent. High photoconversion efficiencies for both kinds of crystals have been observed. In microcrystals, the photomerocyanine form absorbs at 570 nm with a fading rate of 5 h, in nanocrystals the photomerocyanine form absorbs at 535 nm with a fading rate of 41 h. Therefore, the crystalline structure of nanocrystals is different from the microcrystal one.     

Protein fractionation in a multicompartment device using Off-Gel isoelectric focusing

A new protein fractionation technique based on off-gel isoelectric focusing (IEF) is presented, where the proteins are separated according to their isoelectric point (pI) in a multiwell device with the advantage to be directly recovered in solution for further analysis. The protein fractions obtained with this technique have then been characterized with polymer nanoelectrospray for mass spectrometry (MS) analyses or with Bioanalyzer for mass identification. This methodology shows the possibility of developing alternatives to the classical two-dimensional (2-D) gel electrophoresis. One species numerical simulation of the electric field distribution during off-gel separation is also presented in order to demonstrate the principle of the purification. Experiments with pI protein markers have been carried out in order to highlight the kinetics and the efficiency of the technique. Moreover, the resolution of the fractionation was shown to be 0.1 pH unit for the separation of beta-lactoglobulin A and B. In addition, the isoelectric fractionation of an Escherichia coli extract was performed in standard solubilization buffer to demonstrate the performances of the technique, notably for proteomics applications.