A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons

The crystal structures of potassium and cesium bistrifluoroacetates, KH(CF(3)COO)(2) and CsH(CF(3)COO)(2), respectively, were determined at room and cryogenic temperatures with the single crystal neutron diffraction technique. The crystals belong to the monoclinic space groups, I2a and A2a, respectively, and there is no evidence of any structural phase transition. In both crystals, trifluoroacetate entities in centrosymmetric dimers are linked by very short hydrogen bonds lying across a center of inversion. The thermal parameters provide no evidence of any double minimum potential for hydrogen bond protons. Single-minimum potentials were determined via best fitting to the inelastic neutron scattering spectral profiles of the stretching vibrations. They comprise a narrow well for the ground state and a very broad quasiharmonic well for excited states. The spread out of the wave functions of these states shows that protons are no longer confined between the oxygens. Presumably, they are attracted by the lone pairs of oxygen atoms. These potentials emphasize the covalent nature of the OO bond and the ionic character of the hydrogen bond proton.     

Synthesis of quantum dot-tagged submicrometer polystyrene particles by miniemulsion polymerization

Submicrometer fluorescent polystyrene (PS) particles have been synthesized via miniemulsion polymerization using CdSe/ZnS core-shell quantum dots (QDs). The influence of QD concentration, QD coating (either trioctylphosphine oxide (TOPO)-coated or vinyl-functionalized), and surfactant concentration on the polymerization kinetics and the photoluminescence properties of the prepared particles has been analyzed. Polymerization kinetics were not altered by the presence of QDs, whatever their surface coating. Latexes exhibited particle sizes ranging from 100 to 350 nm, depending on surfactant concentration, and a narrow particle size distribution was obtained in all cases. The fluorescence signal of the particles increased with the number of incorporated TOPO-coated QDs. The slight red shift of the emission maximum was correlated with phase separation between PS and QDs, which occurred during the polymerization, locating the QDs in the vicinity of the particle/water interface. QD-tagged particles displayed higher fluorescence intensity with TOPO-coated QDs compared to those with the vinyl moiety. The obtained fluorescent particles open up new opportunities for a variety of applications in biotechnology.     

Reversed phase liquid chromatography of alkyl-imidazolium ionic liquids

Eleven 1-alkyl-3-methyl imidazolium ionic liquid (IL) salts were analyzed in reversed phase mode with a Kromasil C18 column. The mobile phases were water-rich acetonitrile solutions (water content > or =70%, v/v) without any added salts. It is shown that it is possible to separate different ILs sharing the same cation and differing by the anion when salt-free mobile phases are used. When a buffer, acetate or phosphate salt, or any salt, such as sodium chloride or sodium tetrafluorobarate, is added to the mobile phase, the ILs differing only by their anions cannot be separated. ILs with different alkyl chains in the imidazolium cation are separated by mobile phases with or without added salts following a hydrophobic interaction behavior: log k is proportional to nC, the carbon number of the alkyl chain. Important differences in ion/stationary phase interactions are observed depending on the ionic content of the mobile phase. With salt-free mobile phases, the IL/C18 stationary phase interactions correspond to concave isotherms associated with fronting peaks for all ILs. With mobile phase containing 0.01 M of salt, tailing IL peaks correspond to convex adsorption isotherms. Also, the IL retention factor depends on the concentration and nature of the added salt. Hexafluorophosphate chaotropic anions can adsorb on the Kromasil C18 surface dramatically increasing the imidazolium cation retention factors.     

EXAFS and IR structural study of platinum-based anticancer drugs' degradation by diethyl dithiocarbamate

Platinum compounds constitute a discrete class of DNA-damaging anticancer drug agents, including cisplatin, carboplatin, and oxaliplatin. The toxicity of such drugs raises the problem of waste detoxification. Diethyl dithiocarbamate (DDTC) is recommended by the World Heath Organization (WHO) for the destruction of cisplatin, but the degradation product has not been structurally characterized. This paper deals with the extended X-ray absorption fine structure (EXAFS) and IR structural study of the reaction products of DDTC with cisplatin, carboplatin, and oxaliplatin. Cisplatin and carboplatin give the same reaction product: Pt(DDTC)2. In the case of oxaliplatin, we observed the formation of [(diaminocyclohexane)(DDTC)Pt(II)]. In all cases, the replacement of labile ligands by strong ligands should lead to inactive compounds. Our results suggest that the WHO inactivation protocol might be extended to carboplatin and oxaliplatin. Nevertheless, this should be validated by toxicity tests of the degradation products.     

Characterization of counterion and surface influence on micelle formation using tapping mode atomic force microscopy in air

Cylindrical micelles prepared in aqueous solutions from cationic surfactants octadecyl trimethylammonium (OTA+) or cetyltrimethylammonium (CTA+) and parachlorobenzoate (PCB) counterion were successfully imaged after evaporation of water using tapping mode atomic force microscopy (TM-AFM) onto very smooth gold and glass substrates. With the help of the obtained topography AFM images, it was shown that the micellar structures are preserved on gold substrates after evaporation of the solvent despite the new set of stresses due mainly to capillary forces and dehydration. The influence of the substrate on the resulting micellar morphology observed in air was investigated for these two materials: cylindrical micelles were evidenced as loosely adherent on gold surface in the presence of parachlorobenzoate (PCB) and identical, geometrically speaking, to those known to exist in aqueous solutions. In this situation, topographic AFM images allowed us to determine accurately their geometrical characteristics such as diameter and length in the nanometer range. On the other hand, AFM images obtained in air on glass surfaces revealed micellar structures that are different from those existing in the bulk of the solution. Indeed, bilayer-type micelles with a thickness close to twice the surfactant monomer expected length were observed, indicating that the well-established and strong influence of glass on micelle geometry at the glass/solution interface is maintained after evaporation of water. These results have been analyzed on the basis of positive charge of gold deduced from electrochemical impedance spectroscopy (EIS) and Raman spectroscopy measurements on one hand and of the negative charge of glass on the other hand. Although these results appeal to new theoretical considerations dealing with dynamics of evaporation of micellar solution drops and/or with counterion contributions to macromolecular interactions in aqueous solutions and in air, this new AFM imaging method appears to be the more adequate one to image and measure the micelles formed in the presence of water.     

MALDI imaging mass spectrometry of lipids by adding lithium salts to the matrix solution

Mass spectrometry imaging of lipids using MALDI–TOF/TOF mass spectrometers is of growing interest for chemical mapping of organic compounds at the surface of tissue sections. Many efforts have been devoted to the best matrix choice and deposition technique. Nevertheless, the identification of lipid species desorbed from tissue sections remains problematic. It is now well-known that protonated, sodium- and potassium-cationized lipids are detected from biological samples, thus complicating the data analysis. A new sample preparation method is proposed, involving the use of lithium salts in the matrix solution in order to simplify the mass spectra with only lithium-cationized molecules instead of a mixture of various cationized species. Five different lithium salts were tested. Among them, lithium trifluoroacetate and lithium iodide merged the different lipid adducts into one single lithium-cationized species. An optimized sample preparation protocol demonstrated that the lithium trifluoroacetate salt slightly increased desorption of phosphatidylcholines. Mass spectrometry images acquired on rat brain tissue sections by adding lithium trifluoroacetate showed the best results in terms of image contrast. Moreover, more structurally relevant fragments were generated by tandem mass spectrometry when analyzing lithium-cationized species.     

NIR‐to‐NIR Two‐Photon Scanning Laser Microscopy Imaging of Single Nanoparticles Doped by YbIII Complexes

The photophysical and nonlinear optical properties of water‐soluble chromophore‐functionalised tris‐dipicolinate complexes [LnL₃]^3? (Ln=Yb and Nd) are thoroughly studied, revealing that only the Yb^III luminescence can be sensitized by a two‐photon excitation process. The stability of the complex in water is strongly enhanced by embedding in dispersible organosilicate nanoparticles (NPs). Finally, the spectroscopic properties of [NBu₄]₃[YbL₃] are studied in solution and in the solid state. The high brightness of the NPs allows imaging them as single objects using a modified two‐photon microscopy setup in a NIR‐to‐NIR configuration.     

Identification and functional characterization of a novel α-conotoxin (EIIA) from Conus ermineus

Nicotinic acetylcholine receptors (nAChRs) are one of the most important families in the ligand-gated ion channel superfamily due to their involvement in primordial brain functions and in several neurodegenerative pathologies. The discovery of new ligands which can bind with high affinity and selectivity to nAChR subtypes is of prime interest in order to study these receptors and to potentially discover new drugs for treating various pathologies. Predatory cone snails of the genus Conus hunt their prey using venoms containing a large number of small, highly structured peptides called conotoxins. Conotoxins are classified in different structural families and target a large panel of receptors and ion channels. Interestingly, nAChRs represent the only subgroup for which Conus has developed seven distinct families of conotoxins. Conus venoms have thus received much attention as they could represent a potential source of selective ligands of nAChR subtypes. We describe the mass spectrometric-based approaches which led to the discovery of a novel α-conotoxin targeting muscular nAChR from the venom of Conus ermineus. The presence of several posttranslational modifications complicated the N-terminal sequencing. To discriminate between the different possible sequences, analogs with variable N-terminus were synthesized and fragmented by MS/MS. Understanding the fragmentation pathways in the low m/z range appeared crucial to determine the right sequence. The biological activity of this novel α-conotoxin (α-EIIA) that belongs to the unusual α4/4 subfamily was determined by binding experiments. The results revealed not only its selectivity for the muscular nAChR, but also a clear discrimination between the two binding sites described for this receptor.     

In-house validation for analytical methods and quality control for risk evaluation of chlordecone in food

Chlordecone was used until 1993 as a pesticide in the banana plantation of Martinique and Guadeloupe (French Antilles) against the root borer. This organochlorine pesticide was lipophilic, remnant, and toxic for human beings with both acute and chronic effects. Chlordecone was strongly absorbed and stored in soil and weakly decomposed in environment. Surveys conducted in 2001 revealed its presence in soil, rivers, and domestic food products. Local food (fruits and vegetables, cattle, poultry, and fish) was growing on soils, widely contaminated by chlordecone, used in the past as banana plantations. In 2003, French Administration asked for a risk evaluation for the Antilles population. The French Agency for Food Safety, proposed a Provisional Tolerable Daily Intake of 0.0005?mg?kg^?1?b.w.?day^?1, and an Acute Reference Dose of 0.01?mg?kg^?1?b.w.?day^?1, based on a toxicological risk assessment. The French National Reference Laboratory for pesticides has carried out two analytical methods, one for food of animal origin and another for food of fruit and vegetable origin. These methods were validated in the reference laboratory and dispatched to 13 laboratories for a proficiency test before the launch of two studies on Martinique and Guadeloupe food. About 900 samples from Martinique were sent to the network of laboratories for analysis of chlordecone. Performance parameters obtained through the proficiency test were briefly reported. The quality control test proposed in this study was discussed to shed light on the true variability achievable by intra-laboratory and inter-laboratory analysis. The limits of conventional quality-control procedures were discussed, and a process was proposed in order to get better confidence in analytical results.