Mol Biol Cell

The exon junction complex (EJC) is loaded onto mRNAs as a consequence of splicing and regulates multiple posttranscriptional events. MLN51, Magoh, Y14, and eIF4A3 form a highly stable EJC core, but where this tetrameric complex is assembled in the cell remains unclear. Here we show that EJC factors are enriched in domains that we term perispeckles and are visible as doughnuts around nuclear speckles. Fluorescence resonance energy transfer analyses and EJC assembly mutants show that perispeckles do not store free subunits, but instead are enriched for assembled cores. At the ultrastructural level, perispeckles are distinct from interchromatin granule clusters that may function as storage sites for splicing factors and intermingle with perichromatin fibrils, where nascent RNAs and active RNA Pol II are present. These results support a model in which perispeckles are major assembly sites for the tetrameric EJC core. This subnuclear territory thus represents an intermediate region important for mRNA maturation, between transcription sites and splicing factor reservoirs and assembly sites

Catalytic hydrogenation of alkenes under biphasic conditions:understanding solvent effects

Ruthenium(II)-arene diphosphine complexes are shown to be effective precursors for the catalytic hydrogenation of the C=C bond in an aqueous biphase using molecular hydrogen as the hydrogen source. A mechanistic study performed in water, utilizing high gas pressure NMR spectroscopy and electrospray ionization mass spectrometry, and experiments involving the variation of pH, enables indirect identification of the catalytically active species resulting from the substitution of the chloride with the dihydrogen ligand. Similar catalytic experiments are carried out in ionic liquids (ILs), such as [bmim][BF4] and [bmim][TfO] using an analogous precursor, but no conversion is observed and the ruthenium(II) species does not undergo any chemical change during the reaction. The dissociation of chloride, leading to the catalyst, is inhibited in these ILs due to a lower solvation of chloride. An estimate of the bmim-chloride interaction, which is lower than the water-chloride interaction, is determined by 1H NMR spectroscopy. To ensure the formation of the active species in such ILs, the coordinated chloride is abstracted beforehand and replaced by acetonitrile, which restores the catalytic activity. An alternative approach to overcome the chloride dissociation problem consists of improving the solvation of the chloride in the IL medium. To accomplish this, water is added as a co-solvent in the IL phase and, under these conditions, the activity is restored. In order to assess the reactivity of chloride ions in ILs, a relative scale for the solvation of chloride is given for a series of [cation][Tf2N] ILs. This scale, based on the reaction enthalpy for the dissociation of a chloride-templated metallacage, yields the following order of chloride solvation: [C5e3am][Tf2N] < [bmpy][Tf2N] < [bmim][Tf2N] ≤ [bdmim][Tf2N] < [bpy] [Tf2N] &#8810; [mimeOH][Tf2N]. In Tf2N-based ILs, the catalytic activity of hydrogenation reactions using the same Ru(II) precursor in the presence of chloride is totally modified. In a series of [cation][Tf2N] ILs, the rate of formation of the catalyst, which is promoted by chlorides, is shown to depend on the solvation of chloride

The Dielectric Response of Room-Temperature Ionic Liquids: Effect of Cation Variation

In continuation of recent work on the dielec. response of imidazolium-based ionic liqs. (ILs) (J. Phys. Chem. B, 2006, 110, 12682), the authors report on the effect of cation variation on the frequency-dependent dielec. permittivity up to 20 GHz of ionic liqs. The salts are comprised of pyrrolidinium, pyridinium, tetraalkylammonium, and triethylsulfonium cations combined with the bis((trifluoromethyl)sulfonyl)imide anion. The dielec. spectra resemble those obsd. for imidazolium salts with the same anion. In all cases, the major contribution results from a diffusive low-frequency response on the time scale of several 100 ps, which shows a broadly distributed kinetics similar to that of spatially heterogeneous states in supercooled and glassy systems rather than that obsd. in fluid systems. There is evidence for a weak secondary process near 10-20 ps. Perhaps the most interesting difference to imidazolium salts is founded in the missing portions of the spectra due to processes beyond the upper cutoff frequency of 20 GHz. These are lower than that obsd. for imidazolium-based salts and seem to vanish for tetraalkylammonium and triethylsulfonium salts. As for imidazolium salts, the extrapolated static dielec. consts. are on the order of eS equiv. 10-13, Classifying these ILs as solvents of moderate polarity