Enantioselective metal-free reduction of ketones by a user-friendly silane with a reusable chiral additive

1-Hydrosilatrane, a safe and easy-to-handle reducing reagent that can be inexpensively accessed, has been shown to reduce prochiral ketones asymmetrically in the presence of chiral 1,2-aminoalcohols with ees ranging from 8% to 86%. The best result was achieved using ephedrine as the source of chirality, which is readily commercially available. The additive can be recovered through extraction and reused without any erosion of enantioselectivity.     

Determination of Atrazine,Simazine, Alachlor,and Metolachlor in Surface Water Using Dispersive Pipette Extraction and Gas Chromatography–Mass Spectrometry

Four commonly found pesticides (alachlor, atrazine, metolachlor, and simazine) in surface water were determined using dispersive pipette extraction followed by gas chromatography–mass spectrometry. The rapid mixing and equilibrium between the dispersive pipette extraction adsorbent and water sample resulted in fast and efficient extraction. Using only 5?mL of water sample, the estimated time consumption for extraction of each sample was less than 5?min. Method validation was performed to evaluate accuracy, precision, linearity, the limits of detection, and the limits of quantitation. Average recovery of above 90% was obtained with relative standard deviations below 10%, which indicated good accuracy and precision of the dispersive pipette extraction method. Coefficients of determination were all above 0.9901 and showed good linearity. For the four pesticides studied using the current method, the limits of detection ranged from 7 to 40?ng?L^?1, and limits of quantitation were from 20 to 130?ng?L^?1. Method validation results supported the application of the current method for drinking water safety monitoring per National Primary Drinking Water Regulations established by the US Environmental Protection Agency. Water samples from Lake Lanier and Stone Mountain Lake (Georgia, USS) were analyzed with this method as a preliminary work for a larger scale drinking water quality study in the future. Trace amounts of simazine and atrazine were found in lake water samples, but both were below the regulation levels of the US Environmental Protection Agency.     

Extraction of aromatic organic compounds by polyurethane foam

The mechanism of extraction of organic compounds by open-cell polyurethane foam has been investigated through a detailed study with simple aromatic compounds. Comparison with identical extractions into diethyl ether suggests that the basic extraction mechanism is an ether-like solvent extraction process. The addition of salt increases the extraction and changing the dielectric constant of the aqueous solution also affects the extraction. For organic compounds which have a group capable of hydrogen bonding, some additional factor appears to influence the extraction. This appears to be hydrogen bonding with the polyurethane foam; it is stronger with polyether foam and reduced by the presence of a strong intramolecular hydrogen-bonding group placed ortho to the hydrogen-bonding group. Thermodynamic studies support these conclusions.     

Facile patterning of upconversion NaYF4:Yb,Er nanoparticles

Different kinds of highly ordered patterns of NaYF(4):Yb,Er nanoparticles on gold substrates were fabricated using a simple method combining micro-contact printing and "breath figures" techniques. Ordered arrays of water droplets were first formed in the hydrophilic regions of patterned self-assembled monolayers (SAMs). This was subsequently submerged in a chloroform solution of NaYF(4):Yb,Er nanoparticles. The particles were spontaneously assembled at the interface of chloroform/water droplet surface, leading to different kinds of uniform patterns after solvent evaporation. The structures of NaYF(4):Yb,Er particles patterns depended on the dimension of the substrate, the concentration of the NaYF(4):Yb,Er nanoparticles and the water condensation process.     

A versatile palladium catalyst system for Suzuki-Miyaura coupling of alkenyl tosylates and mesylates

A general and effective palladium system for Suzuki-Miyaura coupling of alkenyl electrophiles under mild reaction conditions is reported. With the Pd(OAc)(2)/CM-phos system, a variety of alkenyl tosylates are coupled well with ArB(OH)(2). Moreover, the first successful examples of using alkenyl mesylates in alkenylation are also described.     

Synthesis of helix 69 of Escherichia coli 23S rRNA containing its natural modified nucleosides,m(3)Psi and Psi

The synthesis of 3-methylpseudouridine (m(3)Psi) phosphoramidite, 5'-O-[benzhydryloxybis(trimethylsilyloxy)silyl]-2'-O-[bis(2-acetoxyethoxy)methyl]-3-methylpseudouridine-3'-(methyl-N,N-diisopropyl)phosphoramidite, is reported. Selective pivaloyloxymethyl protection of the Psi N1 followed by methylation at N3 was used to generate the naturally occurring pseudouridine analogue. The m(3)Psi phosphoramidite was used in combination with pseudouridine (Psi) and standard base phosphoramidites to synthesize a 19-nucleotide RNA representing helix 69 of Escherichia coli 23S ribosomal RNA (rRNA) (residues 1906-1924), containing a single m(3)Psi at position 1915 and two Psi's at positions 1911 and 1917. Our synthesis of the fully modified helix 69 RNA demonstrates the ability to make milligram quantities of RNA that can be used for further high-resolution structure studies. Site-selective introduction of the methyl group at the N3 position of pseudouridine at position 1915 causes a slight increase in the thermodynamic stability of the RNA hairpin relative to pseudouridine; RNAs containing either uridine or 3-methyluridine at position 1915 have similar stability. One-dimensional imino proton NMR and circular dichroism spectra of the modified RNAs reveal that the methyl group does not cause any substantial changes in the RNA hairpin structure.     

Dramatic 5'-residue effect on conformer distribution of short oligonucleotide retro models of the cisplatin-DNA cross-link: implications for the Lippard and cross-link distorted base pair steps present in cisplatin-DNA duplex adducts

The cisplatin anticancer drug preferentially attacks the GG sequence of DNA duplexes. Virtually all DNAs containing the key G*G* lesion (G* = N7 platinated G) have large distortions in the cross-link (G*G*) base pair (bp) step and also in the adjacent Lippard (XG*) bp step, making the adducts very different from B-form DNA in the XG*G* region. The XG*G* strand in duplexes also differs in several ways from single-strand (ss) models with G*G* and XG*G* sequences. In the duplex, the X residue has an N sugar, the 5'-G* and 3'-G* bases have slight "R" canting (3'-G* H8 atom toward the 5'-G* base), and there is no or weak H-bonding by the NH3 ligands. In most XG*G* ss models, X has an S sugar, the 5'-G* base normally cants strongly toward the 3'-G* base (L canting), and the NH3 forms an H-bond. Well-defined ss models exist in the solid state, but dynamic motion obscures the properties of the ss models in solution. In this work, we employ retro models (better defined, less dynamic ss models) to understand the differences between duplex and ss models. The retro models in this study lack carrier ligand NH's, thus eliminating H-bonding. To correlate previous ss solid-state models with our solution work, we constructed hybrid molecules by overlaying parts of known structures. The combined model and experimental information indicates that the X N-pucker is not favorable in L-canted ss models, that X residue steric effects (not H-bonding) favor L canting in ss models, that X N-pucker is needed for favorable WC hydrogen bonding and stacking interactions in duplexes, and that X N-pucker minimizes X base clashes with bases in the complementary strand in duplexes. The R canting minimizing clashes between the X and G* residues of the Lippard bp step (independent of X pucker) and the repositioning of the X residue base caused by the change from S-pucker to N-pucker together lead to the unusual features of the Lippard bp step in the duplex.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Mass spectrometry and non-covalent protein-ligand complexes: confirmation of binding sites and changes in tertiary structure

An experimental approach is described for determining protein-small molecule non-covalent ligand binding sites and protein conformational changes induced by ligand binding. The methodology utilizes time resolved limited proteolysis and the high throughput analysis capability of MALDI TOF MS to determine the binding site in a tetanus toxin C-fragment (51 kDa)-doxorubicin (543 Da) non-covalent complex. Comparing relative ion abundances of peptides released from the time resolved limited proteolysis of tetanus toxin C-fragment (TetC) and the TetC-doxorubicin complex every 10 min from 10 to 120 min of digestion revealed that the binding of doxorubicin induced a significant change in surface topology of TetC. Four of the twenty-nine peptides observed by MALDI MS, including amino acids 351-360, 299-304, 305-311 and 312-316, had a lower abundance in the TetC-doxorubicin complex relative to TetC from 10 to 100 min of digestion. A decrease in ion abundance suggests doxorubicin obstructs the access of the protease to one or both termini of these peptides, identifying doxorubicin binding site(s). Conversely, five peptide ions, including amino acids 335-350, 364-375, 364-376, 281-298, and 316-328, all had a greater abundance in the digest of the complex, indicating an increase in accessibility to these sites. These five peptides flank regions of decreased ion abundance, suggesting that doxorubicin not only binds to the surface, but also induces a conformational change in TetC.