Predicting the R/S absolute configuration in asymmetric bifunctional catalysis (ABC)

A predictive model for assigning the R/S absolute configuration in chiral Lewis base-dependent asymmetric bifunctional catalysis (ABC) has been developed. Chiral Lewis base (LB^∗)-dependent ABC abolishes the chiral Lewis acid (LA^∗) component as the stereochemical determining factor in asymmetric catalysis. By correlating the constant LB^∗ chirality to the facial preference for the LA^∗-bound carbonyl group for nucleophile delivery, the R/S absolute configuration of the products can be predicted a priori.     

Determination of oxytetracycline residues in matrixes from a freshwater recirculating aquaculture system

This paper describes related procedures to determine the amount of oxytetracycline (OTC) present in trout tissue (muscle with skin attached), biofilter sand, sediment, and tank water from a recirculating aquaculture system. OTC was extracted from the matrixes by different techniques, depending on complexity of the matrix and desired OTC detection level in that matrix. Listed in order of increasing complexity, OTC was extracted from tank water by dilution with acidic buffer containing ethylenediaminetetraacetic acid (EDTA); from biofilter sand by shaking with 0.1 N HCl; from sediment by homogenization and shaking with buffer/EDTA; and from ground trout by homogenization and shaking with buffer/EDTA (twice), with further cleanup and concentration of the extract on a polymeric solid-phase extraction cartridge. The 4 procedures all used the same reversed-phase gradient chromatography on a polymeric column with UV detection at 350 nm. The lower limit of detection (estimated) and upper limit of validation for each of these 4 matrixes were 0.04-4.0 microg/g (ppm; trout), 0.03-20 ppm (biofilter sand), 1-6000 ppm (sediment), and 0.003-10 ppm (water). Recoveries ranged from 82 to 108%, with relative standard deviation <20% over the applicable concentration ranges. These procedures were used to monitor OTC residues resulting from medicated feed administered to rainbow trout in a recirculating aquaculture system.     

Rapid identification of stress-related fingerprint from whole bacterial cells of <Emphasis Type="Italic">Bifidobacterium lactis</Emphasis> using matrix assisted laser desorption/ionization mass spectrometry

Whole cells of Bifidobacterium lactis were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). Characteristic and reproducible mass spectra were obtained in the mass range from 6 to 19 kDa. After several days of bacterial cell storage at 4 degrees C (D0, D2, and D6), only minor signal differences were observed. Under identical and reproducible conditions, fourteen relevant diagnostic ions were identified. Moreover, control- and stress-related fingerprints were rapidly obtained using MALDI-TOFMS by comparison of protein patterns obtained from non-stressed (control) versus stressed cells (addition of bile salts during growth). After quantitative validation of the MALDI-MS data by a statistical approach, two and eight signals were assigned as control- and stress-specific ions, respectively. This work provides the evidence that MALDI-TOFMS can be used for the identification of stress-related fingerprint of B. lactis bacterial cells and could have a high potential for the assessment of the physiological status of the cells.     

High-resolution X-ray photoelectron spectroscopic studies of alkylated silicon(111) surfaces

Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.     

Use of Ninhydrin for Detection of Silylated Amino Acids

JPC – Journal of Planar Chromatography – Modern TLC -     

Ligand oxidations in high-spin nickel thiolate complexes and zinc analogues

Oxidations of a trigonal-bipyramidal, high-spin Ni(II) dithiolate complex of a pentadentate, N3S2-donor ligand, N1,N9-bis(imino-2-mercaptopropane)-1,5,9-triazanonane) nickel(II), and the structurally analogous Zn(II) complex, lead to oxidations of the ligand. Oxidation of the Ni(II) complex with I2 produces a novel Ni(II) macrocyclic cationic complex containing a monodentate disulfide ligand (2). Crystals of the I3- salt of the complex form in the triclinic space group P(1) with cell dimensions a=8.508(3) A, b=9.681(2) A, c=14.066(4) A, angles alpha=90.97(2) degrees , beta=91.61(3) degrees , gamma=90.83(2) degrees , and Z=2. The structure was refined to R=6.31% and Rw=16.63% (I > 2sigma(I)). Oxidation of the Ni(II) complex with O2 leads to the formation of a novel pentadentate bis-iminothiocarboxylate complex with trigonal-bipyramidal geometry (3). This neutral product crystallizes in the monoclinic space group P21/c with cell dimensions a=13.625(3) A, b=7.605(5) A, c=14.902(4) A, angles alpha=gamma=90 degrees, beta=102.81(2) degrees , and Z=4. The structure was refined to R=7.18% and Rw=17.86% (I > 2sigma(I)). Oxidation of the Zn(II) dithiolate analogue with O2 leads to the formation of the Zn(II) complex of the pentadentate bis-iminothiocarboxylate ligand. The neutral complex is isomorphous with the Ni(II) complex and crystallizes in the monoclinic space group P2(1)/c with cell dimensions a=13.8465(4) A, b=7.6453(2) A, c=15.0165(6) A, angles alpha=gamma=90 degrees , beta=103.2140(11) degrees , and Z=4. The structure was refined to R=3.96% and Rw=9.45% (I > 2sigma(I)). Details of the crystal structures are reported. Kinetics of the O2 reactions show that the reactions of the Ni(II) and Zn(II) dithiolates follow the rate law, Rate=k2[1][O2], with k2=1.81 M(-1) s(-1) for the Ni(II) complex and k2=1.93 x 10(-2) M(-1) s(-1) for the Zn(II) complex. The O2 oxidation of the high-spin Ni(II) thiolate complex was found to follow a similar oxidation mechanism to those of low-spin Ni(II) complexes, which form transient persulfoxide intermediates that yield S-oxidation products. In the case of the high-spin system reported here, the transient persulfoxide intermediate gives rise to an alternative ligand oxidation product, a bis-iminothiocarboxylate complex, because of the reactivity of the ligand, which contains a methylene with acidic H atoms alpha to the thiolate sulfur. The proposed mechanism is supported by studies of the analogous Zn dithiolate complex, which gives rise to the analogous bis-iminothiocarboxylate product (5).