Variable flip angle imaging and fat suppression in combined gradient and spin-echo (GREASE) techniques

Conventional "proton density" and "T2-weighted" spin-echo images are susceptible to motion induced artifact, which is exacerbated by lipid signals. Gradient moment nulling can reduce motion artifact but lengthens the minimum TE, degrading the "proton density" contrast. We designed a pulse sequence capable of optimizing proton density and T2-weighted contrast while suppressing lipid signals and motion induced artifacts. Proton density weighting was obtained by rapid readout gradient reversal immediately after the excitation RF pulse, within a conventional spin-echo sequence. By analyzing the behavior of the macroscopic magnetization and optimizing excitation flip angle, we suppressed T1 contribution to the image, thereby enhancing proton density and T2-weighted contrast with a two- to four-fold reduction of repetition time. This permitted an increased number of averages to be used, reducing motion induced artifacts. Fat suppression in the presence of motion was investigated in two groups of 8 volunteers each by (i) modified Dixon technique, (ii) selective excitation, and (iii) hybrid of both. Elimination of fat signal by the first technique was relatively uniform across the field of view, but it did not fully suppress the ghosts originating from fat motion. Selective excitation, while sensitive to the main field inhomogeneity, largely eliminated the ghosts (0.21 +/- 0.05 vs. 0.29 +/- 0.06, p less than 0.01). The hybrid of both techniques combined with bandwidth optimization, however, showed the best results (0.17 +/- 0.04, p less than 0.001). Variable flip-angle imaging allows optimization of image contrast which, along with averaging and effective fat suppression, significantly improves gradient- and spin-echo imaging, particularly in the presence of motion.     

Studies on the mechanism of action of azinomycin B: definition of regioselectivity and sequence selectivity of DNA cross-link formation and clarification of the role of the naphthoate

Evaluation of the sequence selectivity, noncovalent association, and orientation of the DNA cross-linking agent azinomycin B on its duplex DNA receptor is described. A strong correlation between sequence nucleophilicity and cross-linking yield was observed, and steric effects due to the thymine C5-methyl group were identified. Detailed studies on the role of the azinomycin naphthoate using viscometry, fluorescence contact energy transfer, and DNA unwinding assays point to a nonintercalative binding mode for this group. A kinetic assay for agent regioselectivity was used to determine the orientation of binding and covalent cross-link formation.     

Mass spectrometric determination of selenenylsulfide linkages in rat selenoprotein P

The reversible formation of a selenenylsulfide linkage in mammalian thioredoxin reductase was identified as having a key role in its activity. Identification of selenenylsulfide and/or diselenide linkages is therefore critical to the determination of the structure and function of selenoproteins. A selenopeptide, (298)SGSAITUQCAENLPSLCSUQGLFAEEK(324) (U=selenocysteine), was isolated from a tryptic digest of rat selenoprotein P. Its two cysteine residues and two selenocysteine (Sec) residues were determined to be present in oxidized form by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The selenopeptide was subjected to partial reduction by dithiothreitol with immediate alkylation by iodoacetamide. This process was monitored by MALDI-TOFMS to determine the number of alkylations that had taken place. The partially reduced and alkylated peptides were then analyzed by nano-electrospray ionization tandem mass spectrometry and the results indicated that selenenylsulfide linkages Sec304-Cys314 and Cys306-Sec316 were present. It is concluded that selenoprotein P contains these two selenenylsulfide bonds.     

Gas-Phase Lithium Cation Basicity: Revisiting the High Basicity Range by Experiment and Theory

According to high level calculations, the upper part of the previously published FT-ICR lithium cation basicity (LiCB at 373 K) scale appeared to be biased by a systematic downward shift. The purpose of this work was to determine the source of this systematic difference. New experimental LiCB values at 373 K have been measured for 31 ligands by proton-transfer equilibrium techniques, ranging from tetrahydrofuran (137.2 kJ mol^?1) to 1,2-dimethoxyethane (202.7 kJ mol^?1). The relative basicities (ΔLiCB) were included in a single self-consistent ladder anchored to the absolute LiCB value of pyridine (146.7 kJ mol^?1). This new LiCB scale exhibits a good agreement with theoretical values obtained at G2(MP2) level. By means of kinetic modeling, it was also shown that equilibrium measurements can be performed in spite of the formation of Li⁺ bound dimers. The key feature for achieving accurate equilibrium measurements is the ion trapping time. The potential causes of discrepancies between the new data and previous experimental measurements were analyzed. It was concluded that the disagreement essentially finds its origin in the estimation of temperature and the calibration of Cook’s kinetic method. Graphical Abstract

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An immobilized homogeneous catalyst for efficient and selective hydrogenation of functionalized aldehydes, alkenes, and alkynes

An immobilized cationic rhodium(I) catalyst bearing the diphosphine 1,1'-bis(diisopropylphosphino)ferrocene (DiPFc, 1) allows efficient and chemoselective hydrogenation of a range of functionalized aldehydes, as well as alkenes and alkynes, under mild conditions. This heterogenized catalyst system is convenient to prepare, is stable to air and moisture over extended periods, and is readily recycled.     

A catalyst for efficient and highly enantioselective hydrogenation of aromatic, heteroaromatic, and alpha,beta-unsaturated ketones

PhanePhos-ruthenium-diamine complexes catalyze the asymmetric hydrogenation of a wide range of aromatic, heteroaromatic, and alpha, beta-unsaturated ketones with high activity and excellent enantioselectivity.     

D'Alembert's letter to Euler of 3 march 1766

Just before Leonhard Euler resigned his post at the Berlin Academy to return to St. Petersburg, where he spent the rest of his life, he received a letter from d'Alembert urging him to remain in Berlin. This letter is here published for the first time with commentary on the historical context.