Voltammetric assay of heroin in illicit dosage forms

The oxidation of heroin on a carbon paste electrode has been studied by using voltammetric techniques under both semi-infinite linear diffusion and hydrodynamic conditions. By employing a simple and rapid in situ pre-treatment of the electrode, excellent reproducibility of the current signal was obtained. Subsequently, the current was measured and the concentration of total heroin present could be determined. The method compared favourably with instrumental methods that are more commonly used for the quantification of heroin in clinical laboratories such as chromatographic, spectrophotometric or radiometric techniques. Moreover, the proposed method showed good selectivity and was superior with respect to cost and time of analysis, permitting the determination of heroin in seized drug samples over a wide concentration range.     

Asymmetric synthesis of the AB ring system of lactonamycin

[reaction: see text] An enantiospecific synthesis of the AB fragment of lactonamycin (5) is achieved in eight steps from dimethyl D-tartrate. Ester enolate chemistry features prominently in the sequence.     

QM/MM determination of kinetic isotope effects for COMT-catalyzed methyl transfer does not support compression hypothesis

Secondary alpha-D3 kinetic isotope effects calculated by the hybrid AM1/TIP3P/CHARMM method for the reaction of S-adenosylmethionine with catecholate anion in aqueous solution and catalyzed by rat liver catechol O-methyltransferase at 298 K are 0.94 and 0.85, respectively, in good accord with experiment. The large inverse effect for the enzymatic reaction is not due to compression but arises from significant increases in the stretching and bending force constants involving the isotopically substituted atoms of the transferring methyl group as between the reactant complex and the transition structure, larger than for the reaction in water.     

Theoretical insights in enzyme catalysis

In this tutorial review we show how the methods and techniques of computational chemistry have been applied to the understanding of the physical basis of the rate enhancement of chemical reactions by enzymes. This is to answer the question: Why is the activation free energy in enzyme catalysed reactions smaller than the activation free energy observed in solution? Two important points of view are presented: Transition State (TS) theories and Michaelis Complex (MC) theories. After reviewing some of the most popular computational methods employed, we analyse two particular enzymatic reactions: the conversion of chorismate to prephenate catalysed by Bacillus subtilis chorismate mutase, and a methyl transfer from S-adenosylmethionine to catecholate catalysed by catechol O-methyltransferase. The results and conclusions obtained by different authors on these two systems, supporting either TS stabilisation or substrate preorganization, are presented and compared. Finally we try to give a unified view, where a preorganized enzyme active site, prepared to stabilise the TS, also favours those reactive conformations geometrically closer to the TS.     

低雷诺数下串列布置双圆柱涡激振动特性研究

基于四步半隐式特征线分裂算子有限元方法,对串列布置双圆柱双自由度涡激振动问题进行了数值模拟计算,并分析了间距比、剪切率、频率比以及折减速度4个参数对圆柱结构动力响应的影响.研究发现:不同固有频率比与剪切率对下游圆柱振动幅值影响较大,然而对上游圆柱振动幅值影响较小.上游圆柱在两个自由度方向达到最大值的折减速度不同,然而下...     

Phase-aligned multiple spin-echo averaging: a simple way to improve signal-to-noise ratio of in vivo mouse spinal cord diffusion tensor image

Purpose

To improve signal-noise-ratio of in vivo mouse spinal cord diffusion tensor imaging using-phase aligned multiple spin-echo technique.

Material and methods

In vivo mouse spinal cord diffusion tensor imaging maps generated by multiple spin-echo and conventional spin-echo diffusion weighting were examined to demonstrate the efficacy of multiple spin-echo diffusion sequence to improve image quality and throughput. Effects of signal averaging using complex, magnitude and phased images from multiple spin-echo diffusion weighting were also assessed. Bayesian probability theory was used to generate phased images by moving the coherent signals to the real channel to eliminate the effect of phase variation between echoes while preserving the Gaussian noise distribution. Signal averaging of phased multiple spin-echo images potentially solves both the phase incoherence problem and the bias of the elevated Rician noise distribution in magnitude image. The proposed signal averaging with Bayesian phase-aligned multiple spin-echo images approach was compared to the conventional spin-echo data acquired with doubling the scan time. The diffusion tensor imaging parameters were compared in the mouse contusion spinal cord injury. Significance level (p-value) and effect size (Cohen’s d) were reported between the control and contused spinal cord to inspect the sensitivity of each approach in detecting white matter pathology.

Results

Compared to the spin-echo image, the signal-noise-ratio increased to 1.84-fold using the phased image averaging and to 1.30-fold using magnitude image averaging in the spinal cord white matter. Multiple spin-echo phased image averaging showed improved image quality of the mouse spinal cord among the tested methods. Diffusion tensor imaging metrics obtained from multiple spin-echo phased images using three echoes and two averages closely agreed with those derived by spin-echo magnitude data with four averages (two times more in acquisition time). The phased image averaging correctly reflected pathological features in contusion spinal cord injury.

Conclusion

Our in vivo imaging results indicate that averaging the phased multiple spin-echo images yields an 84% signal-noise-ratio increase over the spin-echo images and a 41% gain over the magnitude averaged multiple spin-echo images with equal acquisition time. Current results from the animal model of spinal cord injury suggest that the phased multiple spin-echo images could be used to improve signal-noise-ratio.     

Nil Bohr0-sets and polynomial recurrence

In Huang et al.  [17] it was proved that for any Nil_d  Bohr₀-set A$A$, there are a minimal system (X,T)$(X,T)$ and a non-empty open subset U$U$ of X$X$ with A⊃{n∈Z:U∩T⁻ⁿU∩?∩T^−dnU≠0?}$A\supset \{n\in \mathbb{Z}:U\cap T^{-n}U\cap ?\cap T^{-dn}U\ne 0?\}$, and for any minimal system (X,T)$(X,T)$ and any open non-empty U⊂X$U\subset X$, the set {n∈Z:U∩T⁻ⁿU∩?∩T^−dnU≠0?}$\{n\in \mathbb{Z}:U\cap T^{-n}U\cap ?\cap T^{-dn}U\ne 0?\}$ is an almost Nil_d Bohr₀-set. The polynomial form of this problem is considered in this paper. It is shown that the latter is still true in the polynomial case, while the former is not in general. We also consider the special case when the system is a nilsystem.