Improving sample representativeness in environmental studies: a major component for the uncertainty budget

For environmental quality assessment, INAA has been applied for determining chemical elements in small (200 mg) and large (200 g) samples of leaves from 200 trees. By applying the Ingamells’ constant, the expected percent standard deviation was estimated in 0.9–2.2% for 200 mg samples. Otherwise, for composite samples (200 g), expected standard deviation varied from 0.5 to 10% in spite of analytical uncertainties ranging from 2 to 30%. Results thereby suggested the expression of the degree of representativeness as a source of uncertainty, contributing for increasing of the reliability of environmental studies mainly in the case of composite samples.     

Structure elucidation and stereoselective total synthesis of pavettamine, the causal agent of gousiekte

The structure elucidation of a novel natural product pavettamine (1), the causal agent of the plant toxicosis gousiekte, is reported. The structure was defined by analysis of NMR and MS data and the relative configuration followed from the ¹³C NMR data of the acetonide derivative. The absolute stereochemistry was established by total synthesis from (2S)-malic acid using chiral sulfoxide methodology as (2S,4R,8R,10S)-1,11-diamino-6-aza-undecane-2,4,8,10-tetraol.     

Pulsed electron-electron double resonance (PELDOR) distance measurements in detergent micelles

Pulsed electron-electron double resonance (PELDOR) spectroscopy is a powerful tool for measuring nanometer distances in spin-labeled systems. A common approach is doubly covalent spin-labeling of a macromolecule and measurement of the inter-spin distance, or to use singly-labeled components of a system that forms aggregates or oligomers. This situation has been described as a spin-cluster. The PELDOR signal, however, does not only contain the desired dipolar coupling between the spin-labels of the molecule or cluster under study. In samples of finite concentration the dipolar coupling between the spin-labels of the randomly distributed molecules or spin-clusters also contributes significantly. In homogeneous frozen solutions or lipid vesicle membranes this second contribution can be considered to be an exponential or stretched exponential decay, respectively. In this study, we show that this assumption is not valid in detergent micelles. Spin-labeled fatty acids that are randomly partitioned into different detergent micelles give rise to PELDOR time traces which clearly deviate from stretched exponential decays. The obtained signals can be modeled quantitatively based on the size of the micelles, their aggregation number, the spin-label concentration and the degree of spin-labeling. As a main conclusion a PELDOR signal deviating from a stretched exponential decay does not necessarily prove the observation of specific distance information on the molecule or cluster. These results are important for the interpretation of PELDOR experiments on membrane proteins or lipophilic peptides solubilized in detergent micelles or small vesicles, which often do not show pronounced dipolar oscillations in their time traces.     

连锁超市供应链风险评价模型及实证研究

随着经济的飞速发展,供应链风险管理问题越来越受到企业的重视．但国内外还缺乏对连锁超市供应链风险的深入研究．事实上,很多连锁超市的供应链上很多环节并不完善．本文分析了连锁超市供应链上所面临的风险,构建了连锁超市供应链风险评价模型,以某连锁超市为例,通过调查问卷与所得数据,并利用熵权法确定了各指标的权重,最后基于模糊综合评价法得出的结果,提出了降低连锁超市供应链风险的对策建议．     

The catalyzed hydrosilation reaction: substituent effects

Ab initio electronic structure calculations using MP2 wavefunctions have been used to investigate a reaction path for the hydrosilation reaction catalyzed by divalent titanium [modeled by TiH₂, TiCl₂, and Ti(C₅H₅)₂]. Optimized structures and energies are presented. All model reactions predict a barrierless reaction path compared to a barrier of 78 kcal/mol for the uncatalyzed reaction. Received: 11 August 1998 / Accepted: 3 September 1998 / Published online: 23 February 1999     

Structure Elucidation and Activity of Kolossin A,the D‐/L‐Pentadecapeptide Product of a Giant Nonribosomal Peptide Synthetase

The largest continuous bacterial nonribosomal peptide synthetase discovered so far is described. It consists of 15 consecutive modules arising from an uninterrupted, fully functional gene in the entomopathogenic bacterium Photorhabdus luminescens. The identification of its cryptic biosynthesis product was achieved by using a combination of genome analysis, promoter exchange, isotopic labeling experiments, and total synthesis of a focused collection of peptide candidates. Although it belongs to the growing class of D ‐/ L ‐peptide natural products, the encoded metabolite kolossin A was found to be largely devoid of antibiotic activity and is likely involved in interspecies communication. A stereoisomer of this peculiar natural product displayed high activity against Trypanosoma brucei rhodesiense, a recalcitrant parasite that causes the deadly disease African sleeping sickness.     

Structure,Biosynthesis, and Occurrence of Bacterial Pyrrolizidine Alkaloids

Pyrrolizidine alkaloids (PAs) are widespread plant natural products with potent toxicity and bioactivity. Herein, the identification of bacterial PAs from entomopathogenic bacteria using differential analysis by 2D NMR spectroscopy (DANS) and mass spectrometry is described. Their biosynthesis was elucidated to involve a non‐ribosomal peptide synthetase. The occurrence of these biosynthesis gene clusters in Gram‐negative and Gram‐positive bacteria indicates an important biological function in bacteria.     

Catalytic Synthesis of N‐Unprotected Piperazines,Morpholines, and Thiomorpholines from Aldehydes and SnAP Reagents

Commercially available SnAP (stannyl amine protocol) reagents allow the transformation of aldehydes and ketones into a variety of N‐unprotected heterocycles. By identifying new ligands and reaction conditions, a robust catalytic variant that expands the substrate scope to previously inaccessible heteroaromatic substrates and new substitution patterns was realized. It also establishes the basis for a catalytic enantioselective process through the use of chiral ligands.     

Structure elucidation and biosynthesis of lysine-rich cyclic peptides in Xenorhabdus nematophila

Thirteen novel PAX (peptide-antimicrobial-Xenorhabdus) peptides were identified in Xenorhabdus nematophila HGB081. Their structures including the absolute configuration were elucidated using a combination of labeling experiments, detailed MS/MS experiments, the advanced Marfey's method, and a detailed analysis of the biosynthesis gene cluster, which was identified as well.