No abstract

No abstract

No abstract

No abstract

Enantiomer migration order (EMO) in chiral capillary electrophoresis (CE) represents a challenging issue, referred to in less than 20% of the articles on CE enantioseparations. This review article will (i) illustrate the actuality of the topic, (ii) discuss some technical problems related to EMO in CE enantioseparations, (iii) examine the principal differences between CE and other separation techniques from the viewpoint of enantiomer elution order, (iv) demonstrate the potential for a designed reversal of EMO in CE, and (v) emphasize the importance of studying EMO for better understanding of chiral CE as well as its more effective application. Along with CE, the results obtained by other instrumental techniques such as nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), X-ray crystallography, as well as molecular modeling calculations will be shortly discussed. Rather than referring to all published examples of the opposite migration order of enantiomers in CE, the emphasis will be on general aspects. Recently, the reversal of the EMO was described in detail in a book chapter (Chankvetadze, B., Capillary Electrophoresis in Chiral Analysis, Wiley & Sons, Chichester, UK 1997, Chapter 12) as well as in three review articles.

In this study, the systemic assessments of the stereoisomers of triazole fungicide difenoconazole are reported for the first time, including absolute stereochemistry, stereoselective bioactivity toward pathogens (Alternaria sonali, Fulvia fulva, Botrytis cinerea, and Rhizoctonia solani), and toxicity toward aquatic organisms (Scenedesmus obliquus, Daphnia magna, and Danio rerio). Moreover, the stereoselective degradation of difenoconazole in vegetables (cucumber, Cucumis sativus and tomato, Lycopersicon esculentum) under field conditions and in soil under laboratory-controlled conditions (aerobic and anaerobic) was investigated. There were 1.33-24.2-fold and 1.04-6.78-fold differences in bioactivity and toxicity, respectively. Investigations on the stereoselective degradation of difenoconazole in vegetables showed that the highest-toxic and lowest-bioactive (2S,4S)-stereoisomer displays a different enrichment behavior in different plant species. Under aerobic or anaerobic conditions, (2R,4R)- and (2R,4S)-difenoconazole were preferentially degraded in the soil. Moreover, difenoconazole was configurationally stable in the test soil matrices. On the basis of biological activity, ecotoxicity, and environmental behavior, it is likely that the use of pure (2R,4S)-difenoconazole instead of the commercial stereoisomer mix may help to increase the bioactivity and reduce environmental pollution.

No abstract

This tutorial review describes the contribution of chiral capillary electrophoresis in combination with other instrumental techniques, especially nuclear magnetic resonance spectroscopy, to a better understanding of the chiral recognition mechanisms by cyclodextrins. Aspects such as affinity pattern of enantiomers towards various cyclodextrins as well as the stoichiometry of the resulting complexes, the equilibrium constants and the structure of complexes are addressed. In addition to the aforementioned techniques, the usefulness of complementary instrumental and molecular modeling techniques for an understanding of the chiral recognition mechanisms of cyclodextrins is also illustrated.