A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H(2)O(2) as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H(2)O(2) and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by (1)H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H(2)O(2) required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid. 相似文献
Microcystins are cyclic peptide toxins with hepatotoxic and tumour-promoting properties which are produced in high quantities
in freshwater cyanobacterial water blooms, and several studies have reported microcystin accumulation in fish with possible
food transfer to humans. In this study, we provide the first comparison of liquid chromatography with single mass-spectrometric
and with tandem mass-spectrometric detection for analyses of microcystins in complex fish tissue samples. Use of traditional
single mass spectrometry (i.e. monitoring of ions with m/z 519.5 for microcystin-RR and m/z 995.5 for microcystin-LR) was found to provide false-positive responses, thus overestimating the concentrations of microcystins
in the tissue samples. More selective tandem mass spectrometry seems to provide more reliable results. The concentrations
of microcystins detected by tandem mass spectrometry in fish from controlled-exposure experiments were more than 50% lower
in comparison with concentrations obtained by single mass spectrometry. Extensive analyses of edible fish parts—muscles (148
fish specimens from eight different species from five natural reservoirs with dense cyanobacterial water blooms)—showed negligible
microcystin concentrations (all analyses below the limit of detection; limit of detection of 1.2–5.4 ng/g fresh weight for
microcystin-RR, microcystin-YR and microcystin-LR in multiple reaction monitoring mode). Our findings have practical consequences
for critical re-evaluation of the health risks of microcystins accumulated in fish. 相似文献
We observe imaging through windows comprising pairs of confocal lenslet arrays that have different focal lengths but that are otherwise identical. Image space is stretched in the longitudinal direction only. Such windows are examples of METATOYs, optical components that can change light-ray direction in ways that appear wave-optically forbidden. 相似文献
Manipulation makes light work : The morphology and rheological properties of a liquid‐crystalline system can be dynamically manipulated with UV light by attaching photoresponsive liquid‐crystalline moieties to a siloxane‐based polymer. Stimulation with UV light induces a conformational change in the molecule, which disrupts the liquid‐crystalline mesophase (see picture), and results in a dramatic change in its rheological properties.
The absorption properties of chromophores in biomolecular systems are subject to several fine‐tuning mechanisms. Specific interactions with the surrounding protein environment often lead to significant changes in the excitation energies, but bulk dielectric effects can also play an important role. Moreover, strong excitonic interactions can occur in systems with several chromophores at close distances. For interpretation purposes, it is often desirable to distinguish different types of environmental effects, such as geometrical, electrostatic, polarization, and response (or differential polarization) effects. Methods that can be applied for theoretical analyses of such effects are reviewed herein, ranging from continuum and point‐charge models to explicit quantum chemical subsystem methods for environmental effects. Connections to physical model theories are also outlined. Prototypical applications to optical spectra and excited states of fluorescent proteins, biomolecular photoreceptors, and photosynthetic protein complexes are discussed.相似文献
Summary: The effect of sodium dodecyl sulfate (SDS) on the mass transfer rate between styrene and water has been investigated. SDS increases the solubility of styrene in water even below the critical micelle concentration (CMC) and therefore increases the thermodynamic driving force for the mass transfer. The mass transfer coefficient however is not altered by SDS, even if the interface is almost saturated with emulsifier. 相似文献
We prove an index theorem for boundary value problems in Boutet de Monvel's calculus on a compact manifold X with boundary. The basic tool is the tangent semi-groupoid generalizing the tangent groupoid defined by Connes in the boundaryless case, and an associated continuous field of C*-algebras over [0,1]. Its fiber in =0, , can be identified with the symbol algebra for Boutet de Monvel's calculus; for ≠0 the fibers are isomorphic to the algebra of compact operators. We therefore obtain a natural map . Using deformation theory we show that this is the analytic index map. On the other hand, using ideas from noncommutative geometry, we construct the topological index map and prove that it coincides with the analytic index map. 相似文献