257 research outputs found
Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical Profiling
Gold carbenes of the general type [LAu=CR2]+ are sufficiently longâlived for spectroscopic inspection only if the substituents compensate for the largely missing stabilization of the carbene center by the [LAu]+ fragment. ÏâDonation by two fluorine substituents (R=F) is insufficient; rather, difluorocarbene complexes are so deprived in electron density that they sequester even âweakly coordinatingâ anions such as triflate or triflimide. This particular bonding situation translates into unmistakable carbenium ion chemistry upon reaction with stilbene as a model substrate
(Un-) Coupling gaze and attention outside central vision
In normal vision, shifts of attention and gaze are tightly coupled. Here we ask if this coupling affects performance also when central vision is not available. To this aim, we trained normal-sighted participants to perform a visual search task while vision was restricted to a gaze-contingent viewing window ("forced field location") either in the left, right, upper, or lower visual field. Gaze direction was manipulated within a continuous visual search task that required leftward, rightward, upward, or downward eye movements. We found no general performance advantage for a particular part of the visual field or for a specific gaze direction. Rather, performance depended on the coordination of visual attention and eye movements, with impaired performance when sustainedattention and gaze have to be moved in oppositedirections. Our results suggest that during early stages of central visual field loss, the optimal location for the substitution of foveal vision does not depend on the particular retinal location alone, as has previously been thought, but also on the gaze direction required by the task the patient wishes to perform
Polyunsaturated C-Glycosidic 4-Hydroxy-2-pyrone Derivatives: Total Synthesis Shows that Putative Orevactaene Is Likely Identical with Epipyroneâ A
Orevactaene and epipyroneâ
A were previously thought to comprise the same polyunsaturated tail but notably different C-glycosylated 4-hydroxy-2-pyrone head groups. Total synthesis now shows that the signature bicyclic framework assigned to orevactaene is a chimera; the compound is almost certainly identical with epipyroneâ
A, whose previously unknown stereochemistry has also been established during this study. Key to success was the ready formation of the bicyclic core of putative orevactaene by a sequence of two alkyne cycloisomerization reactions using tungsten and gold catalysis. Equally important was the flexibility in the assembly process gained by the use of heterobimetallic polyunsaturated modules whose termini could be selectively and consecutively addressed in a practical one-pot cross-coupling sequence
Conformational investigations in flexible molecules using orientational NMR constraints in combination with ÂłJ-couplings and NOE distances
Simulação do impacto das atividades florestais na fragilidade potencial em plantios de pinus.
Este trabalho, realizado em uma ĂĄrea com povoamento de Pinus taeda, teve por objetivo simular e mapear a influĂȘncia do valor de impacto das atividades florestais na fragilidade potencial. VariĂĄveis do solo, de declividade do terreno, valor de impacto e de cobertura vegetal foram utilizadas para gerar os mapas. Os resultados expressos pelo modelo adotado mostraram que as ĂĄreas mais frĂĄgeis apresentam em sua maioria declividades superiores a 75 % e textura que variam de arenosa a mĂ©dia, e as ĂĄreas menos frĂĄgeis apresentam predominantemente relevo plano (atĂ© 3 %) e textura argilosa. Quando simulado o impacto das atividades florestais sobre esse ambiente, verificou-se que na maior parte da ĂĄrea (78 %) todas as atividades florestais podem ser realizadas sem influĂȘncia na degradação do meio ambiente e para algumas ĂĄreas (11 %) o ambiente foi considerado menos frĂĄgil, em razĂŁo da presença de cobertura vegetal
The Catalytic Asymmetric MukaiyamaâMichael Reaction of Silyl Ketene Acetals with α,ÎČ-Unsaturated Methyl Esters
α,ÎČ-Unsaturated esters are readily available but challenging substrates to activate in asymmetric catalysis. We now describe an efficient, general, and highly enantioselective MukaiyamaâMichael reaction of silyl ketene acetals with α,ÎČ-unsaturated methyl esters that is catalyzed by a silylium imidodiphosphorimidate (IDPi) Lewis acid
MHC class II antigen presentation pathway in murine tumours: tumour evasion from immunosurveillance?
Qualitative differences in the MHC class II antigen processing and presentation pathway may be instrumental in shaping the CD4+ T cell response directed against tumour cells. Efficient loading of many MHC class II alleles with peptides requires the assistance of H2-M, a heterodimeric MHC class II-like molecule. In contrast to the HLA-DM region in humans, the ÎČ-chain locus is duplicated in mouse, with the H2-Mb1 (Mb1ÎČ-chain distal to H2-Mb2 (Mb2) and the H2-Ma (Ma) α-chain gene). Here, we show that murine MHC class II and H2-M genes are coordinately regulated in murine tumour cell lines by T helper cell 1 (IFN-Îł) and T helper cell 2 (IL-4 or IL-10) cytokines in the presence of the MHC class II-specific transactivator CIITA as determined by mRNA expression and Western blot analysis. Furthermore, MαÎČ1 and MαÎČ2 heterodimers are differentially expressed in murine tumour cell lines of different histology. Both H2-M isoforms promote equally processing and presentation of native protein antigens to H2-Ad- and H2-Ed-restricted CD4+ T cells. Murine tumour cell lines could be divided into three groups: constitutive MHC class II and CIITA expression; inducible MHC class II and CIITA expression upon IFN-Îł-treatment; and lack of constitutive and IFN-Îł-inducible MHC class II and CIITA expression. These differences may impact on CD4+ T cell recognition of cancer cells in murine tumour models. © 2000 Cancer Research Campaig
GeoestatĂstica integrada com estatĂstica multivariada e geoprocessamento na definição de unidades de manejo para o Pinus taeda.
Controlling suction by vapour equilibrium technique at different temperatures, application to the determination of the water retention properties of MX80 clay
Problems related to unsaturated soils are frequently encountered in
geotechnical or environmental engineering works. In most cases, for the purpose
of simplicity, the problems are studied by considering the suction effects on
volume change or shear strength under isothermal conditions. Under isothermal
condition, very often, a temperature independent water retention curve is
considered in the analysis, which is obviously a simplification. When the
temperature changes are too significant to be neglected, it is necessary to
account for the thermal effects. In this paper, a method for controlling
suction using the vapour equilibrium technique at different temperatures is
presented. First, calibration of various saturated saline solutions was carried
out from temperature of 20 degrees C to 60 degrees C. A mirror psychrometer was
used for the measurement of relative humidity generated by saturated saline
solutions at different temperatures. The results obtained are in good agreement
with the data from the literature. This information was then used to determine
the water retention properties of MX80 clay, which showed that the retention
curve is shifting down with increasing of temperature
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