In recent years, the non‐covalent interaction of halogen bonding (XB) has found increasing application in organocatalysis. However, reports of the activation of metal‐ligand bonds by XB have so far been limited to a few reactions with elemental iodine or bromine. Herein, we present the activation of metal‐halogen bonds by two classes of inert halogen bond donors and the use of the resulting activated complexes in homogenous gold catalysis. The only recently explored class of iodolium derivatives were shown to be effective activators in two test reactions and their activity could be modulated by blocking of the Lewis acidic sites. Bis(benzimidazolium)‐based halogen bonding activators provided even more rapid conversion, while the non‐iodinated reference compound showed little activity. The role of halogen bonding in the activation of metal‐halogen bonds was further investigated by NMR experiments and DFT calculations, which support the mode of activation occurring via halogen bonding.
Water potentials of leaves and nodules of broad bean (Vicia faba L.) cultivated on a sandy mixture were linearly and highly (r2 = 0.99) correlated throughout a water deprivation of plants. A decrease of 0.2 megapascal of the nodule water potential (I,w) induced an immediate 25% inhibition of the highest level of acetylene reduction of broad bean nodules attached to roots. This activity continued to be depressed when water stress increased, but the effect was less pronounced. Partial recovery of optimal C2H2 reduction capacity of mildly water stressed nodules (*,od= -1.2 megapascals) was possible by increasing the external 02 partial pressure up to 60 kilopascals. The dense packing of the cortical cells of nodules may be responsible for the limitation of 02 diffusion to the central tissue. Bacteroids isolated from broad bean nodules exhibited higher N2 fixation activity with glucose than with succinate as an energy-yielding substrate. Bacteroids from stressed nodules appeared more sensitive to O2, and their optimal activity declined with increasing nodule water deprivation. This effect could be partly due to decreased bacteroid respiration capacity with water stress. Water stress was also responsible for a decrease of the cytosolic protein content of the nodule and more specifically of leghemoglobin. The alteration of the bacteroid environment appears to contribute to the decline in N2 fixation under water restricted conditions. In Western Europe, short periods of water deprivation for broad bean cultures can occur during the growing season and reduce both growth and yield. Among the numerous studies devoted to the plant response to water stress, many were focused on the limitation of the leaf photosynthetic capacity (1 1). An immediate effect of drought resulted in a limitation of CO2 diffusion due to the stomatal closure (9,20). Furthermore, photosynthetic metabolism would also particularly affect the partitioning between sucrose and starch as recently pointed out by Quick et al. (17) and Vassey and Sharkey (26).The effect of water deprivation has also been investigated in legumes, specifically in broad beans in direct relation to their capacity to fix N2 (21). In soybeans, the decrease in nitrogenase activity was closely related to the decrease in ' This research was supported by a grant from European Communities Coordinated Agricultural Research. energy charge of the nodules (14) and to the changes in photosynthate pool sizes (8). However, the decline in N2 fixation activity of nodules during stress did not appear correlated with the availability of carbohydrates, since a large accumulation of sucrose was reported in nodules of stressed soybeans (7). On the other hand, water stress induced a 5% decline in photosynthesis in soybean, while nodule C2H2 reduction showed a 70% decrease (6), pointing out the involvement of other mechanisms in the drop of N2 fixation. Thus, the possibility for an increase in the resistance to 02 diffusion through the nodule cortex to the central tissue has been propos...
Screening synthetic combinatorial libraries may facilitate rapid drug lead discovery by substantially increasing the number of molecules tested. Drug discovery efficiency and productivity can be further improved by designing libraries to maximize their molecular diversity or by comparing them to existing collections of compounds and/or libraries to select those that complement the properties already well represented. In this paper we describe two strategies to aid in the design and comparison of combinatorial libraries. The methods employ multi-pharmacophore three-dimensional (3D) descriptors in combination with two recent proposals for dissimilarity-based compound selection and library comparison. This method allows the design to be performed in product space and library comparison to consider all pair-wise intermolecular contributions to the diversity.
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