130,467 research outputs found
The Nowicki conjecture for relatively free algebras
No full textA linear locally nilpotent derivation of the polynomial algebra K[Xm] in m variables over a field K of characteristic 0 is called a Weitzenböck derivation. It is well known from the classical theorem of Weitzenböck that the algebra of constants K[Xm]δ of a Weitzenböck derivation δ is finitely generated. Assume that δ acts on the polynomial algebra K[X2d] in 2d variables as follows: δ(x2i)=x2i−1, δ(x2i−1)=0, i=1,...,d. The Nowicki conjecture states that the algebra K[X2d]δ is generated by x1,x3....,x2d−1, and x2i−1x2j−x2ix2j−1, 1≤
Theoretical Investigation of the Biogenetic Pathway for Formation of Antibacterial Indole Alkaloids from Voacanga africana
No full textThe energetic viability of the previously proposed biogenetic pathway for the formation of two unique monoterpenoid indole alkaloids, voacafricine A and B, which are present in the fruits of Voacanga africana, was investigated using density functional theory computations. The results of these calculations indicate that not only is the previously suggested pathway not energetically viable but also that an alternative biosynthetic precursor is likely
Influence of ionic liquids on the electronic environment of atomically dispersed Ir on (MgO)(100)
No full textRecently, ionic liquids (ILs) have been used as ligands for single-site Ir(CO)(2) complexes bound to metal-oxide supports because of their electron-donor/acceptor capacities. The combined effects of supports and ILs as ligands may pave the way to the tuning of the surrounding electronic properties to increase electron-donor/acceptor efficiency in metal-oxide supported Ir(CO)(2) complexes. Herein, we have used Density Functional Theory to model Ir(CO)(2) complexes bound to MgO supports with and without the presence of an IL to explain the role of ILs in modifying the electronic structure of the supported complex. Comparison of the nu(CO) band stretching frequencies with experimental results has led to the rationalization of the factors driving the interactions between the IL, the support, and the catalyst as well as the justification of the methodology for further studies
Insight into the Thiol-yne Kinetics via a Computational Approach
No full textThiol-yne reactions have drawn attention because of the click nature as well as the regular step-growth network nature of their products, despite the radical-mediated reactant. However, the factors governing the reaction pathways have not been examined using quantum chemical tools in a comprehensive manner. Thereupon, we have systematically investigated the mechanism of thiol-yne reactions, focusing on the structural influences of thiol and alkyne functionalities. The reaction kinetics, structure-reactivity relations, and E/Z diastereoselectivity of the products have been enlightened for the first cycle of the thiol-yne polymerization reaction. For this reason, a diverse set of 11 thiol-yne reactions with four thiols and eight alkynes was modeled by means of density functional theory. We performed a benchmark study and determined the M06-2X/6-31+G(d,p) level of theory as the best cost-effective methodology to model such reactions. Results reveal that spin density, the stabilities of sulfur radicals for propagation, and the stability of alkenyl intermediate radicals for the chain transfer are the determining factors of each reaction rate. Intramolecular p-p stacking interactions at transition-state structures are found to be responsible for Z diastereoselectivity
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mechanistic Investigation of Lysine-Targeted Covalent Inhibition of PI3K delta via ONIOM QM:QM Computations
No full textPhosphoinositide 3-kinase (PI3K) enzymes are important drug targets, especially in oncology, and several inhibitors are currently under investigation in clinical trials for the treatment of lymphocytic leukemia, follicular lymphoma, breast, thyroid, colorectal, and lung cancer. Targeted covalent inhibitors hold significant promise for drug discovery research especially for kinases. Targeting the lysine residues attracts attention as a new strategy in designing targeted covalent inhibitors, since the lysine residue provides several advantages over the traditional cysteine residue. Recently, new highly selective covalent inhibitors of PI3K delta with activated ester warheads, targeting the conserved Lys779 residue, were reported. Based on the observed kinetics, a covalent inhibition mechanism was proposed, but the atomistic details of the reaction are still not understood. Therefore, in the present work, we have conducted quantum chemical ONIOM M06-2X/6-31+G(d,p):PM6 calculations on the active site cluster structure of PI3K delta to elucidate the microscopic details of the mechanism of the aminolysis reaction between Lys779 and the ester inhibitors. Our calculations clearly discriminate the noncovalent methyl ester inhibitor and the covalent inhibitors with activated phenolic esters. For the representative p-NO2, p-F, p-H, and p-OCH3 phenolic esters, the Gibbs free energy profiles of alternative mechanistic paths through either Asp782 or Asp911 demonstrate the modulatory role of active site aspartate residues. The most plausible path alters depending on the electron-withdrawing/donating nature of the psubstituted phenolate leaving group. Inhibitors with sufficiently strong electron-withdrawing group prefer direct dissociation of the leaving group from the tetrahedral zwitterion intermediate, while the ones with electron-donating group favor the formation of a neutral tetrahedral intermediate prior to the dissociation. The relative Gibbs free energy barriers of p-NO2 < p- F < p-H < p-OCH3 substituted phenyl esters display the same qualitative trend as the experimentally measured k(inact)/K-1 values. Our results provide in depth insight into the mechanism, which can pave the way for optimizing the inhibitor efficiency
Peasant household survival strategies: rural transformation in the heartland of Turkey's hazelnut production belt
No full textThis study analyses the dynamics of persistence of the peasantry in a capitalist social formation through a case study of a village (Kayadibi) of hazelnut producers in the Central Black Sea region of Turkey. In the analysis the peasant household is given analytical priority as it is seen to be the single most important social institution through which the peasantry interacts, condition and is conditioned by the wider social, economic and political structures. Within such an analytical framework, this study concentrates on three areas of inquiry concerning the dynamics of survival of peasant modes. This is carried out in the context of the process of rural socio-economic transformation which took place under the impact of capitalism and with the start of hazelnut production for the world market in the early nineteenth century. These are: (1) the historical and contingent factors which contributed to the emergence and decline of big land- ownership and the new forms of development of capitalism in agriculture; (2) the areas of disputes and clashes of interests between the peasantry, the state and the merchants concerning the actual form of organization of the commodity and credit markets and further development or restriction of hazelnut production in the country; and (3) the patterns and mechanisms which enable the peasant households to have continuous access to land, labour and credit. The thesis arrives at the conclusion that the key to the persistence of the peasantry, as a property-owning social category of the society in a capitalist formation, is its strategy of diversifying its sources of income in order to decrease the degree of its dependency on land-bound agricultural production. This is combined with the strategy of consolidating its savings in the means of production in its own possession instead of using them to improve its standards of living and consumption
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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