Deuterium kinetic isotope effects in homogeneous decatungstate catalyzed photooxygenation of 1,1-diphenylethane and 9-methyl-9H-fluorene: evidence for a hydrogen abstraction mechanism

The homogeneous decatungstate catalyzed photooxygenation of 1,1-diphenylethane and 9-methyl-9H-fluorene has been studied mechanistically. The primary and β-secondary kinetic isotope effects provide strong evidence for a stepwise mechanism, with a hydrogen atom abstraction in the rate-determining step.     

LC-NMR coupling technology: recent advancements and applications in natural products analysis

An overview of recent advances in nuclear magnetic resonance (NMR) coupled with separation technologies and their application in natural product analysis is given and discussed. The different modes of LC-NMR operation are described, as well as how technical improvements assist in establishing LC-NMR as an important tool in the analysis of plant-derived compounds. On-flow, stopped-flow and loop-storage procedures are mentioned, together with the new LC-SPE-NMR configuration. The implementation of mass spectrometry in LC-NMR is also useful on account of the molecular weight and fragmentation information that it provides, especially when new plant species are studied. Cryogenic technology and capillary LC-NMR are the other important recent developments. Since the plant kingdom is endless in producing potential drug candidates, development and optimization of LC-NMR techniques convert the study of natural products to a less-time-consuming task, speeding up identification.     

Probing the structure and energetics of dislocation cores in SiGe alloys through Monte Carlo simulations

We present a methodology for the investigation of dislocation energetics in segregated alloys based on Monte Carlo simulations which equilibrate the topology and composition of the dislocation core and its surroundings. An environment-dependent partitioning of the system total energy into atomic contributions allows us to link the atomistic picture to continuum elasticity theory. The method is applied to extract core energies and radii of 60 degrees glide dislocations in segregated SiGe alloys which are inaccessible by other methods.     

Anomalous bias dependence of spin torque in magnetic tunnel junctions

We predict an anomalous bias dependence of the spin transfer torque parallel to the interface, Tparallel, in magnetic tunnel junctions, which can be selectively tuned by the exchange splitting. It may exhibit a sign reversal without a corresponding sign reversal of the bias or even a quadratic bias dependence. We demonstrate that the underlying mechanism is the interplay of spin currents for the ferromagnetic (antiferromagnetic) configurations, which vary linearly (quadratically) with bias, respectively, due to the symmetric (asymmetric) nature of the barrier. The spin transfer torque perpendicular to interface exhibits a quadratic bias dependence.     

Local density inhomogeneities and dynamics in supercritical water: A molecular dynamics simulation approach

Molecular dynamics atomistic simulations in the canonical ensemble (NVT-MD) have been used to investigate the "Local Density Inhomogeneities and their Dynamics" in pure supercritical water. The simulations were carried out along a near-critical isotherm (Tr = T/Tc = 1.03) and for a wide range of densities below and above the critical one (0.2 rho(c) - 2.0 rho(c)). The results obtained reveal the existence of significant local density augmentation effects, which are found to be sufficiently larger in comparison to those reported for nonassociated fluids. The time evolution of the local density distribution around each molecule was studied in terms of the appropriate time correlation functions C(Delta)rhol(t). It is found that the shape of these functions changes significantly by increasing the density of the fluid. Finally, the local density reorganization times for the first and second coordination shell derived from these correlations exhibit a decreasing behavior by increasing the density of the system, signifying the density effect upon the dynamics of the local environment around each molecule.     

On the trace of idempotent matrices over group algebras

Let G be a group and E an idempotent matrix with entries in the group algebra CG. In this paper, we consider the embedding of CG into the von Neumann algebra G and use the center-valued trace on the latter, in order to obtain some information about the coefficients of the Hattori-Stallings rank of E. Our results generalize the inequalities obtained previously by Kaplansky [11], Passi, Passmann, Luthar and Alexander [1,10,12], while providing at the same time a unified and coherent presentation of these, via the notion of moments that are associated with E.     

beta-Barrel transmembrane proteins: Geometric modelling, detection of transmembrane region, and structural properties

The location of the membrane lipid bilayer relative to a transmembrane protein structure is important in protein engineering. Since it is not present on the determined structures, it is essential to automatically define the membrane embedded protein region in order to test mutation effects or to design potential drugs. beta-Barrel transmembrane proteins, present in nature as outer membrane proteins (OMPs), comprise one of the two transmembrane protein fold classes. Lately, the number of their determined structures has increased and this enables the implementation and evaluation of structure-based annotation methods and their more comprehensive study. In this paper, we propose two new algorithms for (i) the geometric modelling of beta-barrels and (ii) the detection of the transmembrane region of a beta-barrel transmembrane protein. The geometric modelling algorithm combines a non-linear least square minimization method and a genetic algorithm in order to find the characteristics (axis, radius) of a shape with axial symmetry which best models a beta-barrel. The transmembrane region is detected by profiling the external residues of the beta-barrel along its axis in terms of hydrophobicity and existence of aromatic and charged residues. TbB-Tool implements these algorithms and is available in . A non-redundant set of 22 OMPs is used in order to evaluate the algorithms implemented and the results are very satisfying. In addition, we quantify the abundance of all amino acids and the average hydrophobicity for external and internal beta-stranded residues along the axis of beta-barrel, thus confirming and extending other researchers' results.     

Direct influence of hydrogen-bonding on the reduction potential of a CuII center

Two hydrogen-bonds from geometrically constrained OH groups to coordinated oxygen donors shift the reduction potential of a Cu(II) complex by +270 mV as compared to the structurally analogous reference complex missing the OH groups.     

A descent hybrid conjugate gradient method based on the memoryless BFGS update

In this work, we present a new hybrid conjugate gradient method based on the approach of the convex hybridization of the conjugate gradient update parameters of DY and HS+, adapting a quasi-Newton philosophy. The computation of the hybrization parameter is obtained by minimizing the distance between the hybrid conjugate gradient direction and the self-scaling memoryless BFGS direction. Furthermore, a significant property of our proposed method is that it ensures sufficient descent independent of the accuracy of the line search. The global convergence of the proposed method is established provided that the line search satisfies the Wolfe conditions. Our numerical experiments on a set of unconstrained optimization test problems from the CUTEr collection indicate that our proposed method is preferable and in general superior to classic conjugate gradient methods in terms of efficiency and robustness.