Borohydride anions as terminal ligands on a Fe/Mo/S cluster. Synthesis, structure, and characterization of the [(Cl4-cat)(PPr3)MoFe3S4(BH4)2]2(Bu4N)4 double-fused cubane

The synthesis and structure of the first Mo/Fe/S/BH(4) cluster is reported. Reaction of (Cl(4)-cat)(2)Mo(2)Fe(6)S(8)(PPr(3))(6) with 4 equiv of Bu(4)NBH(4) results in the formation of [(Cl(4)-cat)(PPr(3))MoFe(3)S(4)(BH(4))(2)](2)(Bu(4)N)(4) (Cl(4)-cat = tetrachloro-catecholate) which has been fully characterized. X-ray structural determination of this double-fused cubane reveals four BH(4)(-) ligands bound to four Fe atoms in a bidentate fashion. A synopsis of the solution characterization as well as the reactivity of this cluster is also presented.     

Nonlinear seismo-acoustic land mine detection and discrimination

A novel technique for detection and discrimination of artificial objects, such as land mines, pipes, containers, etc., buried in the ground, has been developed and tested. The developed approach utilizes vibration (using seismic or airborne acoustic waves) of buried objects, remote measurements of soil surface vibration (using laser or microwave vibrometers), and processing of the measured vibration to extract mine's "vibration signatures." The technique does not depend upon the material from which the mine is fabricated whether it be metal, plastic, wood, or any other material. It depends upon the fact that a mine is a "container" whose purpose is to contain explosive materials and associated detonation apparatus. The mine container is in contact with the soil in which it is buried. The container is an acoustically compliant article, whose compliance is notably different from the compliance of the surrounding soil. Dynamic interaction of the compliant container and soil on top of it leads to specific linear and nonlinear effects used for mine detection and discrimination. The mass of the soil on top of a compliant container creates a classical mass-spring system with a well-defined resonance response. Besides, the connection between mass (soil) and spring (mine) is not elastic (linear) but rather nonlinear, due to the separation of the soil/mine interface in the tensile phase of applied dynamic stress. These two effects, constituting the mine's vibration signature have been measured in numerous laboratory and field tests, which proved that the resonance and nonlinear responses of a mine/soil system can be used for detection and discrimination of buried mines. Thus, the fact that the mine is buried is turned into a detection advantage. Because the seismo-acoustic technique intrinsically detects buried containers, it can discriminate mines from noncompliant false targets such as rocks, tree roots, chunks of metal, bricks, etc. This was also confirmed experimentally in laboratory and field tests.     

Regular 4-polytopes from the Livingstone graph of Janko’s first group

The Janko group J ₁ has, up to duality, exactly two regular rank four polytopes, of respective Schl?fli types {5,3,5} and {5,6,5}. The aim of this paper is to give geometric constructions of these two polytopes, starting from the Livingstone graph.     

Fluid–solid coupling on a cluster of GPU graphics cards for seismic wave propagation

We develop a hybrid multiGPUs and CPUs version of an algorithm to model seismic wave propagation based on the spectral-element method in the case of models of the Earth containing both fluid and solid layers. Thanks to the overlapping of communications between processing nodes on the computer with calculation by means of non-blocking message passing, we obtain excellent weak scalability of this finite-element code on a cluster of 192 GPUs and speedup factors of more than one order of magnitude compared to the same code run on a cluster of traditional CPUs. This enables us to show a new geophysical phenomenon concerning wave propagation of diffracted shear waves in a layer called D″ located at the base of the Earth's mantle, namely that in this layer the transverse and radial components of these waves can undergo a relative shift even in an isotropic Earth model, whereas this observation in real seismological data was interpreted until now as an indication of the presence of anisotropy in this layer.     

Barrier Crossing in Dihydrofolate Reductasedoes not involve a rate-promoting vibration

We have studied atomic motions during the chemical reaction catalyzed by the enzyme dihydrofolate reductase of Escherichia coli (EcDHFR), an important enzyme for nucleic acid synthesis. In our earlier work on the enzymes human lactate dehydrogenase and purine nucleoside phosphorylase, we had identified fast sub-ps motions that are part of the reaction coordinate. We employed Transition Path Sampling (TPS) and our recently developed reaction coordinate identification methodology to investigate if such fast motions couple to the reaction in DHFR on the barrier-crossing timescale. While we identified some protein motions near the barrier crossing event, these motions do not constitute a compressive promoting vibration, and do not appear as a clearly identifiable protein component in reaction.