Phosphate activation in the ground state of purine nucleoside phosphorylase

Phosphate and ribose 1-phosphate (R1P) bound to human purine nucleoside phosphorylase (PNP) have been studied by FTIR spectroscopy for comparison with phosphate bound with a transition state analogue. Bound phosphate is dianionic but exists in two distinct binding modes with similar binding affinities. The phosphate of bound R1P is also dianionic. Bound R1P slowly hydrolyzes to ribose and phosphate even in the absence of nucleobase. The C-OP bond is cleaved in bound R1P, the same as in the PNP-catalyzed reaction. Free R1P undergoes both C-OP and CO-P solvolysis. A hydrogen bond to one P-O group is stronger than those to the other two P-O groups in both the PNP.R1P complex and in one form of the PNP.PO4 complex. The average hydrogen bond strength to the PO bonds in the PNP.R1P complex is less than that in water but stronger than that in the PNP.PO4 complex. Hydrolysis of bound R1P may be initiated by distortion of the phosphate moiety in bound R1P. The unfavorable interactions on the phosphate moiety of bound R1P are relieved by dissociation of R1P from PNP or by hydrolysis to ribose and phosphate. The two forms of bound phosphate in the PNP.PO4 complex are interpreted to be phosphate positioned as the product in the nucleoside synthesis direction and as the reactant in the phosphorolysis reaction; their interconversion can occur by the transfer of a proton from one PO bond to another. The electronic structure of phosphate bound with a transition state analogue differs substantially from that in the Michaelis complexes.     

Moving targets: recognition of alkyl groups

Alkyl chains can adopt seemingly unusual conformations, such as helices, when bound to natural and synthetic hydrophobic receptors. This plasticity allows the alkanes to assume shapes that are congruent to the receptor's space and fill that space properly. We describe here the use of cavitands and capsules as tools that expose the forces involved in the molecular recognition of hydrocarbons. Studies using NMR spectroscopy reveal how attractive interactions and solvophobic forces are maximized in solution through unprecedented contortions of alkanes and hint at a new generation of nanoscale mechanical devices.     

Coupling-isomerization-N,S-ketene acetal-addition sequences--a three-component approach to highly fluorescent pyrrolo[2,3-b]pyridines, [1,8]naphthyridines, and pyrido[2,3-b]azepines

Annelated 2-amino pyridines such as pyrrolo[2,3-b]pyridines, [1,8]naphthyridines, and pyrido[2,3-b]azepines can be synthesized in moderate to good yields in a consecutive one-pot three-component process by a coupling-isomerization-enamine-addition-cyclocondensation sequence of an electron-poor (hetero)aryl halide, a terminal propargyl N-tosylamine, and an N,S-ketene acetal. After the coupling-isomerization sequence, a Diels-Alder reaction with inverse electron demand of the intermediate enimine and the N,S-ketene acetal and subsequent aromatization furnish annelated 2-amino pyridines 4 that were unambiguously characterized by numerous X-ray structure analyses. These heterocycles are highly fluorescent and partially pH sensitive, and their electronic structure was studied with spectroscopic and computational methods.     

Decompositions of injection patterns for nodal flow allocation in renewable electricity networks

The large-scale integration of fluctuating renewable power generation represents a challenge to the technical and economical design of a sustainable future electricity system. In this context, the increasing significance of long-range power transmission calls for innovative methods to understand the emerging complex flow patterns and to integrate price signals about the respective infrastructure needs into the energy market design. We introduce a decomposition method of injection patterns. Contrary to standard flow tracing approaches, it provides nodal allocations of link flows and costs in electricity networks by decomposing the network injection pattern into market-inspired elementary import/export building blocks. We apply the new approach to a simplified data-driven model of a European electricity grid with a high share of renewable wind and solar power generation.     

A compact solution for ion beam therapy with laser accelerated protons

The recent advancements in the field of laser-driven particle acceleration have made Laser-driven Ion Beam Therapy (L-IBT) an attractive alternative to the conventional particle therapy facilities. To bring this emerging technology to clinical application, we introduce the broad energy assorted depth dose deposition model which makes efficient use of the large energy spread and high dose-per-pulse of Laser Accelerated Protons (LAP) and is capable of delivering homogeneous doses to tumors. Furthermore, as a key component of L-IBT solution, we present a compact iso-centric gantry design with 360° rotation capability and an integrated shot-to-shot energy selection system for efficient transport of LAP with large energy spread to the patient. We show that gantry size could be reduced by a factor of 2–3 compared to conventional gantry systems by utilizing pulsed air-core magnets.     

Signature of a type-a glass transition and intrinsic confinement effects in a binary glass-forming system

We study dynamically highly asymmetric binary mixtures comprised of small methyl tetrahydrofuran (MTHF) molecules and polystyrene. Combined use of dielectric spectroscopy, ^{2}H nuclear magnetic resonance, incoherent quasielastic neutron scattering, and depolarized dynamic light scattering allows us to selectively probe the dynamics of the components in a broad dynamic range. It turns out that the mixtures exhibit two glass transitions in a wide concentration range although being fully miscible on a macroscopic scale. In between both glass transition temperatures, the dynamics of the small molecules show strong confinement effects, e.g., a crossover from Vogel-Fulcher to Arrhenius behavior of the time constants. Moreover, the dynamical behavior of small molecules close to the slow matrix is consistent with mode coupling theory predictions for a type-A glass transition, which was expected from recent theoretical and simulation studies in comparable systems.     

Synthesis and Biological Activity of Oxidation Products of the Antiprogestine Mifepristone

Summary.  Selenium dioxide oxidation allows the selective introduction of a hydroxyl group at position 6 of the steroid skeleton of the antihormone (11β, 17β)-11-(4-dimethylamino-phenyl)-17-hydroxyl-17-(1-propynyl)-estra-4,9-dien-3-one (mifepristone, RU 486) and leads in a one-step procedure to two diastereomeric oxidation products. Their structure (6α- and 6β-hydroxy-mifepristone) was determined by NMR spectroscopy. The antiprogestinic activity of the oxidation products is comparable to that of mifepristone. Received August 25, 2000. Accepted (revised) October 24, 2000     

Influence of the transport direction on gas permeation in two-layer ceramic membranes

Experimental and theoretical results of studying gas permeation through porous membranes are presented. In order to mimic an asymmetric membrane two porous ceramic disks with different pore radii were arranged in series. Besides the possibility to perform conventional permeation measurements, the applied experimental setup permits the determination of the pressure at the interface between the two discs. To predict the performance of the asymmetric structure, in preliminary experiments structure parameters were determined for both membranes separately. For the same total pressure difference across the two-disk arrangement, different interlayer pressures and fluxes were predicted and detected experimentally depending on the flow direction.     

Embeddings of Gromov hyperbolic spaces

It is shown that a Gromov hyperbolic geodesic metric space X with bounded growth at some scale is roughly quasi-isometric to a convex subset of hyperbolic space. If one is allowed to rescale the metric of X by some positive constant, then there is an embedding where distances are distorted by at most an additive constant.?Another embedding theorem states that any -hyperbolic metric space embeds isometrically into a complete geodesic -hyperbolic space.?The relation of a Gromov hyperbolic space to its boundary is further investigated. One of the applications is a characterization of the hyperbolic plane up to rough quasi-isometries. Submitted: October 1998, Revised version: January 1999.