Inelastic scattering matrix elements, cross sections and rate constants for transition B(P1/2) + H2(j = 0) ↔ B(P3/2) + H2(j = 0)

The non-adiabatic wave packet collisions of B(²P_1/2) + H₂(j = 0) ↔ B(²P_3/2) + H₂(j = 0) were calculated using the time dependent Channel Packet Method to compute transition probabilities, cross sections and rate constants. While the H₂ angular momentum j was fixed to 0, the total angular momentum of the system J, was varied from 1/2 to 153/2. The feature of the Stückelberg oscillation was shown in transition probabilities. The transition from B(²P_3/2) to B(²P_1/2) state was shown to be favored over the reverse process. The ratio of the computed rate constants was well compared with that of the analytic result obtained from the Boltzmann factor and the detailed balance.     

Synthesis and characterization of a chromium(V) cis-dioxo bis(1,10-phenanthroline) complex and crystal and molecular structures of its chromium(III) precursor

The first structurally characterized Cr(V) dioxo complex, cis-[CrV(O)2(phen)2](BF4) (2, phen=1,10-phenanthroline) has been synthesized by the oxidation of a related Cr(III) complex, cis-[Cr(III)(phen)2(OH2)2](NO3)3.2.5H2O (1, characterized by X-ray crystallography), with NaOCl in aqueous solutions in the presence of excess NaBF4, and its purity has been confirmed by electrospray mass spectrometry (ESMS), EPR spectroscopy, and analytical techniques. Previously reported methods for the generation of Cr(V)-phen complexes, such as the oxidation of 1 with PbO2 or PhIO, have been shown by ESMS to lead to mixtures of Cr(III), Cr(V), Cr(VI), and in some cases Cr(IV) species, 3. Species 3 was assigned as [CrIV(O)(OH)(phen)2]+, based on ESMS and X-ray absorption spectroscopy measurements. A distorted octahedral structure for 2 (CrO, 1.63 A; Cr-N, 2.04 and 2.16 A) was established by multiple-scattering (MS) modeling of XAFS spectra (solid, 10 K). The validity of the model was verified by a good agreement between the results of MS XAFS fitting and X-ray crystallography for 1 (distorted octahedron; Cr-O, 1.95 A; Cr-N, 2.06 A). Unlike for the well-studied Cr(V) 2-hydroxycarboxylato complexes, 2 was equally or more stable in aqueous media (hours at pH=1-13 and 25 degrees C) compared with polar aprotic solvents. A stable Cr(III)-Cr(VI) dimer, [Cr(III)(Cr(VI)O4)(phen)2]+ (detected by ESMS), is formed during the decomposition of 2 in nonaqueous media. Comparative studies of the oxidation of 1 by NaOCl or PbO2 have shown that [Cr(V)(O)2(phen)2]+ was the active species responsible for the previously reported oxidative DNA damage, bacterial mutagenicity, and increased incidence of micronuclei in mammalian cells, caused by the oxidation products of 1 with PbO2. Efficient oxidation of 1 to a genotoxic species, [Cr(V)(O)2(phen)2]+, in neutral aqueous media by a biological oxidant, hypochlorite, supports the hypothesis on a significant role of reoxidation of Cr(III) complexes, formed during the intracellular reduction of Cr(VI), in Cr(VI)-induced carcinogenicity. Similar oxidation reactions may contribute to the reported adverse effects of a popular nutritional supplement, Cr(III) picolinate.     

Facile conversion of aptamers into sensors using a 2'-ribose-linked fluorophore

In vitro selection can be used to identify nucleic acid receptors, called aptamers, that bind diverse small molecule targets. Aptamers therefore represent an attractive platform for creating sensors. Here, we report a straightforward, semi-rational approach for converting arbitrary aptamers into reagentless, single fluorophore biosensors. The local electrostatic environment at the 2'-ribose position is exquisitely sensitive to whether a nucleotide is conformationally restrained or not. Thus, by tethering an environmentally sensitive fluorescent group at an appropriate 2'-ribose group, we are able to generically detect ligand-induced conformational changes in aptamers. Three aptamers, including one for which no significant structural information can be inferred, were converted into robust small molecule sensors that function well in simple buffers, human urine, and bovine blood serum.     

Preparation of a chemiluminescent imidoester for the non-radioactive labelling of proteins.

A chemiluminescent aryl acridinium ester was synthesized which possesses an imidate ester group capable of reacting with proteins under mild conditions. The compound can be detected at levels as low as 5.2 x 10(-19) mol using commercially available luminometers and can therefore be used to produce high specific activity labelled antibodies for use in immunochemiluminometric assays. The imidate ester compares favourably with a previously reported N-succinimidyl ester in terms of its labelling properties but is easier to synthesize, requiring one less step. The compound was used to label affinity purified to synthesize, requiring one less step. The compound was used to label affinity purified sheep antibodies to human parathyroid hormone to demonstrate its utility in a two-site immunochemiluminometric assay for the measurement of intact parathyroid hormone.     

Interparticle interactions and direct imaging of colloidal phases assembled from microsphere-nanoparticle mixtures

We investigate the interparticle interactions, phase behavior, and structure of microsphere-nanoparticle mixtures that possess high size and charge asymmetry. We employ a novel Monte Carlo simulation scheme to calculate the effective microsphere interactions in suspension, yielding new insight into the origin of the experimentally observed behavior. The initial settling velocity, final sediment density, and three-dimensional structure of colloidal phases assembled from these binary mixtures via gravitational settling of silica microspheres in water and index-matched solutions exhibit a strong compositional dependence. Confocal laser scanning microscopy is used to directly image and quantify their structural evolution during assembly. Below a lower critical nanoparticle volume fraction (phi(nano) < phi(L,C)), the intrinsic van der Waals attraction between microspheres leads to the formation of colloidal gels. These gels exhibit enhanced consolidation as phi(nano) approaches phi(L,C). When phi(nano) exceeds phi(L,C), an effective repulsion arises between microspheres due to the formation of a dynamic nanoparticle halo around the colloids. From this stable fluid phase, the microspheres settle into a crystalline array. Finally, above an upper critical nanoparticle volume fraction (phi(nano) > phi(U,C)), colloidal gels form whose structure becomes more open with increasing nanoparticle concentration due to the emergence of an effective microsphere attraction, whose magnitude exhibits a superlinear dependence on phi(nano).     

The generalized tilt formula

A convex hull construction in Minkowski space defines a canonical cell decomposition for a cusped hyperbolicn-manifold. An algorithm to compute the canonical cell decomposition uses the concept of the tilt of ann-simplex relative to each of its (n–1)-dimensional faces. An essential tool for computing tilts is the tilt theorem. The tilt theorem was previously known only in dimensionsn3, and the proof was needlessly complicated. Here we offer a new, simplified proof which applies in all dimensions. We also offer a second geometric interpretation of the tilt.     

A two-region diffusion model for current-induced instabilities of step patterns on vicinal Si(1 1 1) surfaces

We study current-induced step bunching and wandering instabilities with subsequent pattern formations on vicinal surfaces. A novel two-region diffusion model is developed, where we assume that there are different diffusion rates on terraces and in a small region around a step, generally arising from local differences in surface reconstruction. We determine the steady state solutions for a uniform train of straight steps, from which step bunching and in-phase wandering instabilities are deduced. The physically suggestive parameters of the two-region model are then mapped to the effective parameters in the usual sharp step models. Interestingly, a negative kinetic coefficient results when the diffusion in the step region is faster than on terraces. A consistent physical picture of current-induced instabilities on Si(1 1 1) is suggested based on the results of linear stability analysis. In this picture the step wandering instability is driven by step edge diffusion and is not of the Mullins–Sekerka type. Step bunching and wandering patterns at longer times are determined numerically by solving a set of coupled equations relating the velocity of a step to local properties of the step and its neighbors. We use a geometric representation of the step to derive a nonlinear evolution equation describing step wandering, which can explain experimental results where the peaks of the wandering steps align with the direction of the driving field.     

Concerning the negative thermal expansion for extended chain polyethylene

Thermal expansion coefficients have been measured on ultraoriented polyethylene fibers. Samples were prepared by a solid-state (crystalline) extrusion in an Instron Capillary Rheometer at 136°, 2100 atm. and at a draw ratio of 50. The expansion coefficients between –150 and +50°, perpendicular to the fiber axis, were positive and of the order of 10^–4 °C^–1. Conversely, parallel to that direction, the observed values were both negative and small, of the order –10^–6 °C^–1. Length changes were reversible and reproducible between –150 and < +50°. Negative coefficients were attributed to the presence of highly-extended and fully-aligned chains along the fiber. Valuable information about chain conformation, dynamics and lattice perfection can be obtained for the ultraoriented morphologies simply by measuring the linear thermal expansion.

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Zusammenfassung Thermische Expansionkoeffizienten wurden an ultraorientierten Polyäthylenfasern gemessen. Die Proben wurden durch Festphasen -(kristalliner) Extrusion in einem Instron Kapillar-Rheometer bei 136°, 2100 Atm. und einem Ziehverhältnis von 50 hergestellt. Die zur Achse der Phase perpendikulären Expansionskoeffizienten zwischen –150 und +50° waren positiv und in der Größenordnung von 10^–4°C^–1. Umgekehrt waren die parallel zu dieser Richtung beobachteten Werte sowohl negativ wie auch kleiner, in der Größenordnung von 10^–6 °C^–1. Die Längenveränderungen zwischen –150 und > +50° waren reversibel und reproduzierbar. Negative Koeffizienten wurden der Anwesenheit stark gedehnter und voll geordneter Ketten längs der Fasern zugeschrieben. Wertvolle Informationen betreffs Kettenkonformation, Dynamik und Gitterstruktur ultraorientierter Morphologien können durch einfaches Messen der linearen thermischen Expansion erhalten werden.

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Résumé On a mesuré les coefficients d'expansion thermique de fibres de polyéthylène ultra-orientées. Les échantillons ont été préparés par extrusion à l'état solide (état cristallisé) dans un rhéomètre capillaire Instron, à 136°, 2100 atm., avec un rapport d'étirement de 50. Entre –150 et +50° les coefficients d'expansion perpendiculaires à l'axe des fibres sont positifs et de l'ordre des 10^–4 °C^–1. Les valeurs observées parallèlement à cette direction sont, par contre, négatives et faibles, de l'ordre de –10^–6 °C^–1. Entre –150 et > 50° les changements de longueur sont réversibles et reproductibles. On a attribué les coefficients négatifs à la présence de chaînes fortement allongées et complètement alignées le long des fibres. On peut obtenir des renseignements précieux sur la conformation des chaînes, la dynamique et la perfection des réseaux dans le cas de morphologies ultra-orientées en mesurant simplement l'expansion thermique linéaire.

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. () 136°, 2100 50. –150 +50° 10^–4 °^{– 1}. , , — –10^–6 °^{– 1}. –150 > +50°. - . , .

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On leave from Universidad Nacional del Sur, Bahia Bianca, Argentina.

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Acknowledgement is hereby given to Professor M. Jacovi and Mr. W. Perkins for their help and to the Office of Naval Research for financial support.