Search for lepton-flavor-violating decays of the neutral kaon

The Fermilab KTeV experiment has searched for lepton-flavor-violating decays of the K(L) meson in three decay modes. We observe no events in the signal region for any of the modes studied, and we set the following upper limits for their branching ratios at the 90% C.L.: BR(K(L) --> pi(0) micro(+/-) e(-/+)) <7.6 x 10(-11); BR(K(L) --> pi(0)pi(0) micro(+/-) e(-/+)) <1.7 x 10(-10); BR(pi(0) --> micro(+/-) e(-/+)) <3.6 x 10(-10). This result represents a factor of 82 improvement in the branching ratio limit for K(L) --> pi(0) micro(+/-) e(-/+) and is the first reported limit for K(L) --> pi(0)pi(0) micro(+/-) e(-/+).     

Continuum modeling of twinning,amorphization, and fracture: theory and numerical simulations

A continuum mechanical theory is used to model physical mechanisms of twinning, solid-solid phase transformations, and failure by cavitation and shear fracture. Such a sequence of mechanisms has been observed in atomic simulations and/or experiments on the ceramic boron carbide. In the present modeling approach, geometric quantities such as the metric tensor and connection coefficients can depend on one or more director vectors, also called internal state vectors. After development of the general nonlinear theory, a first problem class considers simple shear deformation of a single crystal of this material. For homogeneous fields or stress-free states, algebraic systems or ordinary differential equations are obtained that can be solved by numerical iteration. Results are in general agreement with atomic simulation, without introduction of fitted parameters. The second class of problems addresses the more complex mechanics of heterogeneous deformation and stress states involved in deformation and failure of polycrystals. Finite element calculations, in which individual grains in a three-dimensional polycrystal are fully resolved, invoke a partially linearized version of the theory. Results provide new insight into effects of crystal morphology, activity or inactivity of different inelasticity mechanisms, and imposed deformation histories on strength and failure of the aggregate under compression and shear. The importance of incorporation of inelastic shear deformation in realistic models of amorphization of boron carbide is noted, as is a greater reduction in overall strength of polycrystals containing one or a few dominant flaws rather than many diffusely distributed microcracks.     

Total Synthesis of Tricolorin A

Tricolorin A (1), a structurally amazing resin glycoside with promising bioactivities from Ipomoea tricolor cav. (convolvulaceae), was synthesized in a total of 45 steps, with the longest linear sequence of 20 steps and overall yield of 0.65% from D-mannitol. The AB disaccharide 19-membered lactone 2 was constructured by a regioselective macrolactonization using Corey-Nicolaou protocol. The macrolactone tetrasaccharide 33 was realized either by "one-pot two-step" glycosylation procedure or by a stepwise assembly employing the "armed-disarmed" glycosylation strategy.     

Conformational switching between diastereoisomeric atropisomers of arenedicarboxamides induced by complexation with Lewis acids

Tertiary diamides of xanthene-1,8-dicarboxylic acid and biphenyl-2,2'-dicarboxylic acid exhibit a thermodynamic preference for anti stereochemistry which is inverted in the presence of Ti- or Sn-based Lewis acids, allowing interconversion between kinetically stable syn and anti diastereoisomeric atropisomers.     

Metal-specific allosteric activation and deactivation of a diamine

The reaction of a potentially tetradentate bis(pyridyl-thiazole) ligand with acetone is allosterically activated upon complexation with Cd(II) but deactivated by reaction with Cu(I), demonstrating metal-specific allosteric controlled reactivity.     

An innovative molybdenum column liner for oxygen and hydrogen stable isotope analysis by pyrolysis

The most widely used method for pyrolysing samples for hydrogen or oxygen isotopic analysis involves heating them to greater than 1300 degrees C in a helium stream passed through a glassy carbon tube in an alumina casing. There are a number of difficulties with this. Glassy carbon tubes are expensive and interaction between the carbon tube and the outer casing produces unwanted carbon monoxide by reduction of the alumina at high temperatures. The latter effect is overwhelming if temperatures of 1400 degrees C or greater are used for pyrolysis. We experimented with lining alumina casings with pure molybdenum sheet. It is relatively cheap, conforms well to the interior of the reactor tube (to avoid carrier and sample bypassing of the carbon pack), resists high temperatures and neither oxidises excessively nor absorbs the gases. The main disadvantages are that silver sample cups must be used and that the molybdenum degrades over time by formation of the carbide. We can maintain sharp peaks, high precision and good accuracy over more than 700 solid samples for both hydrogen and oxygen. The reactors last longer for water injections. The molybdenum in the columns does not contribute greatly to memory effects. The precision of analysis is dependent on other factors as well as the pyrolysis column, but for oxygen we typically achieve approximately <0.2 per thousand (sucrose), <0.25 per thousand (water) and <0.25 per thousand (leaf), sometimes using only a linear correction of drift, after dividing the run into 1 to 3 segments.     

Yoctomole analysis of ganglioside metabolism in PC12 cellular homogenates

We report an ultrasensitive method for the analysis of glycosphingolipid catabolism. The substrate G(M1) and the set of seven metabolites into which it can be degraded (G(A1), G(M2), G(A2), G(M3), LacCer, GlcCer, and Cer) were labeled with the highly fluorescent dye tetramethylrhodamine. CE with LIF detection was used to assay these compounds with 150 +/- 80 yoctomole mass (1 ymol = 10(-24) mol = 0.6 copies) detection limits and 5 +/- 3 pM concentration detection limits. An alignment algorithm based on migration of two components was employed to correct for drift in the separation. The within-day and between-day precision in peak height was 20%, in peak width 15%, and in adjusted migration time 0.03%. After normalization to total sample injected, the RSD in peak height reduced to 2-6%, which approaches the limit set by molecular shot noise in the number of molecules taken for analysis. PC12 cells were incubated with the labeled G(M1). Fluorescent microscopy demonstrated uptake by the cells. CE was used to separate a cellular homogenate prepared from these cells. A set of peaks was observed, which were tentatively identified based on comigration with the standards. Roughly 120 pL of homogenate was injected, which contained a total of 150 zmol of labeled substrate and products. Metabolite that preserves the fluorescent label can be detected at the yoctomole level, which should allow characterization of this metabolic pathway in single cells.     

The sixteen CB(11)H(n)Me(12-n)(-) anions with fivefold substitution symmetry: anodic oxidation and electronic structure

The 15 symmetrically methylated derivatives of the CB11H12(-) anion (1a) have been synthesized and found to vary greatly in ease of oxidation. Cyclic voltammetry in liquid SO2 yielded fully reversible oxidation potentials for five of those that have no adjacent unsubstituted vertices in positions 7-12; three others showed some indication of reversibility. The anions 1a-16a and the Jahn-Teller distorted neutral radicals 1r-16r have been characterized by ab initio and density functional theory calculations. In the state average CASSCF(13,12)/6-31+G* approximation, the ground state potential energy surface of 1r contains five symmetry-related pairs of minima. The computational results account for the reversible redox potentials very well when the solvent is included explicitly (RI-DFT(BP)/TZVP, COSMO). For display and for a semiquantitative understanding of methyl substituent effects in terms of perturbation theory, the molecular orbitals of 1a have been expressed in the symmetry-adapted cluster basis. The results serve as an underpinning for a set of additive empirical increments for redox potential prediction. Relative to the usual hydrogen standard, a single methyl group facilitates oxidation by approximately 50, 70, 70, and 10 mV in positions 1, 2, 7, and 12, respectively. This electron donor effect on the redox potential is due to a contribution, whereas those of (inductive and direct field) type are negligible.