Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Solid state spin densities in triplet-exciton TCNQ salts

The combination of molecular spin densities and of structural data is shown to provide a sensitive test for the fine structure splittings and principal axes of triplet spin excitons in organic ion-radical crystals, in support of weakly-perturbed molecular sites in these solids. The exchange pathway in Rb(TCNQ) and the occurrence of dimer radicals, with fractional charges, in several tetrameric (TCNQ)₄^2? stacks illustrate the comparisons afforded by computations of fine structure parameters for triplets states in the solid.     

Probing two-photon properties of molecules: large non-Condon effects dominate the resonance hyper-Raman scattering of rhodamine 6G

Experimentally measured resonance hyper-Raman (RHR) spectra spanning the S(1) ← S(0), S(2) ← S(0), and S(3) ← S(0) transitions in rhodamine 6G (R6G) have been recorded. These spectra are compared to the results of first-principles calculations of the RHR intensity that include both Franck-Condon (A-term) and non-Condon (B-term) scattering effects. Good agreement between the experimental and theoretical results is observed, demonstrating that first-principles calculations of hyper-Raman intensities are now possible for large molecules such as R6G. Such agreement indicates that RHR spectroscopy will now be a routine aid for probing multiphoton processes. This work further shows that optimization of molecular properties to enhance either A- or B-term scattering might yield molecules with significantly enhanced two-photon properties.     

Modification of porous suspension‐PVC particles by stabilizer‐free aqueous dispersion polymerization of absorbed acrylate monomers

The present study describes modification of porous PVC particles by polymerization of a monomer/crosslinker/peroxide solution absorbed within the PVC particles. The modifying crosslinked polymers include butyl acrylate (BA) crosslinked with ethylene glycol dimethacrylate (EGDMA) and ethylhexyl acrylate (EHA) crosslinked with EGDMA. The monomer solution is blended with the PVC particles by dry‐blending. The monomer absorbed particles are then polymerized in a stabilizer‐free aqueous dispersion‐polymerization. The modified semi‐IPN PVC particles have better stability than the neat PVC particles in packed columns for absorption of halo‐organics from water, etc. The modified semi‐IPN PVC particles are melt processable and thus have the potential of being interesting and useful modified rigid PVC materials. The modified PVC particles characterization includes polymerization yield, non‐extractables and porosity measurements and also morphology and dynamic mechanical behavior (DMTA). PBA and PEHA polymerization has shown high yield levels. The high conversion of BA and EHA within the particle, is partly due to their low solubility in water. The levels of non‐extractable fractions found are indicative of low chemical interaction between the polyacrylate/PVC phases in the particle. The modified PVC particle's porosity levels indicate that BA and EHA partly polymerize within the PVC particles' bulk and partly in the pores as crusts covering the PVC pore surfaces. This finding is supported by SEM observations of unetched and etched freeze fractured surfaces. Higher crosslinking levels of the polyacrylate modification promote compatibility with the PVC particles' bulk. DMTA measurements show two loss modulus peaks for the 0.5%EGDMA blends in the glass transition temperature region, suggesting imcompatibility. However, at 5%EGDMA a single transition is found exhibiting enhanced compatibility owing to the high degree of crosslinking, which prevents phase separation. Copyright © 2002 John Wiley & Sons, Ltd.     

A note on the largest eigenvalue of a large dimensional sample covariance matrix

Let {v_ij; i, J = 1, 2, …} be a family of i.i.d. random variables with E(v₁₁⁴) = ∞. For positive integers p, n with p = p(n) and p/n → y > 0 as n → ∞, let M_n = (1/n) V_n V_n^T , where V_n = (v_ij)_{1 ≤ i ≤ p, 1 ≤ j ≤ n}, and let λ_max(n) denote the largest eigenvalue of M_n. It is shown that a.s. This result verifies the boundedness of E(v₁₁⁴) to be the weakest condition known to assure the almost sure convergence of λ_max(n) for a class of sample covariance matrices.     

First- and second-harmonic radiations from a near-millimeter-wave gyromonotron

First- and second-harmonic radiations in frequency regions centered around 120 and 240 GHz, respectively, have been observed from a gyromonotron employing magnetic fields between 34 and 54 kG and a magnetron injection gun preducing a 30-kV, 1- to 2-A electron beam. Extensive redesign of the cavity, electron beam tunnel holders, and vacuum envelope resulted in greatly improved tube performance over that obtained previously. Nine first-harmonic modes and three second-harmonic modes were identified with specific TE cavity modes. These identifications were based on agreement between the measured frequency of a radiation and the calculated resonant frequency of a TE mode, and one or both of the following: (a) the agreement between the magnetic field at which the radiation occurred and that predicted by linear theory calculations of threshold current versus magnetic field for that mode; and (b) the similarity between the far-field antenna pattern measured with calibrated, waveguide-mounted detectors and that predicted for that mode. Output power was measured with a Scientech 362 Calorimeter and also estimated by integrating the far-field antenna patterns measured with the calibrated detectors. The power ranged from a fraction of a watt to 200 W for second-harmonic modes, and from tens of watts to 2 kW for first-harmonic modes.     

On the closability of Dirichlet forms

Summary Necessary and sufficient conditions are found for closability of a two dimensional Dirichlet form which reduces to whenever f is supported in the complement of the x-axis.     

String cosmology: a review

We give an overview of the status of string cosmology. We explain the motivation for the subject, outline the main problems, and assess some of the proposed solutions. Our focus is on those aspects of cosmology that benefit from the data of an ultraviolet-complete theory.     

Synthesis of the enantiomers of 4-substituted γ-lactones with known absolute configuration

The highly pure enantiomers of several 4-alkyl (or alkenyl)-γ-lactones of known absolute configuration were synthesized from glutamic acid enantiomers. The key step is selective tosylate displacement rather than ring opening of the lactone tosylate (7→1) by lithium dialkylcuprate or dialkenylcuprate. The enantiomeric purity of synthesized γ-caprolactone was confirmed within the limitations of Pirkle's chiral solvating agent. The enantiomers of synthesized (Z)-6-dodecen-4-olide were used for reference to determine the enantiomeric composition of the pheromone isolated from the black-tailed deer.