Olefin terpolymerizations. II. 4-vinylcyclohexene and cyclooctadiene

4-Vinylcyclohexene (VCH) and cyclooctadiene (COD) were investigated as termonomers in EPDM (ethylene/propylene/diene) synthesis by using rac-ethylenebis (1-η⁵-indenyl) zir-conium dichloride ( 1 ) as a catalyst precursor. Homopolymerizations of VCH, vinylcycloh-exane and cyclohexene were compared. The parameter K_πκ_p, which is the apparent rate constant for Ziegler-Natta polymerization, is about the same for VCH and vinylcyclohexanebut is 10 times smaller for cyclohexene. Therefore, the linear olefinic double bond is more active than the cyclic internal double bond. VCH reduces ethylene polymerization rate but not propylene polymerization rate in copolymerizations. In terpolymerizations, VCH tends to suppress ethylene incorporation especially at elevated polymerization temperature and Lowers the polymer MW by about two-fold. COD has very low activity as a termonomer. © 1995 John Wiley & Sons, Inc.     

Amphoteric trap modeling of multidielectric scaled SONOS nonvolatile memory structures

We have characterized multidielectric scaled SONOS nonvolatile memory structures with the quasi-static linear voltage ramp (LVR) technique and dynamic pulse measurements. We have formulated physically-based ERASE/WRITE and retention methods with deep level amphoteric traps which capture and emit carriers to the bands in the silicon nitride film. Amphoteric trap parameters are extracted by the LVR technique. ERASE/WRITE and retention amphoteric trap model simulations agree well with the experimental dynamic pulse measurements. Experimental scaled SONOS structures have been fabricated with tunnel oxide X_OT=20 Å, nitride X_N=30 Å and blocking oxide X_OB=55 Å and demonstrated a static flatband shift of 3.6 V with ±5 V programming voltages. These structures may be used as the nonvolatile memory element in high density VLSI circuits.     

On the Role of Mixed Strategies in Elementary Decision Analysis and Related Decision-Support-System Treatments

We first demonstrate that mixed strategies are relevant in decision analysis for a maximin decision-maker quite apart from any game-theory considerations. This rectifies the apparent misconception that results from MS/OR textbooks which discuss mixed strategies only in the game-theory setting. Next we show an example of an implementable mixed strategy, by which we mean a mixed decision strategy which does not require randomization for its implementation. This application is to portfolio construction.     

Asymmetric zirconocene precursors for catalysis of propylene polymerization

Racemic isopropylidene (1-η⁵-cyclopentadienyl)(1-η⁵-indenyl) dichlorozirconium and the 3-methylindenyl derivative have been synthesized and characterized. These precursors activated with methylaluminoxane produce poly(propylene) with hemiisotactic microstructures. © 1994 John Wiley & Sons, Inc.     

Iron chelation properties of an extracellular siderophore exochelin MN

The coordination chemistry of an extracellular siderophore produced by Mycobacterium neoaurum, exochelin MN (ExoMN), is reported along with its pK(a) values, Fe(III) and Fe(II) chelation constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titration techniques. Exochelin MN is of particular interest as it can efficiently transport iron into pathogenic M. leprae, which is responsible for leprosy, in addition to its own parent cells. The Fe(III) coordination properties of ExoMN are important with respect to understanding the Fe(III) acquisition and uptake mechanism in pathogenic M. leprae, as the siderophores from this organism are very difficult to isolate. Exochelin MN has two hydroxamic acid groups and an unusual threo-beta-hydroxy-l-histidine available for Fe(III) chelation. The presence of threo-beta-hydroxy-l-histidine gives rise to a unique mode of Fe(III) coordination. The pK(a) values for the two hydroxamic acid moieties, the histidine imidazole ring and the alkylammonium groups on ExoMN, correspond well with the literature values for these moieties. Proton-dependent Fe(III)- and Fe(II)-ExoMN equilibrium constants were determined using a model involving sequential protonation of the Fe(III)- and Fe(II)-ExoMN complexes. These data were used to develop a model whereby deprotonation reactions on the surface of the complex in the second coordination shell result in first coordination shell isomerization. The overall formation constants were calculated: log beta(110) = 39.12 for Fe(III)-ExoMN and 16.7 for Fe(II)-ExoMN. The calculated pFe value of 31.1 is one of the highest among all siderophores and their synthetic analogues and indicates that ExoMN is thermodynamically capable of removing Fe(III) from transferrin. The E(1/2) for the Fe(III)ExoMN/Fe(II)ExoMN(-) couple was determined to be -595 mV from quasi-reversible cyclic voltammograms at pH = 10.8, and the pH-dependent E(1/2) profile was used to determine the Fe(II)-ExoMN protonation constants.     

High performance liquid chromatographic determination of n-butyl glycidyl ether

A novel high-performance liquid chromatographic (HPLC) technique for the determination of n-butyl glycidyl ether (n-BGE) at concentrations down to 1 ppb (1 μg/liter) has been developed. This HPLC procedure is simple, highly sensitive, and rapid within the limits described.     

PHYCOBILIPROTEINS: COMPARISON OF SOLUTION AND SINGLE CRYSTAL FLUORESCENCE FOR C-PHYCOCYANIN AND B-PHYCOERYTHRIN*

Abstract— Trimeric and hexameric solution forms of C-phycocyanin (CPC) from the cyanophyte Agme-nellum quadruplicatum have been isolated and their spectral properties compared to those obtained from single crystals. Although the absorbance peak of a suspension of small C-phycocyanin crystals is red-shifted only 7 nm relative to the solution forms, the single crystal fluorescence is red-shifted 60 nm relative to the solution forms. The crystal fluorescence spectrum exhibits a single peak at LD_max= 708 nm when excited at 514.5 or 530.9 nm and two peaks (LD_max= 661 and 708 nm) when excitation occurs at 568.2 nm. Fluorescence depolarization measurements indicate that extensive energy transfer could occur for both solution and crystal forms with the latter being dependent upon the relative orientation of the crystal with respect to the excitation dipole. Similar results were obtained with B-phycoerythrin (BPE) from the red alga Porphyridium cruentum where the single crystal fluorescence is red-shifted =50nm relative to the solution spectra with two peaks (LD_max= 583 and 617 nm) observed whose relative intensities are dependent on the excitation wavelength (LD_max– 514.5 and 530.9 nm). Single crystal fluorescent lifetimes exhibited considerable shortening relative to that observed for the solution forms. The implications of these results are discussed with respect to the possible relationships of the crystalline structures to the assembly forms present within phycobilisomes.     

Potential cerebral perfusion agents. Synthesis and evaluation of berberine analogues

The synthesis and tissue distribution studies in rats of tetra[³H]-hydroberberine ([³H] 2 ) and 8-(p-[¹²⁵I]iodobenzyl)tetrahydroberberine ([¹²⁵I] 6 ) are described. Compound 2 was synthesized by sodium borohydride reduction of berberine hydrochloride ( 1 ). Treatment of berberine hydrochloride with p-bromo-benzylmagnesium bromide gave 8-(p-bromobenzyl)dihydroberberine ( 4 ) which after sodium borohydride reduction and iodine-125 bromine exchange gave [¹²⁵I] 6 . The unsubstituted tetrahydro compound [³H] 2 showed significantly higher brain uptake (2.2% dose/gm after 5 minutes) as compared to the corresponding 8-substituted derivative [¹²⁵I] 6 . Both radiolabeled compounds washed out from the brain relatively quickly.