Influence of catalyst depletion or deactivation on polymerization kinetics. III. Solution polymerization of acrylonitrile in N,N-dimethylformamide at −30°C.

The experimental results on homogeneous polymerization of acrylonitrile initiated with the sodium triethylthioisopropoxyaluminate, NaAlEt₃S(i-Pr), catalyst in DMF at ?30°C. are compared with the prediction of equations based on a postulated mechanism. The agreement between the calculated and observed number-average molecular weight combined with the kinetic data and the relationship between the conversion and the initial catalyst concentration provides a rigorous test concerning the validity of the equations and the mechanism of the polymerization. A plausible mechanism is postulated as follows: The initiation must be relatively fast in accordance with the rate equations and the growing polymer undergoes propagation, transfer (to monomer), and deactivation simultaneously. The infrared spectrum of the low molecular weight polymer prepared at a high catalyst concentration showed strong absorption at 2337, 2205, and 1620 cm.^?1 but no absorption at 900 cm.^?1, indicating that there are two nitriles in the polymer, one of which is conjugated. The possibility of having ? CH?CH₂ groups in the polymer is ruled out by the absence of the band at 900 cm.^?1. In view of these facts, it is concluded that the polymer has a ? CH?CHCN endgroup resulting from the transfer reaction.     

Analysis of ephedra-alkaloids using sweeping and cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography methods

Two stacking methods of capillary electrophoresis (CE) were developed for the separation of very dilute solutions of ephedra-alkaloids, namely ephedrine, pseudoephedrine, methylephedrine, methylpseudoephedrine, norephedrine, and norpseudoephedrine. A sweeping method which uses a carrier comprised of phosphoric acid, sodium dodecyl sulfate (SDS), diethylamine and acetonitrile permits the detection of the alkaloids down to the 10(-1) microg/mL level, and the cation-selective exhaustive injection and sweeping micellar electrokinetic chromatography (CSEI-Sweep-MEKC) method using phosphoric acid, SDS, and acetronitrile as electrolytes can detect down to the 10(-3) microg/mL level. The former requires the conductance of the sample solution to be adjusted beforehand, and only five peaks were observed, two of which were overlapped. The latter is capable of separating the six alkaloids but has a somewhat poorer reproducibility. Using an optimized injection time, it was found that the more diluted a solution is, the greater the sweeping effect will be. The CSEI-Sweep-MEKC method with a 600 s injection time and a 10(-1) microg/mL solution concentration provides an amplification effect of approximately 10(4). The method is suitable for analyses of dilute herb drug extracts and mouse sera. The effect of buffers on the separation and validation of the methods in this study are also discussed.     

Potent and selective structure-based dibenzofuran inhibitors of transthyretin amyloidogenesis: kinetic stabilization of the native state

Transthyretin (TTR) amyloidogenesis requires rate-limiting tetramer dissociation and partial monomer denaturation to produce a misassembly competent species. This process has been followed by turbidity to identify transthyretin amyloidogenesis inhibitors including dibenzofuran-4,6-dicarboxylic acid (1). An X-ray cocrystal structure of TTR.1(2) reveals that it only utilizes the outer portion of the two thyroxine binding pockets to bind to and inhibit TTR amyloidogenesis. Herein, structure-based design was employed to append aryl substituents at C1 of the dibenzofuran ring to complement the unused inner portion of the thyroxine binding pockets. Twenty-eight amyloidogenesis inhibitors of increased potency and dramatically increased plasma TTR binding selectivity resulted. These function by imposing kinetic stabilization on the native tetrameric structure of TTR, creating a barrier that is insurmountable under physiological conditions. Since kinetic stabilization of the TTR native state by interallelic trans suppression is known to ameliorate disease, there is reason to be optimistic that the dibenzofuran-based inhibitors will do the same. Preventing the onset of amyloidogenesis is the most conservative strategy to intervene clinically, as it remains unclear which of the TTR misassembly intermediates results in toxicity. The exceptional binding selectivity enables these inhibitors to occupy the thyroxine binding site(s) in a complex biological fluid such as blood plasma, required for inhibition of amyloidogenesis in humans. It is now established that the dibenzofuran-based amyloidogenesis inhibitors have high selectivity, affinity, and efficacy and are thus excellent candidates for further pharmacologic evaluation.     

Tetracyclic triterpenoids from Melia azedarach, L.—III

From petroleum ether extracts of the bark of M. azedarach four novel tetracyclic triterpenoids have been isolated and characterized to be C₃₀ compounds of the euphane (20R) series. Previously known triterpenoids from Meliaceae have all been 20S compounds. Kulinone 1 is the first known euphane or tirucallane (20S) derivative oxygenated in the D ring; kulactone 2, kulolactone 3 and methyl kulonate 4 are additionally oxygenated in the side chain. Compounds 2 and 3 have a 2-oxa-trans-bicyclo[3,3,0]-octanone structure.     

Fumigant Activity of Bacterial Volatile Organic Compounds against the Nematodes Caenorhabditis elegans and Meloidogyne incognita

Plant-parasitic nematodes infect a diversity of crops, resulting in severe economic losses in agriculture. Microbial volatile organic compounds (VOCs) are potential agents to control plant-parasitic nematodes and other pests. In this study, VOCs emitted by a dozen bacterial strains were analyzed using solid-phase microextraction followed by gas chromatography–mass spectrometry. Fumigant toxicity of selected VOCs, including dimethyl disulfide (DMDS), 2-butanone, 2-pentanone, 2-nonanone, 2-undecanone, anisole, 2,5-dimethylfuran, glyoxylic acid, and S-methyl thioacetate (MTA) was then tested against Caenorhabditis elegans. DMDS and MTA exhibited much stronger fumigant toxicity than the others. Probit analysis suggested that the values of LC₅₀ were 8.57 and 1.43 μg/cm³ air for DMDS and MTA, respectively. MTA also showed stronger fumigant toxicity than DMDS against the root-knot nematode Meloidogyne incognita, suggesting the application potential of MTA.     

Design of a new thin-film electro-optic switch

A new thin-film electro-optic switch employing two stable states of polarization is proposed and investigated. The design can be further employed to realize a TE-TM polarization converter by constructing a Mach-Zehnder type interferometer integrated optical switch. The limitations and design procedures for electro-optic devices are also discussed.     

Distributed-feedback optical parametric oscillation by use of a photorefractive grating in periodically poled lithium niobate

We report a demonstration of distributed-feedback (DFB) optical parametric oscillation (OPO) by writing photorefractive gratings in periodically poled lithium niobate (PPLN). The photorefractive DFB structures were fabricated by illumination of PPLN with UV light through a photomask and by writing of PPLN with UV-light gated interfering laser beams at 532 nm. Evidence of OPO was observed from the spectral narrowing at the 1438.8- and the 619.3-nm signal wavelengths from 1064- and 532-nm-pumped PPLN crystals with the DFB grating periods phase matched to the 4084.5- and 3774-nm idler wavelengths, respectively.