Strain and temperature accelerated relaxation in polycarbonate

The nonlinear viscoelastic behavior of glassy polymers and its relationship to ductile yielding is studied by single- and double-step stress relaxation experiments. In the latter case a small stress relaxation step is superimposed on a specimen at an elevated state of temperature or strain. The results show that the changes in the relaxation behaviors in the two cases closely parallel each other. The relaxation behavior at strains near yield closely approximates that at low strain but near Tg. The small strain relaxation response can be described well by a Kohlrausch-Williams-Watts (KWW) type function. The interpretation of these data in terms of a coupling model which includes the KWW form is discussed.     

A simple and highly efficient P,O-type ligand for Suzuki-Miyaura cross-coupling of aryl halides

A simple and efficient hemilabile-type phosphine ligand, found to be highly effective in Suzuki-Miyaura cross-coupling of aryl chlorides with generally low Pd-catalyst loading (0.05%), was prepared in one step based on an economically attractive approach from commercially available benzamide starting material.     

Elicitation of lignin peroxidase inStreptomyces lividans

Using a novel starch-based medium (DJMM) which elicits high expression of lignin peroxidase (ALiP-P3) fromStreptomyces viridosporus T7A, significant levels of ALiP-P3 (between 1135 and 1784 nmol/g cell-min) were excreted byS. lividans TK23, TK24, and TK64 with the supernatants capable of degrading dichlorophenol (these strains were previously reported to produce low levels of LiP). TheS. lividans wildtype strains produced 1/9 to 1/6 the cell-specific LiP activity previously detected inS. viridosporus T7A cultures grown in the same starch-based medium; however, by using DJMM to increase the cell density, the volumetric activity of wild-typeS. lividans TK23, TK24, and TK64 strains was increased 11 to 20-fold compared to cultivations in a yeast-extract-based medium. Consequently, this increase of LiP production allows the direct analysis of LiP activity in the supernatants of these strains without the need for enzyme concentration through ultrafiltration. Immunoblot analysis verified that a single 56.5 kDa band, secreted by all three strains, was extremely similar in size and immunologic reactivity to the 59.5 kDa ALiP-P3 isoform of S.viridosporus T7A. In addition, Western blot analysis was used to show that a previously cloned 4.1 kb chromosomal fragment ofS. viridosporus T7A DNA did not contain the ALiP-P3 structural genes.     

Studies on taxadiene synthase: interception of the cyclization cascade at the isocembrene stage with GGPP analogues

[reaction: see text] The cyclization of GGPP to taxadiene, catalyzed by taxadiene synthase, has been suggested to proceed through a series of monocyclic isocembrenyl- and bicyclic verticillyl-carbocationic intermediary stages. A set of GGPP analogues with abolished or perturbed pi-nucleophilicity at the delta10 double bond (GGPP numbering) was synthesized and incubated with taxadiene synthase to intercept the cyclization cascade at the monocyclic stage. Each analogue was transformed by taxadiene synthase in vitro to hydrocarbon products in varying yields, and the structures of the major product in each reaction were solved by GCEIMS and one- and two-dimensional (1H and 13C) NMR and found to be 14-membered monocyclic isocembrenyl diterpenes, indicating that the first C-C bond formation catalyzed by taxadiene synthase could be uncoupled from the other subsequent bond formation events by using suitably designed substrate analogues. The formation and isolation of these isocembrenyl diterpene products using taxadiene synthase supports proposals that the isocembrenyl cation is an intermediate in the cyclization of GGPP to taxadiene.     

Chelation-assisted Rh(I)-catalyzed ortho-alkylation of aromatic ketimines or ketones with olefins

Described herein is the Rh(I)-catalyzed ortho-alkylation of aromatic ketimines or ketones with olefins. This method showed high reactivity and selectivity to monoalkylation for a variety of olefins including 1-alkenes with an allylic proton, alpha,omega-dienes, and internal olefins. For a mechanistic study, H/D exchange experiments were carried out, which demonstrated that the ortho C-H bond could be easily cleaved even at the low temperature of 45 degrees C. The key step of this reaction is the formation of a stable five-membered metallacycle by a chelation-assisted ortho C-H bond activation. Furthermore, the direct ortho-alkylation of aromatic ketones with the Rh(I) complex was successfully achieved by adding 50 mol % of benzylamine as a chelation-assistant tool.     

Si-C(N) sigma linkages and N --> Si dative bonding at pyridine/Si(111)-7 x 7

We experimentally demonstrated that pyridine/Si(111)-7 x 7 can act as an electron donor/acceptor pair as a result of the charge transfer from the electron-rich N atom of pyridine to the electron-deficient adatom of the Si surface, evidenced by the upshift of 1.8 eV (state A) for the N(1s) core level upon the formation of a datively bonded complex compared to physisorbed molecules. Another state (B) whose N(1s) binding energy downshifts by 1.2 eV was assigned to an adduct through Si-C and Si-N covalent linkages, formed via a [4 + 2]-like addition mechanism on Si(111)-7 x 7. Binding molecules through the formation of the dative bond resulted from significant electron transfer opens a new approach for the creation of Si-based molecular architectures and modification of semiconductor interfacial properties with unsaturated organic molecules.     

A microfluidic system for high-speed reproducible DNA sizing and quantitation

Microfabrication technology was used to develop a system consisting of disposable glass chips containing etched channels, reagents including polymer matrix and size standards, computer-controlled instrumentation for performing electrophoretic separations and fluorescence detection of double-stranded DNA, and software for automated data analysis. System performance was validated for separation and quantitation reproducibility using samples varying in amount and size of DNA fragments, buffer composition, and salt concentrations. Several applications of the microfluidic system for DNA analysis have been demonstrated, such as of polymerase chain reaction (PCR) products, sizing of plasmid digests, and detection of point mutations by restriction fragment length polymorphism (RFLP) mapping.