A reinterpretation of the mechanism of the simplest reaction at an sp3-hybridized carbon atom: H + CD4 --> CD3 + HD

A comparison between theory and experiment for the benchmark H + CD4 --> HD + CD3 abstraction reaction yields a reinterpretation of the reaction mechanism and highlights the unexpected contribution of a stripping mechanism. Whereas the best analytic surface fails to reproduce experiment, a first-principles direct-dynamics (on the fly) treatment is in good agreement, showing that the H + CD4 reaction exhibits extreme sensitivity to modest differences in the potential energy surface. We find that bent H-D-C transition state geometries play an important role in the dynamics. A simple model that relates the scattering angle impact parameter and cone of acceptance accounts well for the overall reaction dynamics.     

Utilization and transport of l-arabinose by non-Saccharomyces yeasts

l-Arabinose is one of the sugars found in hemicellulose, a major component of plant cell walls. The ability to convert l-arabinose to ethanol would improve the economics of biomass to ethanol fermentations. One of the limitations for l-arabinose fermentation in the current engineered Saccharomyces cerevisiae strains is poor transport of the sugar. To better understand l-arabinose transport and use in yeasts and to identify a source for efficient l-arabinose transporters, 165 non-Saccharomyces yeast strains were studied. These yeast strains were arranged into six groups based on the minimum time required to utilize 20 g/L of l-arabinose. Initial transport rates of l-arabinose were determined for several species and a more comprehensive transport study was done in four selected species. Detailed transport kinetics in Arxula adeninivorans suggested both low and high affinity components while Debaryomyces hansenii var. fabryii, Kluyveromyces marxianus and Pichia guilliermondii possessed a single component, high affinity active transport systems.     

Intramolecular sulfoxide electrophilic sulfenylation in 2- and 3-indoleanilides

When N-[2-(alkylsulfinyl)phenyl]-1H-indole-2-carboxamides with varying degrees of indolic and amidic N-alkylation are heated in an inert solvent or treated with trifluoroacetic anhydride; only compounds in which the amidic nitrogen is methylated cyclize to indolo[3,2-b]-1,5-benzothiazepinones (9, 10). Successful cyclization is attributed to the ability of N-Me amides to readily adopt a conformation conducive to cyclization, which other derivatives are unable to achieve. The analogous 3-indoleanilide, N,N-dimethyl N-[2-(ethylsulfinyl)phenyl]-1H-indole-3-carboxamide (17a), undergoes SES/rearrangement to produce 10 upon heating in p-xylene. An intermediate 3H-indolinium spirocyclic species is proposed to account for this result.     

Dynamic kinetic resolution of racemic tropic acid ethyl ester and its derivatives

The dynamic kinetic resolution of racemic mixtures of tropic acid ethyl ester under substrate racemizing conditions was studied using lipase PS with a ruthenium catalyst. Isopropenyl acetate was used as an acyl donor, since it was found to be compatible with both catalysts; this resulted in an efficient dynamic kinetic resolution. With this process, a variety of racemic tropic acid ethyl esters were transformed to optically active acetoxy-2-arylpropionic acid ethyl esters with 60-88% yields and 53-92% ee.     

Novel photopolymerizable biodegradable triblock polymers for tissue engineering scaffolds: synthesis and characterization

For use in micro-patterned scaffolds in tissue engineering, novel diacrylated triblock macromers (PLA-b-PCL-b-PLA, PGA-b-PCL-b-PGA and PCL-b-PEO-b-PCL) were synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). All diacrylated polymers were designed as triblock copolymers and involved biodegradable blocks of relatively non-polar epsilon-caprolactone (CL) and polar monomers such as glycolide (GA), lactide (LA) or ethylene oxide (EO). All triblock polymers were prepared in molecular weights of a few kilo daltons via the anionic ring-opening polymerization (ROP) of the corresponding lactide, glycolide or caprolactone using stannous octoate [Sn(Oct)(2)] as catalyst. The polymers had low polydispersity indices, ranging from 1.23 to 1.56. Biodegradable polymeric networks were prepared with conversions of 72-84% via photopolymerization of the triblock diacrylated polymers with 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator. PLA-b-PCL-b-PLA copolymers crumbled easily and were not suitable for micro-patterning. PGA-b-PCL-b-PGA copolymers had higher water contact angles than PCL-b-PEO-b-PCL and were also cytocompatible with Fibroblasts 3T3.     

Nuclear resonance vibrational spectra of five-coordinate imidazole-ligated iron(II) porphyrinates

Nuclear resonance vibrational spectra have been obtained for six five-coordinate imidazole-ligated iron(II) porphyrinates, [Fe(Por)(L)] (Por = tetraphenylporphyrinate, octaethylporphyrinate, tetratolylporphyrinate, or protoporphyrinate IX and L = 2-methylimidazole or 1,2-dimethylimidazole). Measurements have been made on both powder and oriented crystal samples. The spectra are dominated by strong signals around 200-300 cm(-1). Although the in-plane and out-of-plane vibrations are seriously overlapped, oriented crystal spectra allow their deconvolution. Thus, oriented crystal experimental data, along with density functional theory (DFT) calculations, enable the assignment of key vibrations in the spectra. Molecular dynamics are also discussed. The nature of the Fe-N(Im) vibrations has been elaborated further than was possible from resonance Raman studies. Our study suggests that the Fe motions are coupled with the porphyrin core and peripheral groups motions. Both peripheral groups and their conformations have significant influence on the vibrational spectra (position and shape).     

Rapid computation of intermolecular interactions in molecular and ionic clusters: self-consistent polarization plus symmetry-adapted perturbation theory

A method that we have recently introduced for rapid computation of intermolecular interaction energies is reformulated and subjected to further tests. The method employs monomer-based self-consistent field calculations with an electrostatic embedding designed to capture many-body polarization (the "XPol" procedure), augmented by pairwise symmetry-adapted perturbation theory (SAPT) to capture dispersion and exchange interactions along with any remaining induction effects. A rigorous derivation of the XPol+SAPT methodology is presented here, which demonstrates that the method is systematically improvable, and moreover introduces some additional intermolecular interactions as compared to the more heuristic derivation that was presented previously. Applications to various non-covalent complexes and clusters are presented, including geometry optimizations and one-dimensional potential energy scans. The performance of the XPol+SAPT methodology in its present form (based on second-order intermolecular perturbation theory and neglecting intramolecular electron correlation) is qualitatively acceptable across a wide variety of systems-and quantitatively quite good in certain cases-but the quality of the results is rather sensitive to the choice of one-particle basis set. Basis sets that work well for dispersion-bound systems offer less-than-optimal performance for clusters dominated by induction and electrostatic interactions, and vice versa. A compromise basis set is identified that affords good results for both induction and dispersion interactions, although this favorable performance ultimately relies on error cancellation, as in traditional low-order SAPT. Suggestions for future improvements to the methodology are discussed.     

The caffeine contents of dietary supplements commonly purchased in the US: analysis of 53 products with caffeine-containing ingredients

As part of a study initiating the development of an analytically validated Dietary Supplement Ingredient Database (DSID) in the United States (US), a selection of dietary supplement products were analyzed for their caffeine content. Products sold as tablets, caplets, or capsules and listing at least one caffeine-containing ingredient (including botanicals such as guarana, yerba mate, kola nut, and green tea extract) on the label were selected for analysis based on market share information. Two or three lots of each product were purchased and analyzed using high-pressure liquid chromatography (HPLC). Each analytical run included one or two National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) and two products in duplicate. Caffeine intake per serving and per day was calculated using the maximum recommendations on each product label. Laboratory analysis for 53 products showed product means ranging from 1 to 829 mg caffeine/day. For products with a label amount for comparison (n = 28), 89% (n = 25) of the products had analytically based caffeine levels/day of between −16% and +16% of the claimed levels. Lot-to-lot variability (n = 2 or 3) for caffeine in most products (72%) was less than 10%.