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PubMed1999

Production of organically bound selenium yeast by continuous fermentation.

This paper describes a protocol for incorporation of sodium selenite or sodium selenate into Saccharomyces cerevisiae biomass by continuous fermentation in a medium with minimal sulfur and methionine concentrations. Selenium incorporation was followed by atomic absorption analysis and methylene blue reduction time (MBRT). Continuous fermentation at 0.2 h(-1) dilution rate and sodium selenite addition gradient up to 0.69 g/L of Na(2)SeO(3) yielded 1. 89 g/L of biomass with 1904 microg of selenium/g of dry biomass. However, MBRT was 0.1 min, which indicated that the majority of selenium was in the inorganic form. On the other hand, continuous fermentation at 0.2 h(-1) dilution rate and sodium selenate gradient up to 0.28 g/L of Na(2)SeO(4) yielded 0.76 g/L of dry biomass with 687 microg of selenium/g of dry biomass, and MBRT was 26 min, which indicated a high concentration of organically bound selenium. Overall, the results indicate a Se/S ratio of 3.9:1 and a dry biomass/Se ratio of 5.5:1 as optimal for continuous production of organically bound selenium.

Arsenicals↗
PubMed2004

Et2AlCl-promoted asymmetric phenylseleno group transfer radical cyclization reactions of unsaturated beta-hydroxy esters.

We have developed a new method for asymmetric phenylseleno group transfer radical cyclization of unsaturated beta-hydroxy esters. Various unsaturated alpha-phenylseleno beta-hydroxy esters underwent radical cyclization in the presence of Et(2)AlCl in benzene with sunlamp irradiation at 25-30 degrees C to give mono- and bicyclic group-transferred products in an efficient and highly regioselective and diastereoselective manner. To rationalize the high diastereoselectivities observed in this reaction, we propose a model based on chelation control of the aluminum alkoxides that are formed in situ. We devised a general method to prepare chiral radical precursors from which we obtained highly optically pure mono- and bicyclic group transfer products. The synthetic advantages of this method are demonstrated by our formal total synthesis of (-)-wilforonide. This paper presents the first examples of stereoselective group transfer radical cyclizations that occur via 1,2-asymmetric induction.

Aluminum Compounds↗
PubMed2006

Synthesis and antioxidant profile of all-rac-alpha-selenotocopherol.

all-rac-alpha-Selenotocopherol (6c) has been synthesized in 11 steps in 6.6% total yield. Key steps include chloromethylation to approach the persubstituted aromatic 9b and cyclization of alcohol precursor 10 by radical homolytic substitution at selenium to form the selenotocopherol heterocycle. Determination of the OH bond dissociation enthalpy (BDE) of 6c by electron paramagnetic resonance (EPR) equilibration techniques gave a value of 78.1 +/- 0.3 kcal mol(-1), approximately 1 kcal mol(-1) higher than that of alpha-tocopherol. Kinetic studies performed by measuring oxygen uptake of the induced oxidation of styrene in the presence of an antioxidant showed that selenotocopherol (6c) was a slightly poorer inhibitor than alpha-tocopherol, in agreement with the BDE values. In contrast to simpler selenotocopherol analogues, 6c was not regenerable in the presence of a stoichiometric coreductant in a two-phase lipid peroxidation model.

Antioxidants↗
PubMed2006

Microwave-assisted copper-catalyzed preparation of diaryl chalcogenides.

Diaryl chalcogenide synthesis employing diaryl dichalcogenides and aryl halides as starting materials in the presence of excess magnesium and a catalytic amount of CuI/bipyridyl is significantly improved by microwave heating. Reaction times can be reduced from 2 to 3 days to 6-8 h. Both aryl bromides and aryl chlorides can be used as substrates in the substitution reaction. The procedure is useful not only for diaryl sulfide and diaryl selenide synthesis but also for the preparation of unsymmetrical diaryl tellurides. Starting from suitable aryl halides, the novel microwave-assisted procedure was used for the facile preparation of novel chalcogen analogues (PhS-, PhSe-, and PhTe-) of various antioxidants (ethoxyquin and 3-pyridinol). Attempts to use dialkyl dichalcogenides for the coupling of alkylchalcogeno moieties to aryl halides were only successful in the case of long-chain (such as n-octyl) disulfides and diselenides.

Catalysis↗
PubMed2005

Self-assembled monolayers of alkaneselenolates on (111) gold and silver.

Self-assembled monolayers (SAMs) formed from didodecyl diselenide (C12SeSeC12) and didodecyl selenide (C12SeC12) on (111) Au and Ag substrates were extensively characterized by several complementary techniques. C12SeSeC12 was found to form contamination-free, densely packed, and well-ordered C12Se SAMs on both substrates, whereas the adsorption of C12SeC12 occurred only on Au and resulted in the formation of a SAM-like C12SeC12 film with a low packing density and a conformational disorder. The properties of the C12Se SAMs were compared with those of dodecanethiolate (C12S) SAMs. The packing density, orientational order, and molecular inclination in C12Se/Au and C12S/Au were found to be very similar. In contrast, C12Se/Ag exhibited significantly lower packing density, a lower degree of the conformational and orientational order, and a larger molecular inclination than C12S/Ag. The results suggest a sp3 bonding configuration for the selenium atom on Au and Ag and indicate a larger corrugation of the headgroup-substrate binding energy surface in C12Se/Ag than in C12S/Ag.

Gold↗
PubMed2006

Air-stable PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystal quantum dots and their applications.

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

Air↗
PubMed2005

Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots.

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.

Colloids↗
PubMed2005

Local charge transport in two-dimensional PbSe nanocrystal arrays studied by electrostatic force microscopy.

Two-dimensional PbSe nanocrystal arrays on silicon nitride membranes were investigated using electrostatic force microscopy (EFM) and transmission electron microscopy (TEM). Changes in lattice and transport properties upon annealing in a vacuum were revealed. Local charge transport behavior was directly imaged by EFM and correlated to nanopatterns observed with TEM. Charge transport through nanochannels in complex two-dimensional nanocrystal networks was identified. Our results demonstrate the importance of measurements of local transport details complementary to the conventional current-voltage (I-V) measurements.

Crystallization↗
PubMed2006

Impact of ripening on manganese-doped ZnSe nanocrystals.

We examine the impact of growth kinetics on the incorporation of Mn dopants into ZnSe nanocrystals. We synthesize such particles, also known as colloidal quantum dots, and use optical spectroscopy to extract information about the average number of Mn impurities per nanocrystal as the reaction proceeds. We find that this number increases with particle growth until the Zn and/or Se precursors are depleted in the reaction solution. If the reaction is continued further, then ripening of the colloid begins and the average number of Mn per nanocrystal decreases, even as the particles slowly increase in size. We show that this effect, which is detrimental for enhanced doping, can be avoided if the reactant concentration is maintained by addition of more reactants. We consider several explanations and conclude that intraparticle ripening, in which material is redistributed on the same nanocrystal due to evolution of the particle shape, is the most consistent with experimental observations.

Colloids↗
PubMed2006

Shape control of PbSe nanocrystals using noble metal seed particles.

We demonstrate that the shape of PbSe nanocrystals can be controlled systematically by seeding their growth with noble metal nanoparticles (Au, Ag, or Pd) and varying the seed and precursor concentrations. Cylinders (quantum rods), cubes, crosses, stars, and branched structures were produced in high yield at 150 degrees C in reaction times of a few minutes. Although their absorption spectrum does not exhibit sharp features, the quantum rods exhibit significant photogeneration efficiency, enabling infrared sensitization of a polymeric photoconductive nanocomposite.

Crystallization↗
PubMed2006

Impact ionization can explain carrier multiplication in PbSe quantum dots.

The efficiency of conventional solar cells is limited because the excess energy of absorbed photons converts to heat instead of producing electron-hole pairs. Recently, efficient carrier multiplication has been observed in semiconductor quantum dots. In this process, a single, high-energy photon generates multiple electron-hole pairs. Rather exotic mechanisms have been proposed to explain the efficiency of carrier multiplication in PbSe quantum dots. Using atomistic pseudopotential calculations, we show here that the more conventional impact ionization mechanism, whereby a photogenerated electron-hole pair decays into a biexciton in a process driven by Coulomb interactions between the carriers, can explain both the rate (<<1 ps) and the energy threshold ( approximately 2.2 times the band gap) of carrier multiplication, without the need to invoke alternative mechanisms.

Computer Simulation↗
PubMed2001

A new, iterative strategy of oligosaccharide synthesis based on highly reactive beta-bromoglycosides derived from selenoglycosides.

Stereoselective conversion of a selenoglycoside to a beta-bromoglycoside in the absence of a glycosyl acceptor followed by the coupling with another selenoglycoside affords the corresponding glycosylated selenoglycoside, which could be directly used for the next glycosylation. The iteration of this sequence allows the synthesis of a variety of oligosaccharides including an elicitor active heptasaccharide. [reaction: see text]

Bromine↗
PubMed2002

Galacto, gluco, manno, and disaccharide-based C-glycosides of 2-amino-2-deoxy sugars.

[reaction: see text] Starting from readily available precursors, selenoglycosides derived from GalNAc, GlcNAc, and ManNAc were prepared by either a one- or a two-step process. The anomeric selenides underwent facile C-Se homolysis to provide the corresponding anomeric radicals, which were trapped with alkenes to give C-glycosides. This provides a general entry to alpha-C-glycosides based on 2-amino-2-deoxy sugars that is also applicable to disaccharide variants.

Amino Sugars↗
PubMed2005

New reactions of selenocarboxylates.

By treatment with Woollins' reagent in toluene solution, carboxylic acids are converted to selenocarboxylic acids. The latter react in situ to provide new products of acid- or base-promoted substitution, addition, and amidation.

Carboxylic Acids↗
PubMed2004

A fluorinated selenide linker for solid-phase synthesis of n-pentenyl glycosides.

[reaction: see text] A fluorine-labeled selenide linker for installing terminal isolated olefins has been synthesized in high overall yield. The resin-bound linker could be glycosylated both with glycosyl trichloroacetimidates and glycosyl fluorides. The linker did not decompose after oxidation with tBuOOH but underwent beta-elimination when it was subjected to heat. This allowed the released n-pentenyl glycoside 15 to be isolated in excellent yield and purity after filtration.

Glucosides↗