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Louis A. Silks - One of the best experts on this subject based on the ideXlab platform.
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chiral n acetyl selone promoted aldol reactions
Synthetic Communications, 2009Co-Authors: Louis A. Silks, R. Michalczyk, David B Kimball, Duane Hatch, Morgane Ollivaultshiflett, Eddie MoodyAbstract:Abstract The chiral oxazolidineselone functionality was found to be an excellent partner in the stereospecific acetate aldol reaction with aldehydes via the titanium enolates. Good stereocontrol was obtained as determined by NMR Spectroscopy. The oxazolidineselone also provided a straightforward way to establish the stereopurity of the coupling reaction through 77Se NMR Spectroscopy.
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Determining the Solution State Orientation of a Ti Enolate via Stable Isotope Labeling, NMR Spectroscopy, and Modeling Studies
Journal of the American Chemical Society, 2003Co-Authors: David B Kimball, R. Michalczyk, Eddie Moody, Morgane Ollivault-shiflett, Karl De Jesus, Louis A. SilksAbstract:Our group has used Ti-promoted aldol additions with an oxazolidineselone as the chiral auxiliary with much success. In these reactions, the Se atom in the auxiliary both promotes stereospecific addition as well as reports on, through the use of 77Se NMR Spectroscopy, the ratio of diastereomers produced and the geometry of intermediates as the reaction proceeds. Through stable isotope labeling and NMR Spectroscopy, we are able to experimentally observe a Ti enolate in solution and gain insight into its structure and reactivity. Results from molecular modeling calculations are also presented for comparison with NMR data.
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Synthesis and Applications of Chiral Selones
Phosphorus Sulfur and Silicon and the Related Elements, 1998Co-Authors: Louis A. Silks, R. Bruce Dunlap, Jerome D. OdomAbstract:ABSTRACT The use of chiral selone derivatizing reagents (CDA's) and 77Se NMR Spectroscopy for the observation of remotely disposed chiral centers has proven to be a highly sensitive method for the determination of enantiomeric excesses. A variety of functional groups can be attached to these CDA's making them the most diverse class of CDA's yet developed. We are now investigating the chemistry unique to these CDA's such as new aldol reactions
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Simple enantiomeric excess determination of amines using chiral selones: unusual N–H⋯Se bonding detected by HMQC 1H/77Se NMR Spectroscopy
Chem. Commun., 1996Co-Authors: Griselda Hernández, Jerome D. Odom, R. Bruce Dunlap, Louis A. SilksAbstract:One-pot coupling of a chiral selone derivatizing agent to a series of chiral and racemic amines mediated with triphosgene gives adducts in yields ranging from 80–99%; 77Se NMR Spectroscopy evaluation of the diastereoisomeric adduct conveniently allows the determination of the enantiomeric excesses of the parent chiral amines.
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simple enantiomeric excess determination of amines using chiral selones unusual n h se bonding detected by hmqc 1h 77Se NMR Spectroscopy
Chemical Communications, 1996Co-Authors: Griselda Hernández, J D Odom, Bruce R Dunlap, Louis A. SilksAbstract:One-pot coupling of a chiral selone derivatizing agent to a series of chiral and racemic amines mediated with triphosgene gives adducts in yields ranging from 80–99%; 77Se NMR Spectroscopy evaluation of the diastereoisomeric adduct conveniently allows the determination of the enantiomeric excesses of the parent chiral amines.
Sabyasachi Sen - One of the best experts on this subject based on the ideXlab platform.
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structural and topological evolution in sixse1 x glasses results from 1d and 2d 29si and 77Se NMR Spectroscopy
Journal of Physical Chemistry B, 2017Co-Authors: Maxwell A T Marple, Ivan Hung, Zhehong Gan, Sabyasachi SenAbstract:The coordination environments of Si and Se atoms and their connectivity in binary SixSe1–x glasses with 0.05 ≤ x ≤ 0.33 are investigated using a combination of one- and two-dimensional 29Si and 77Se nuclear magnetic resonance (NMR) and Raman Spectroscopy. The high-resolution correlated isotropic and anisotropic 29Si and 77Se NMR spectra allow for the identification and quantitation of a variety of Si and Se environments. The results suggest that the structure of these glasses are characterized by a network with essentially perfect short-range chemical order, but with strong clustering at the intermediate range. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe4/2 tetrahedra. These clusters percolate via coalescence near x ≥ 0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x = 0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. ...
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Structural and Topological Evolution in SixSe1–x Glasses: Results from 1D and 2D 29Si and 77Se NMR Spectroscopy
The journal of physical chemistry. B, 2017Co-Authors: Maxwell A T Marple, Ivan Hung, Zhehong Gan, Sabyasachi SenAbstract:The coordination environments of Si and Se atoms and their connectivity in binary SixSe1–x glasses with 0.05 ≤ x ≤ 0.33 are investigated using a combination of one- and two-dimensional 29Si and 77Se nuclear magnetic resonance (NMR) and Raman Spectroscopy. The high-resolution correlated isotropic and anisotropic 29Si and 77Se NMR spectra allow for the identification and quantitation of a variety of Si and Se environments. The results suggest that the structure of these glasses are characterized by a network with essentially perfect short-range chemical order, but with strong clustering at the intermediate range. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe4/2 tetrahedra. These clusters percolate via coalescence near x ≥ 0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x = 0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. ...
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Selenium Chain Length Distribution in GexSe100–x Glasses: Insights from 77Se NMR Spectroscopy and Quantum Chemical Calculations
The journal of physical chemistry. B, 2016Co-Authors: Derrick C. Kaseman, Karina Moreira Oliveira, Teresa A. Palazzo, Sabyasachi SenAbstract:The statistics of selenium chain length distribution in GexSe100–x glasses with 5 ≤ x ≤ 20 are investigated using a combination of high-resolution, two-dimensional 77Se nuclear magnetic resonance (NMR) Spectroscopy and quantum chemical calculations. This combined approach allows for the distinction of various selenium chain environments on the basis of subtle but systematic effects of next-nearest neighbors of Se atoms in −Se–Se–Se– linkages on the 77Se chemical shift tensor parameters. Simulation of the experimental 77Se NMR spectral line shapes indicates that Se chain speciation in these chalcogenide glasses follows the Flory–Schulz distribution, originally developed for organic chain polymers.
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selenium chain length distribution in gexse100 x glasses insights from 77Se NMR Spectroscopy and quantum chemical calculations
Journal of Physical Chemistry B, 2016Co-Authors: Derrick C. Kaseman, Karina Moreira Oliveira, Teresa A. Palazzo, Sabyasachi SenAbstract:The statistics of selenium chain length distribution in GexSe100–x glasses with 5 ≤ x ≤ 20 are investigated using a combination of high-resolution, two-dimensional 77Se nuclear magnetic resonance (NMR) Spectroscopy and quantum chemical calculations. This combined approach allows for the distinction of various selenium chain environments on the basis of subtle but systematic effects of next-nearest neighbors of Se atoms in −Se–Se–Se– linkages on the 77Se chemical shift tensor parameters. Simulation of the experimental 77Se NMR spectral line shapes indicates that Se chain speciation in these chalcogenide glasses follows the Flory–Schulz distribution, originally developed for organic chain polymers.
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Synthesis, crystal structure, and advanced NMR characterization of a low temperature polymorph of SiSe2
J. Mater. Chem. A, 2016Co-Authors: Jian Wang, Sabyasachi Sen, Maxwell A T Marple, Kathleen Lee, Kirill KovnirAbstract:A low temperature polymorphic modification of silicon diselenide was synthesized via solid-state reaction and its crystal structure was determined by single crystal X-ray diffraction. In contrast to the well-characterized high-temperature form of SiSe2, where edge-sharing SiSe4 tetrahedra form one-dimensional chains, in the crystal structure of the low-temperature polymorph two-dimensional puckered layers are formed via edge- and corner-sharing SiSe4 tetrahedra. Such rearrangements have a significant impact on the electronic structure of SiSe2 as revealed by quantum chemical calculations and optical Spectroscopy. The experimentally determined indirect bandgap for the low-temperature modification of SiSe2 is 2.00(5) eV. Multinuclear (77Se, 29Si) NMR Spectroscopy combined with first principles calculations is applied to characterize the NMR chemical shift tensor parameters for the Si and Se sites in the crystal structure. These results will enable quantitative application of combined 29Si and 77Se NMR Spectroscopy in future structural studies of Si–Se glasses, whose structures consist of both corner- and edge-sharing SiSe4/2 tetrahedra.
Sharon Rozovsky - One of the best experts on this subject based on the ideXlab platform.
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77Se NMR Spectroscopy of Selenoproteins
Selenium, 2016Co-Authors: Sharon RozovskyAbstract:One of the most essential contributions of selenium to biology is the specialized chemistry performed by selenoproteins. Elucidating the mechanisms by which selenoproteins govern the reactivity of their selenocysteine (Sec) requires exploring how the protein environment primes Sec interactions with substrates, prevents inactivation, and otherwise optimizes the use of this unique amino acid. 77Se nuclear magnetic resonance (NMR) Spectroscopy is a particularly powerful technique to study the chemical properties of selenocysteine, its conformational preferences and mobility, and the molecular interactions by which it is stabilized. Recent advances have simplified sample preparation and data analysis, extending the utilization of 77Se in NMR studies of biological samples. These improvements include the development of efficient procedures for enriching proteins with the 77Se isotope, the reports on NMR parameters of different selenoproteins that greatly expand the available basis for data analysis, and the progress in utilizing theoretical calculations for data interpretation. We discuss these areas of progress in 77Se NMR of biological systems, and we consider the range of questions for which 77Se NMR is most useful.
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Conversion of the Sulfhydryl Oxidase Augmenter of Liver Regeneration into a Selenoprotein
Biophysical Journal, 2013Co-Authors: Stephanie Aron Schaefer, Ming Dong, Brian J. Bahnson, Colin Thorpe, Sharon RozovskyAbstract:Augmenter of liver regeneration (ALR) is a flavin-dependent sulfhydryl oxidase with roles in mitochondrial oxidative protein folding and cellular signaling. To study ALR's reaction mechanism we have prepared a form of the enzyme in which sulfur was replaced with selenium. The selenium-rich ALR is catalytically active, thermally stable, and its structure is almost identical to that of the native ALR. The presence of selenium in the active site leads to the formation of a charge-transfer complex during turnover, as detected by visible Spectroscopy. To further demonstrate the role selenium plays in ALR's active site, we have utilized E. coli's selenium insertion machinery to introduce selenium in a site-specific manner. using this method we are able to convert ALR's redox active CxxC motif to a selenocysteine containing CxxU motif. Our results demonstrate that the selenocysteine proximal to the FAD cofactor is sufficient to cause a charge-transfer complex during turnover. In addition, 77Se NMR Spectroscopy was used to probe locations typically occupied by sulfur - an insensitive nucleus that is not amendable for NMR studies of proteins. Biological 77Se NMR has so far been underutilized due to the challenges of isotopically enriching protein with 77Se. Here, we have developed a method to introduce 77Se by heterologous expression in E. coli. We report the NMR spectra of ALR bound to oxidized and reduced FAD. An unidentified resonance appears only in the presence of the reducing agent and disappears readily upon exposure to air and subsequent reoxidation of the flavin. Hence, 77Se NMR Spectroscopy can be used to directly probe the chemical environment surrounding the sulfur/selenium sites as a function of their redox state.
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77Se NMR of Selenoproteins
Biophysical Journal, 2013Co-Authors: Sharon RozovskyAbstract:Selenoproteins are a specialized group of enzymes that contain the rare amino acid selenocysteine (Sec). The majority of selenoproteins are oxidoreductases in which the reactive Sec forms a selenylsulfide bond with a partner cysteine, positioned up to two amino acids away. It is predicted that the nature of the residues forming this redox motif and the size of the resulting ring control Sec's reactivity. We demonstrate that it is possible to probe Sec's local environment and flexibility in these redox motifs using 77Se NMR Spectroscopy. We are able to record the 77Se chemical shifts and relaxation properties of proteins with representative redox motifs as a function of increasing ring size at the oxidized state. Theoretical calculations of the chemical shielding tensor are used to interpret the conformational preferences in these redox motifs. We discuss how the local environment fine tunes Sec's reactivity.View Large Image | View Hi-Res Image | Download PowerPoint Slide
Jerome D. Odom - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and Applications of Chiral Selones
Phosphorus Sulfur and Silicon and the Related Elements, 1998Co-Authors: Louis A. Silks, R. Bruce Dunlap, Jerome D. OdomAbstract:ABSTRACT The use of chiral selone derivatizing reagents (CDA's) and 77Se NMR Spectroscopy for the observation of remotely disposed chiral centers has proven to be a highly sensitive method for the determination of enantiomeric excesses. A variety of functional groups can be attached to these CDA's making them the most diverse class of CDA's yet developed. We are now investigating the chemistry unique to these CDA's such as new aldol reactions
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Simple enantiomeric excess determination of amines using chiral selones: unusual N–H⋯Se bonding detected by HMQC 1H/77Se NMR Spectroscopy
Chem. Commun., 1996Co-Authors: Griselda Hernández, Jerome D. Odom, R. Bruce Dunlap, Louis A. SilksAbstract:One-pot coupling of a chiral selone derivatizing agent to a series of chiral and racemic amines mediated with triphosgene gives adducts in yields ranging from 80–99%; 77Se NMR Spectroscopy evaluation of the diastereoisomeric adduct conveniently allows the determination of the enantiomeric excesses of the parent chiral amines.
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(R,R)-N,N'-dimethylcyclohexyl-1,2-diazaseleno-phospholidine as a chiral derivatizing agent for the evaluation of chiral alcohols.
Chirality, 1994Co-Authors: Karen L. House, R. Bruce Dunlap, Louis A. Silks, Megan J. O'connor, Jerome D. OdomAbstract:Previously, a diazaphospholidine has been synthesized and evaluated as a chiral derivatizing reagent for the determination of the optical purity of chiral alcohols via 31P NMR Spectroscopy (Alexakis et al., J. Org. Chem. 57:1224–1237, 1992). Our laboratory is interested in the advantageous and practical applications of 77Se NMR spectroscopic studies in many facets of chemistry and biochemistry. To this end we have used this diazaphospholidine as a starting point and have investigated chiral alcohols coupled to an optically pure diazaselenophospholidine. The diastereomers formed were then evaluated by 77Se NMR Spectroscopy, and these results were compared to the 31P NMR results published by Alexakis and co-workers. It was found that addition of the Se atom produced diastereomers that were air stable and, in many cases, the individual diastereomers could be distinguished by 77Se NMR Spectroscopy. Preliminary results indicate that the 77Se nucleus is somewhat more sensitive to remotely disposed chiral centers than is the 31P nucleus. Furthermore, because of their stability, these compounds do not readily decompose and can, therefore, be studied by a variety of chromatographic and spectroscopic techniques. © 1994 Wiley-Liss, Inc.
R. Bruce Dunlap - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and Applications of Chiral Selones
Phosphorus Sulfur and Silicon and the Related Elements, 1998Co-Authors: Louis A. Silks, R. Bruce Dunlap, Jerome D. OdomAbstract:ABSTRACT The use of chiral selone derivatizing reagents (CDA's) and 77Se NMR Spectroscopy for the observation of remotely disposed chiral centers has proven to be a highly sensitive method for the determination of enantiomeric excesses. A variety of functional groups can be attached to these CDA's making them the most diverse class of CDA's yet developed. We are now investigating the chemistry unique to these CDA's such as new aldol reactions
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Simple enantiomeric excess determination of amines using chiral selones: unusual N–H⋯Se bonding detected by HMQC 1H/77Se NMR Spectroscopy
Chem. Commun., 1996Co-Authors: Griselda Hernández, Jerome D. Odom, R. Bruce Dunlap, Louis A. SilksAbstract:One-pot coupling of a chiral selone derivatizing agent to a series of chiral and racemic amines mediated with triphosgene gives adducts in yields ranging from 80–99%; 77Se NMR Spectroscopy evaluation of the diastereoisomeric adduct conveniently allows the determination of the enantiomeric excesses of the parent chiral amines.
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Simple enantiomeric excess determination of alcohols using chiral selones and 77Se NMR Spectroscopy
Tetrahedron: Asymmetry, 1995Co-Authors: J D Odom, R. Bruce Dunlap, Louis A. SilksAbstract:Abstract Coupling of a chiral selone derivatizing agent to chiral and racemic alcohols mediated with triphosgene gives adducts in yields ranging from 78–100%. 77 Se NMR Spectroscopy conveniently allows the determination of the enantiomeric excesses of the parent chiral alcohol.
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(R,R)-N,N'-dimethylcyclohexyl-1,2-diazaseleno-phospholidine as a chiral derivatizing agent for the evaluation of chiral alcohols.
Chirality, 1994Co-Authors: Karen L. House, R. Bruce Dunlap, Louis A. Silks, Megan J. O'connor, Jerome D. OdomAbstract:Previously, a diazaphospholidine has been synthesized and evaluated as a chiral derivatizing reagent for the determination of the optical purity of chiral alcohols via 31P NMR Spectroscopy (Alexakis et al., J. Org. Chem. 57:1224–1237, 1992). Our laboratory is interested in the advantageous and practical applications of 77Se NMR spectroscopic studies in many facets of chemistry and biochemistry. To this end we have used this diazaphospholidine as a starting point and have investigated chiral alcohols coupled to an optically pure diazaselenophospholidine. The diastereomers formed were then evaluated by 77Se NMR Spectroscopy, and these results were compared to the 31P NMR results published by Alexakis and co-workers. It was found that addition of the Se atom produced diastereomers that were air stable and, in many cases, the individual diastereomers could be distinguished by 77Se NMR Spectroscopy. Preliminary results indicate that the 77Se nucleus is somewhat more sensitive to remotely disposed chiral centers than is the 31P nucleus. Furthermore, because of their stability, these compounds do not readily decompose and can, therefore, be studied by a variety of chromatographic and spectroscopic techniques. © 1994 Wiley-Liss, Inc.
