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Douglas R. Powell

Bio: Douglas R. Powell is an academic researcher from University of Oklahoma. The author has contributed to research in topics: Hydrogen bond & Ligand. The author has an hindex of 55, co-authored 411 publications receiving 13222 citations. Previous affiliations of Douglas R. Powell include University of Texas Health Science Center at San Antonio & Technical University of Berlin.


Papers
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Journal ArticleDOI
TL;DR: A new geometric parameter for four-coordinate compounds, tau(4), is proposed as an improved, simple metric for quantitatively evaluating the geometry of four- coordinate complexes and compounds.
Abstract: Four Cu(I) complexes were synthesized with a family of pyridylmethylamide ligands, HLR [HLR = N-(2-pyridylmethyl)acetamide, R = null; 2,2-dimethyl-N-(2-pyridylmethyl)propionamide, R = Me3; 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide, R = Ph3)]. Complexes 1–3 were synthesized from the respective ligand and [Cu(CH3CN)4]PF6 in a 2 : 1 molar ratio: [Cu(HL)2]PF6 (1), [Cu2(HLMe3)4](PF6)2 (2), [Cu(HLPh3)2]PF6 (3). Complex 4, [Cu(HL)(CH3CN)(PPh3)]PF6, was synthesized from the reaction of HL with [Cu(CH3CN)4]PF6 and PPh3 in a 1 : 1 : 1 molar ratio. X-Ray crystal structures reveal that complexes 1, 3 and 4 are mononuclear Cu(I) species, while complex 2 is a Cu(I) dimer. The copper ions are four-coordinate with geometries ranging from distorted tetrahedral to seesaw in 1, 2, and 4. Complexes 1 and 2 are very air sensitive and they display similar electrochemical properties. The coordination geometry of complex 3 is nearly linear, two-coordinate. Complex 3 is exceptionally stable with respect to oxidation in the air, and its cyclic voltammetry shows no oxidation wave in the range of 0–1.5 V. The unusual inertness of complex 3 towards oxidation is attributed to the protection from bulky triphenyl substituent of the HLPh3 ligand. A new geometric parameter for four-coordinate compounds, τ4, is proposed as an improved, simple metric for quantitatively evaluating the geometry of four-coordinate complexes and compounds.

2,281 citations

Journal ArticleDOI
22 May 1997-Nature
TL;DR: It is reported that β-amino acids can be used to design β-peptides with a very different secondary structure, a 12-helix, which demonstrates that by altering the nature of β- peptide residues, one can exert rational control over the secondary structure.
Abstract: Proteins and RNA are unique among known polymers in their ability to adopt compact and well-defined folding patterns. These two biopolymers can perform complex chemical operations such as catalysis and highly selective recognition, and these functions are linked to folding in that the creation of an active site requires proper juxtaposition of reactive groups. So the development of new types of polymeric backbones with well-defined and predictable folding propensities ('foldamers') might lead to molecules with useful functions. The first step in foldamer development is to identify synthetic oligomers with specific secondary structural preferences. Whereas alpha-amino acids can adopt the well-known alpha-helical motif of proteins, it was shown recently that beta-peptides constructed from carefully chosen beta-amino acids can adopt a different, stable helical conformation defined by interwoven 14-membered-ring hydrogen bonds (a 14-helix; Fig. 1a). Here we report that beta-amino acids can also be used to design beta-peptides with a very different secondary structure, a 12-helix (Fig. 1a). This demonstrates that by altering the nature of beta-peptide residues, one can exert rational control over the secondary structure.

533 citations

Journal ArticleDOI
TL;DR: Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed.
Abstract: Reaction of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(μ-H) (6) with H2 formed [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at −10 °C showed second-order kinetics with ΔH‡ = 12.0 kcal mol-1 and ΔS‡ = −28 eu. The rate of reduction was not accelerated by CF3CO2H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects kRuH/kRuD = 1.5 ± 0.2 and kOH/kOD = 2.2 ± 0.1 for PhCHO reduction at 0 °C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of kOHRuH/kODRuD = 3.6 ± 0.3. [2,5-Ph2-3,4-Tol2(η5-C4COSiEt3)Ru(CO)2H] (12) and NEt4+[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru(CO)2H]- (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeNCHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cycl...

341 citations

Journal ArticleDOI
TL;DR: In this paper, the use of 2,2{prime}-bis[(diphenylphosphino)methyl]1,1{prime})-BISBI ligands as ligands in rhodium-catalyzed hydroformylation has been studied.
Abstract: The use of 2,2{prime}-bis[(diphenylphosphino)methyl]1,1{prime}-biphenyl (BISBI, 1), trans-1,2-bis[(diphenylphosphino)-methyl]cyclopropane (T-BDCP, 2), and other diphosphines with large natural bite angles as ligands in rhodium-catalyzed hydroformylation has been studied. The X-ray crystal structure of (BISBI)Rh(PPh{sub 3})(CO)H{center_dot}CH{sub 2}Cl{sub 2} (7{center_dot}CH{sub 2}Cl{sub 2}) indicated a trigonal bipyramidal structure with three phosphorus atoms in the equatorial plane. The P-Rh-P bite angle of the BISBI ligand in 7{center_dot}CH{sub 2}Cl{sub 2} of 124.8{degrees} is much smaller than the 152{degrees}P-Fe-P bite angle found in (BISBI)Fe(CO){sub 3} and indicates that the BISBI ligand is rather flexible. NMR studies indicate that rapid exchange ({Delta}G = 15.5 kcal mol{sup -1}) occurs between the coordinated PPh{sub 3} of 7 and free PPh{sub 3}. 7 reacted with CO to produce (BISBI)Rh(CO){sub 2}H(9), which was shown by IR and NMR studies to have a trigonal bipyramidal structure with BISBI in the equatorial plane and hydride in an apical position. The solution structures of the T-BDCP complexes (T-BDCP)Rh(PPh{sub 3})(CO)H(13) and (T-BDCP)Rh(CO){sub 2}H(14) were shown by spectroscopy to be similar to the related BISBI compounds. A correlation between the size of the natural bite angle of chelating diphosphines and the regioselectivity for formation of straight-chain aldehydes in the rhodium-catalyzed hydroformylation of 1-hexane was observed. 30 refs., 9 figs.,more » 2 tabs.« less

337 citations

Journal ArticleDOI
TL;DR: In this article, a review of the binding properties of amine-based hosts is presented, which is divided into acyclic and macrocyclic categories, the latter based on cyclic dimension.

336 citations


Cited by
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01 Jun 2012
TL;DR: SPAdes as mentioned in this paper is a new assembler for both single-cell and standard (multicell) assembly, and demonstrate that it improves on the recently released E+V-SC assembler and on popular assemblers Velvet and SoapDeNovo (for multicell data).
Abstract: The lion's share of bacteria in various environments cannot be cloned in the laboratory and thus cannot be sequenced using existing technologies. A major goal of single-cell genomics is to complement gene-centric metagenomic data with whole-genome assemblies of uncultivated organisms. Assembly of single-cell data is challenging because of highly non-uniform read coverage as well as elevated levels of sequencing errors and chimeric reads. We describe SPAdes, a new assembler for both single-cell and standard (multicell) assembly, and demonstrate that it improves on the recently released E+V-SC assembler (specialized for single-cell data) and on popular assemblers Velvet and SoapDeNovo (for multicell data). SPAdes generates single-cell assemblies, providing information about genomes of uncultivatable bacteria that vastly exceeds what may be obtained via traditional metagenomics studies. SPAdes is available online ( http://bioinf.spbau.ru/spades ). It is distributed as open source software.

10,124 citations

Journal ArticleDOI
TL;DR: This review covers the literature published in 2014 for marine natural products, with 1116 citations referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms.

4,649 citations

Journal ArticleDOI
TL;DR: 1. Advantages and disadvantages of Chemical Redox Agents, 2. Reversible vs Irreversible ET Reagents, 3. Categorization of Reagent Strength.
Abstract: 1. Advantages of Chemical Redox Agents 878 2. Disadvantages of Chemical Redox Agents 879 C. Potentials in Nonaqueous Solvents 879 D. Reversible vs Irreversible ET Reagents 879 E. Categorization of Reagent Strength 881 II. Oxidants 881 A. Inorganic 881 1. Metal and Metal Complex Oxidants 881 2. Main Group Oxidants 887 B. Organic 891 1. Radical Cations 891 2. Carbocations 893 3. Cyanocarbons and Related Electron-Rich Compounds 894

3,432 citations

Journal ArticleDOI
TL;DR: Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.5.2.
Abstract: 5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

3,114 citations

Journal ArticleDOI
TL;DR: The fibrillar structure of type I collagen-the prototypical collagen fibril-has been revealed in detail and will guide further development of artificial collagenous materials for biomedicine and nanotechnology.
Abstract: Collagen is the most abundant protein in animals. This fibrous, structural protein comprises a right-handed bundle of three parallel, left-handed polyproline II-type helices. Much progress has been made in elucidating the structure of collagen triple helices and the physicochemical basis for their stability. New evidence demonstrates that stereoelectronic effects and preorganization play a key role in that stability. The fibrillar structure of type I collagen—the prototypical collagen fibril—has been revealed in detail. Artificial collagen fibrils that display some properties of natural collagen fibrils are now accessible using chemical synthesis and self-assembly. A rapidly emerging understanding of the mechanical and structural properties of native collagen fibrils will guide further development of artificial collagenous materials for biomedicine and nanotechnology.

2,742 citations