Página 1 dos resultados de 6102 itens digitais encontrados em 0.018 segundos

Structural effects and hydrogen bonds on N,N `-di(methoxycarbonylsulfenyl)urea, [CH(3)OC(O)SNH](2)CO, studied by experimental and theoretical methods

VALLEJOS, Sonia Torrico; ERBEN, Mauricio F.; BOESE, Roland; PIRO, Oscar E.; CASTELLANO, Eduardo Ernesto; VEDOVA, Carlos O. Della
Fonte: ELSEVIER SCIENCE BV Publicador: ELSEVIER SCIENCE BV
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
56.19%
Pure N,N`-di(methoxycarbonylsulfenyl)urea, [CH(3)OC(O)SNH](2)CO, is quantitatively prepared by the hydrolysis reaction of CH(3)OC(O)SNCO and characterized by (1)H NMR, GC-MS and FTIR spectroscopy techniques. Structural and conformational properties are analyzed using a combined approach with data obtained from X-ray diffraction, vibrational spectra and theoretical calculation methods. The IR and Raman spectra for normal and deuterated species are reported. The crystal structure of [CH(3)OC(O)SNH](2)CO was determined by X-ray diffraction methods. The substance crystallizes in the orthorhombic P2(1)2(1)2 space group with a = 9.524(2), b = 12.003(1), c = 4.481 (1) angstrom, and Z = 2 moieties in the unit cell. The molecule is sited on a twofold crystallographic axis (C(2)) parallel to c and shows the anti-anti conformation (S-N single bonds antiperiplanar with respect to the opposite C-N single bonds in sulfenyl-urea-sic group). Neighboring molecules are arranged in a chain motif that extends along the C(2)-axis and is held by bifurcated NH center dot center dot center dot O center dot center dot center dot HN intermolecular bonds. A local planar symmetry is observed in the crystal for the central -SN(H)C(O)N(H)S- skeleton. Experimental and calculated data allow to trace this structural feature to the occurrence of N-H center dot center dot center dot O=C hydrogen bonding interactions. Calculated vibrational and structural properties are in good agreement with the experimentally determined features. (C) 2008 Elsevier B.V. All rights reserved.; Volkswagen-Stiftung; Volkswagen-Stiftung; Deutsche Forschungsgemeinschaft (DFG); Deutsche Forschungsgemeinschaf (DFG); ANPCYT-DAAD for the German-Argentinean cooperation Awards (PROALAR); Deutscher Akademischer Austauschdienst (DAAD); Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT); Deutscher Akademischer Austauschdienst (DAAD); DAAD Regional Program of Chemistry for Argentina; Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina (CONICET); Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET); Comision de Investigaciones Cientificas de la Provincia de Buenos Aires (CIC)...

Hydrogen bonding donation of N-methylformamide with dimethylsulfoxide and water

Borges, Alexandre; Cordeiro, João M.M.
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Artigo de Revista Científica Formato: 40-44
Português
Relevância na Pesquisa
56.19%
20% N-methylformamide (NMF) mixtures with water and with dimethylsulfoxide (DMSO) have been studied. A comparison between the hydrogen bonding (H-bond) donation of N-methylformamide with both solvents in the mixtures is presented. Results of radial distribution functions, pair distribution energies, molecular dipole moment correlation, and geometry of the H-bonded species in each case are shown. The results indicate that the NMF-solvent H-bond is significantly stronger with DMSO than with water. The solvation shell is best organized in the DMSO mixture than in the aqueous one. © 2013 Elsevier B.V. All rights reserved.

On the hydrogen bonding between N-methylformamide and acetone and tetrahydrofuran

Almeida, Glauco G.; Borges, Alexandre; Cordeiro, Joao M. M.
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 25-29
Português
Relevância na Pesquisa
56.28%
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Liquid mixtures of N-methylformamide (NMF)-acetone (ACT) and NMF-tetrahydrofuran (THF) 20 mol% NMF have been investigated using Monte Carlo simulations, aiming to obtain new details on the hydrogen bonding donation of the peptide bond in non-aqueous media. There are strong evidences of the H-bonding between NMF and each of the solvents, but the results indicate significant differences between the H-bond interactions in each mixture. THF solvates better the amide than ACT. The NMF-THF H-bonds are more directed and the structure of the solvation shell is much more ordered in the NMF-THF mixture than in the NMF-ACT one. (C) 2014 Elsevier B. V. All rights reserved.

A Vibrational Spectral Maker for Probing the Hydrogen-Bonding Status of Protonated Asp and Glu Residues

Nie, Beining; Stutzman, Jerrod; Xie, Aihua
Fonte: Biophysical Society Publicador: Biophysical Society
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
46.55%
Hydrogen bonding is a fundamental element in protein structure and function. Breaking a single hydrogen bond may impair the stability of a protein. We report an infrared vibrational spectral marker for probing the hydrogen-bond number for buried, protonated Asp or Glu residues in proteins. Ab initio computational studies were performed on hydrogen-bonding interactions of a COOH group with a variety of side-chain model compounds of polar and charged amino acids in vacuum using density function theory. For hydrogen-bonding interactions with polar side-chain groups, our results show a strong correlation between the C=O stretching frequency and the hydrogen bond number of a COOH group: ∼1759–1776 cm−1 for zero, ∼1733–1749 cm−1 for one, and 1703–1710 cm−1 for two hydrogen bonds. Experimental evidence for this correlation will be discussed. In addition, we show an approximate linear correlation between the C=O stretching frequency and the hydrogen-bond strength. We propose that a two-dimensional infrared spectroscopy, C=O stretching versus O-H stretching, may be employed to identify the specific type of hydrogen-bonding interaction. This vibrational spectral marker for hydrogen-bonding interaction is expected to enhance the power of time-resolved Fourier transform infrared spectroscopy for structural characterization of functionally important intermediates of proteins.

Role of Hoogsteen Edge Hydrogen Bonding at Template Purines in Nucleotide Incorporation by Human DNA Polymerase ι

Johnson, Robert E.; Haracska, Lajos; Prakash, Louise; Prakash, Satya
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Publicado em /09/2006 Português
Relevância na Pesquisa
46.56%
Human DNA polymerase ι (Pol ι) differs from other DNA polymerases in that it exhibits a marked template specificity, being more efficient and accurate opposite template purines than opposite pyrimidines. The crystal structures of Pol ι with template A and incoming dTTP and with template G and incoming dCTP have revealed that in the Pol ι active site, the templating purine adopts a syn conformation and forms a Hoogsteen base pair with the incoming pyrimidine which remains in the anti conformation. By using 2-aminopurine and purine as the templating residues, which retain the normal N7 position but lack the N6 of an A or the O6 of a G, here we provide evidence that whereas hydrogen bonding at N6 is dispensable for the proficient incorporation of a T opposite template A, hydrogen bonding at O6 is a prerequisite for C incorporation opposite template G. To further analyze the contributions of O6 and N7 hydrogen bonding to DNA synthesis by Pol ι, we have examined its proficiency for replicating through the 6O-methyl guanine and 8-oxoguanine lesions, which affect the O6 and N7 positions of template G, respectively. We conclude from these studies that for proficient T incorporation opposite template A, only the N7 hydrogen bonding is required...

Hydrogen bonding motifs of protein side chains: descriptions of binding of arginine and amide groups.

Shimoni, L.; Glusker, J. P.
Fonte: Cold Spring Harbor Laboratory Press Publicador: Cold Spring Harbor Laboratory Press
Tipo: Artigo de Revista Científica
Publicado em /01/1995 Português
Relevância na Pesquisa
46.55%
The modes of hydrogen bonding of arginine, asparagine, and glutamine side chains and of urea have been examined in small-molecule crystal structures in the Cambridge Structural Database and in crystal structures of protein-nucleic acid and protein-protein complexes. Analysis of the hydrogen bonding patterns of each by graph-set theory shows three patterns of rings (R) with one or two hydrogen bond acceptors and two donors and with eight, nine, or six atoms in the ring, designated R2(2)(8), R2(2)(9), and R1(2)(6). These three patterns are found for arginine-like groups and for urea, whereas only the first two patterns R2(2)(8) and R2(2)(9) are found for asparagine- and glutamine-like groups. In each case, the entire system is planar within 0.7 A or less. On the other hand, in macromolecular crystal structures, the hydrogen bonding patterns in protein-nucleic acid complexes between the nucleic acid base and the protein are all R2(2)(9), whereas hydrogen bonding between Watson-Crick-like pairs of nucleic acid bases is R2(2)(8). These two hydrogen bonding arrangements [R2(2)(9)] and R2(2)(8)] are predetermined by the nature of the groups available for hydrogen bonding. The third motif identified, R1(2)(6), involves hydrogen bonds that are less linear than in the other two motifs and is found in proteins.

Dependence of Amide Vibrations on Hydrogen Bonding

Myshakina, Nataliya S.; Ahmed, Zeeshan; Asher, Sanford A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
46.63%
The effect of hydrogen bonding on the amide group vibrational spectra has traditionally been rationalized by invoking a resonance model where hydrogen bonding impacts the amide functional group by stabilizing its [-O-C=NH+] structure over the [O=C-NH] structure. However, Triggs and Valentini’s UV-Raman study of solvation and hydrogen bonding effects on ε-caprolactum, N,N-dimethylacetamide (DMA) and N-methylacetamide (NMA) (J .Phys. Chem., 1992; 96, 6922-31) cast doubt on the validity of this model by demonstrating that contrary to the resonance model prediction, carbonyl hydrogen bonding does not impact the AmII’ frequency of DMA. In this study we utilize density functional theory (DFT) calculations to examine the impact of hydrogen bonding on the C=O and N-H functional groups of NMA, which is typically used as a simple model of the peptide bond. Our calculations indicate that, as expected, the hydrogen bonding frequency dependence of the AmI vibration predominantly derives from the C=O group, whereas the hydrogen bonding frequency dependence of the AmII vibration primarily derives from N-H hydrogen bonding. In contrast, the hydrogen bonding dependence of the conformation sensitive AmIII band derives equally from both C=O and N-H groups...

A fast atom bombardment mass spectrometry study of tris (3, 6-dioxaheptyl) amine-alkali metal halide complexes and hydrogen bonded complexes

Théberge, Roger.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
Português
Relevância na Pesquisa
56.19%
The objective of this thesis was to demonstrate the potential of fast atom bombardment mass spectrometry (FABMS) as a probe of condensed phase systems and its possible uses for the study of hydrogen bonding. FABMS was used to study three different systems. The first study was aimed at investigating the selectivity of the ligand tris(3,6-dioxaheptyl) amine (tdoha) for the alkali metal cations. FABMS results correlated well with infrared and nmr data. Systems where a crown ether competed with tdoha for a given alkali metal cation were also investigated by fast atom bombardment. The results were found to correlate with the cation affinity of tdoha and the ability of the crown ether to bind the cation. In the second and third studies, H-bonded systems were investigated. The imidazole-electron donor complexes were investigated and FABMS results showed the expected H-bond strength of the respective complexes. The effects of concentration, liquid matrix, water content, deuterium exchange, and pre-ionization of the complex were also investigated. In the third system investigated, the abundance of the diphenyl sulfone-ammonium salt complexes (presumably H-bonded) in the FABMS spectrum were found to correlate with qualitative considerations such as steric hindrance and strength of ion pairs.

Strong hydrogen bonding in organic synthesis

So, Kwok-Hung.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
Português
Relevância na Pesquisa
56.19%
The preparation of phenacyl and para-phenylphenacyl esters, the reactions of carboxylic acids, phenols, 2-nitropropane and alcohols with alkyl halides in the presence of fluoride anion are described. The reactions are thought to be accelerated by the formation of hydrogen bonds between the fluoride anion and the organic electron acceptor. The fluoride ,carboxylic acids, fluoride-phenols and fluoride-2-nitropropane are better reaction systems than the fluoride-alcohol. The source of the fluoride anion and the choice of solvents are also discussed.

The importance of hydrogen bonding in the alkylation of phenols

Alauddin, Mian Mohannad.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
Português
Relevância na Pesquisa
56.26%
Hydrogen bond assisted alkylation of phenols is compared with the classical base assisted reactions. The influence of solvents on the fluoride assisted reactions is discussed,· with emphasis on the localization of hydrogen bond charge density. Polar aprotic solvents such as DMF favour a-alkylation, and nonpolar aprotic solvents such as toluene favourC-alkylation of phenol. For more reactive and soluble fluorides, such as tetrabu~ylammoniumfluoride, the polar aprotic solvent favours a-alkylation and nonpolar aprotic solvent favours fluorination. Freeze-dried potassium fluoride is a better catalytic agent in hydrogen bond assisted alkylation reactions of phenol than the oven-dried fluoride. The presence of water in the alkylation reactions reduces the expected yield drastically. The tolerance of the reaction to water has also been studied. The use ofa phase transfer catalyst such as tetrabutylammonium bromide in the alkylation reactions of phenol in the presence of potassium fluoride is very effective under anhydrous conditions. Sterically hindered phenols such as 2,6-ditertiarybutyl-4-methyl phenol could not be alkylated even by using the more reactive fluorides, such as tetrabutylammonium fluoride in either polar or nonpolar aprotic solvents. Attempts were also made to alkylate phenols in the presence of triphenylphosphine oxide.

Hydrogen-atom abstraction from a model amino acid: dependence on the attacking radical

Amos, Ruth I. J.; Chan, Bun; Easton, Christopher J.; Radom, Leo
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica Formato: 6 pages
Português
Relevância na Pesquisa
56.33%
We have used computational chemistry to examine the reactivity of a model amino acid toward hydrogen abstraction by HO•, HOO•, and Br•. The trends in the calculated condensed-phase (acetic acid) free energy barriers are in accord with experimental relative reactivities. Our calculations suggest that HO• is likely to be the abstracting species for reactions with hydrogen peroxide. For HO• abstractions, the barriers decrease as the site of reaction becomes more remote from the electron-withdrawing α-substituents, in accord with a diminishing polar deactivating effect. We find that the transition structures for α- and β-abstractions have additional hydrogen-bonding interactions, which lead to lower gas-phase vibrationless electronic barriers at these positions. Such favorable interactions become less important in a polar solvent such as acetic acid, and this leads to larger calculated barriers when the effect of solvation is taken into account. For Br• abstractions, the α-barrier is the smallest while the β-barrier is the largest, with the barrier gradually becoming smaller further along the side chain. We attribute the low barrier for the α-abstraction in this case to the partial reflection of the thermodynamic effect of the captodatively stabilized α-radical product in the more product-like transition structure...

Intramolecular hydrogen bonding of (+)-biotin and biotin derivatives in organic solvents

Crisp, Geoffrey T.; Jiang, Yu-Lin
Fonte: Arkat USA Publicador: Arkat USA
Tipo: Artigo de Revista Científica
Publicado em //2001 Português
Relevância na Pesquisa
66.35%
The intramolecular hydrogen bonding of (+)-biotin and biotin derivatives in mixtures of CDCl3 and DMSO-d6 was investigated by 1H NMR spectroscopy. The dynamic changes observed in the chemic al shifts for the 1-NH and 3-NH protons with changes in solvent composition and temperature confirmed the presence of an intramolecular hydrogen bond between the valeryl side chain hydrogen bond acceptor and the 3-NH proton in a range of biotin derivatives.; Geoffrey T. Crisp and Yu-Lin Jiang; Copyright © ARKAT USA, Inc; The document attached has been archived with permission from the publisher.

NEW PHASES IN THE HYDROUS FERRIC SULFATE SYSTEM, A SUPPORTING ARGUMENT THAT THE MINERAL LAUSENITE IS OF FORMULA FE2(SO4)3∙5H2O AND THE CRYSTAL STRUCTURE REFINEMENT AND HYDROGEN BONDING SCHEME OF THE MINERALS QUENSTEDTITE AND ROMERITE

Westland, ROBIN
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
66.35%
A series of experiments were conducted to explore the hydration state of the mineral lausenite. The experiments of Posnjak and Merwin (1922) were recreated and the conclusion is reached that lausenite is of mineral formula Fe2(SO4)3∙5H2O, rather than Fe2(SO4)3∙6H2O as suggested by Posnjak and Merwin (1922). This conclusion is based on the X-ray diffraction data which found that the Posnjak and Merwin experiments produced a phase whose pattern matched that of the pentahydrate identified by Majzlan (2005). The refractive index data and morphology of this phase is also a match for the mineral described previously as a hexahydrate by Posnjak and Merwin (1922) and Lausen (1928). Synchrotron X-ray diffraction data was collected from a possible new phase in the hydrous ferric sulfate system. The data were analyzed and a unit cell was identified with monoclinic unit cell dimensions of a = 7.532(3)Å b = 12.551(6)Å c = 7.077(4)Å and β = 96.775(8)° with a unit cell volume of 664.4Å3. This phase was determined to grow only at temperatures above 85°C and at a RH of ~23%. The atomic structures of the minerals quenstedtite [Fe2(SO4)3∙11H2O] and romerite [Fe2+Fe3+2(SO4)4∙14H2O] are refined, hydrogen positions are identified and hydrogen bonding scheme is discussed. The weakest hydrogen bonds are found to occur between layers of differently coordinated tetrahedral groups in both romerite and quenstedtite. The transition from romerite to quenstedtite involves the oxidation of the ferrous iron in romerite converting into a Fe3+(SO4)∙5H2O cluster.; Thesis (Master...

Vibrational analysis and structural implications of H-bonding in isolated and aggregated 2-amino-1-propanol: a study by MI-IR and Raman spectroscopy and molecular orbital calculations

Fausto, R.; Cacela, C.; Duarte, M. L.
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Artigo de Revista Científica Formato: aplication/PDF
Português
Relevância na Pesquisa
56.28%
Isolated 2-amino-1-propanol (2AP) was studied by matrix-isolation infrared spectroscopy (MI-IR) in Ar and Kr and ab initio 6-31G* calculations undertaken at the HF-SCF and MP2 levels of theory. For the first time, five different conformational states of 2AP could be experimentally observed, which could be correlated with the most stable forms predicted by the calculations. The first and second lowest energy forms correspond to conformers which exhibit a considerably strong intramolecular OH...N hydrogen bond (g'Gg' and gG'g), while the less abundant forms observed in the matrices (gGg', gGt and g'G'g) are characterized by having a weak intramolecular NH...O or OH...N bond. These results were reinforced by infrared solution studies of the compound in tetrachloromethane and tetrachloroethylene. The experimental data obtained for the pure liquid, where OH...N intermolecular hydrogen bonding dominates, indicate that the preferred conformation of the monomeric unit within the aggregates is similar to conformer gGt.; http://www.sciencedirect.com/science/article/B6TGS-415RNK9-11/1/bc743f78fe244248cd0deab43f335345

Bond energy/eond order relationships for N-O linkages and a quantitative measure of ionicity: the rôle of nitro groups in hydrogen bonding

Johnstone, Robert A. W.; Loureiro, Rui M. S.; Labat, G.; Cristiano, Maria Lurdes Santos
Fonte: Michigan Publishing Publicador: Michigan Publishing
Tipo: Artigo de Revista Científica
Publicado em //2010 Português
Relevância na Pesquisa
56.37%
The nitro group is active in metabolic systems and can be found as an integral part of a number of useful curative drugs and many toxic substances. The basis for much of this activity is not fully understood. It is not necessarily caused directly by through-bond electronic effects but may also be due to direct H-bonding to nitro or to indirect interference by the nitro group with existing H-bonding. An unusual effect of a nitro substituent on kinetic results from urethane addition/elimination reactions (Scheme 1) has been ascribed to some form of self-association, which was neither specified nor quantified. To investigate self-association phenomena caused by a nitro group, a bond energy/bond order formula for N–O bonds has been developed and then used to interpret relative amounts of covalent and ionic contributions to total N–O bond energy. Calculated bond energies were then used to obtain enthalpies of formation for H-bonds to nitro groups in crystals and in solution. Similar results from solution data reveal that direct H-bonding to nitro is much weaker than in crystals, unless intramolecular H-bonding can occur. The results revealed that the 'self-association' effects observed for nitro substituents in urethanes (Scheme 1) were not caused by nitro participating directly in intermolecular bonding to NH of another urethane but by an indirect intramolecular action of the nitro group on pre-existing normal NH–O amide/amide type H-bonding.

The linear relationship between Koopmans' and hydrogen bond energies for some simple carbonyl molecules

Bruns,Roy E.; Haiduke,Roberto L. A.; Amaral,Antonia T. do
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/11/2002 Português
Relevância na Pesquisa
56.28%
Recently Galabov and Bobadova-Parvanova have shown that the energy of hydrogen bond formation calculated at the HF/6-31G(d,p) level is highly correlated with the molecular electrostatic potential at the acceptor site for a number of simple carbonyl compounds. Here it is shown that the electrostatic potential can be replaced by Koopmans' energy. The correlation between this energy and the hydrogen bond formation energy is just as high as the one observed by Galabov and Bobadova-Parvanova. The Siegbahn simple potential relating Koopmans' energies and GAPT charges shows that the hydrogen bond energy is not simply correlated with the charge of the acceptor site because the charges on the neighboring atoms are also important in the hydrogen bonding process.

A Study of Hydrogen Bonding

Conklin, Mary Ann
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
66.59%
The proton magnetic resonance technique lends itself to the study of hydrogen bond equilibria. Through PMR spectra, hydrogen bonding can be detected. The specific proton which participates intimately in the hydrogen bond may be identified. The following work has investigated hydrogen bonding systems in which the proton donor is trifluoroacetic acid and the proton acceptor is an N-heterocyclic base, such as quinazoline, quinoline, and isoquinol ine. From measurements of the chemical shift versus concentration, and from measurements of the line width at half peak amplitude versus concentration for the time average exchange signal, the extent of hydrogen bonding may be determined and the participating equilibria may be characterized. Two equilibrium constants have been calculated using the PMR data obtained in this study. Solvent effects upon the hydrogen bonding equilibria have been noted in several cases. These effects were similar to those found in other previous investigations and can be explained. The effect of hydrogen bonding upon the N-heterocyclic ring has been observed. The ring protons are chemically shifted upon hydrogen bond association of the N-heterocycle with the acid. This observation reveals a charge redistribution in the heterocycle upon complex formation. There is insufficient data to calculate the charge densities of these systems...

Winter Flounder "Antifreeze" Proteins: Synthesis and Ice Growth Inhibition of Analogs that Probe the Relative Importance of Hydrophobic and Hydrogen-Bonding Interactions

Haymet, A D J; Ward, Leanne G; Harding, Margaret
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
56.19%
Two series of mutant polypeptides of the type I, 37-residue winter flounder 'antifreeze' protein have been synthesized and analyzed by nanoliter osmometry, the 'ice hemisphere' test, measurement of ice growth hysteresis and circular dichroism (CD) spectroscopy. In series 1 peptides the central two threonines and all four threonines of the native protein were mutated to serine. In series 2 peptides two additional salt bridges (K7, E11 and K29, E33) were incorporated, and all four threonine residues in this sequence were mutated simultaneously to each of serine, valine, alanine, and glycine, respectively. The CD studies showed that all mutants are 100% helical in structure at low temperature, except for the glycine derivative which was estimated to be 70% α-helical. Dilute solutions of serine-substituted series 1 peptides showed no detectable, nonbasal faceting, or hysteresis behavior, indicating either no or extremely weak interaction with ice. The analogous serine-substituted mutant in series 2, as well as the glycine derivative, displayed unfaceted growth and showed no hysteresis. Hysteresis values, ice growth patterns, and the helicity measurements showed that the additional salt bridges present in series 2 peptides do not alter significantly the properties of the protein. The valine-substituted mutant gave a distinct etching pattern in which polypeptide accumulates on the {2 0 2 1 } plane of ice 1h...

Stereocontrol of the Intramolecular Diels-Alder Reaction by Internal Hydrogen Bonding

Cayzer, Tory; Paddon-Row, Michael; Sherburn, Michael
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
56.19%
A novel approach for exo/endo stereocontrol of intramolecular Diels-Alder reactions is described. Substrates carrying a hydroxymethyl group attached to the diene and an ester group attached to the dienophile participate in hydrogen bonding in the transition state. This non-covalent interaction causes either a significant enhancement or diminution in the observed kinetic endo/exo product ratio. Thus, the parent pentadienyl maleate 12 undergoes intramolecular Diels-Alder reaction to give an approx. 5:1 mixture of trans-and cis-fused bicyclic cycloadducts, whereas the C2-hydroxymethyl analogue 1 delivers a 1:1 ratio of products. In contrast, the parent pentadienyl fumarate 13 gives a 3:2 trans: cis ratio, which is improved to 9:1 in the C2-hydroxymethyl analogue 4. These stereoselectivities are accurately predicted from transition structure populations calculated using B3LYP/6-31+G(d) theory. The presence of an intramolecular H-bond confers a transannular Diels-Alder-like appearance upon the transition states of these reactions.

Hydrogen bonding patterns in a series of 3-spirocyclic oxindoles

Lemmerer,Andreas; Michael,Joseph P.
Fonte: South African Journal of Chemistry Publicador: South African Journal of Chemistry
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2010 Português
Relevância na Pesquisa
66.28%
The crystal structures of the new compounds spiro[cyclohexane-1,3'-indol]-2'(1'H)-one (1), (rel-1R,2S)-spiro[bicyclo[2.2.1] heptane-2,3'-indol]-2'(1'H)-one (2) and spiro[indole-3,2'-tricyclo[3.3.1.1(3,7)]decan]-2(1H)-one (3) have been determined by low temperature single crystal X-ray diffraction. The effects of substitution on the hydrogen bonding pattern is compared between all three compounds.