Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Hindrance

  • steric hindrance

  • Selected Abstracts

    Polymers or Supramolecules Generated From a New V-Shaped Bis-monodentate Ligand and the Effect of Steric Hindrance on Coordination Modes of the Ligand

    Caihua Zhou
    Abstract A new V-shaped bis-monodentate ligand L (L = 2,3,-dipyridylamine) (1) has been designed and synthesized by alkylation reaction of pyridylamine. An investigation of the charge distributions of the coordination atoms and single-point energy calculations of four conformers of ligand L based on the geometry of conformers optimized by the DFT (density functional theory) method was carried out. The results show that the four conformers of ligand L take on two stable and two less stable configurations. Theory forecasts that two relatively stable configurations present in complexes as probable coordination motifs of the ligand, and that steric hindrance of pyridine nitrogen atoms in isomers will affect its coordination ability together with the electronic factor. This forecast has been demonstrated by the coordination chemistry of ligand L, that is, configuration (a) and (b) of the ligand occur in the following reported complexes, which combines with AgI or CuII through two coordination modes (bidentate bridging or a monodentate mode) resulting in coordination polymers {[Ag (L)2]NO3}n (2), [Cu2(L)2(maa)4]n (maa = methacrylic acid) (3), and the mononuclear molecule [Cu(L)4](ClO4)2·2CH3CH2OH (4). The ligand assumes different coordination modes in the three complexes because of different levels of steric hindrance of the pyridine nitrogen atoms in the conformers. Interestingly, polymers 2 and 3 assume a 1D helical structure and a linear framework, respectively, and 4 has a 2D supramolecular architecture induced from hydrogen bond interactions. In addition, the magnetic properties of 3 have been explored, which shows a strong antiferromagnetic interaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    Making Use of Bond Strength and Steric Hindrance in Nanoscale "Synthesis",

    ANGEWANDTE CHEMIE, Issue 50 2009
    Yanhu Wei Dr.
    Die chemischen Konzepte Bindungsstärke und sterische Hinderung werden auf den Nanometerbereich erweitert, um ,Reaktionen" von Kompositnanopartikeln zu primitiven ,Nanomolekülen" genau definierter Form und Größe zu steuern. Dabei wird das Zusammenspiel zwischen der Stärke der Bindung zwischen den reaktiven Partikeldomänen und den auf der Sperrigkeit der nichtreaktiven Teile beruhenden Entropieeffekten genutzt. [source]

    Adaptation of Sudanese Refugees in an Australian Context: Investigating Helps and Hindrances

    Jane Shakespeare-Finch
    The present study investigates the experiences of Sudanese refugees by exploring the themes that characterize participants' experiences in Sudan, en route, and at their Australian destination. In particular, the research identifies several factors that may be seen as ,helps' or ,hindrances' to Sudanese refugees' adaptation. Participants were 12 Sudanese refugees aged between 19 and 40 years old who had been residing in Australia for five years or less. A qualitative phenomenological approach to data collection and analysis was employed. Examination of the interview transcripts revealed that all participants identified both ,hindrances' and ,helps' toward adaptation and indicated that positive adaptation is not only possible, but probable for Sudanese refugees in spite of their past experiences of trauma and present resettlement difficulties. Several practical implications were elicited from the research including a need for programs that actively promote refugees' adaptation by encouraging the broadening of social networks. [source]

    "Developing Good Taste in Evidence": Facilitators of and Hindrances to Evidence-Informed Health Policymaking in State Government

    Context: Policymaking is a highly complex process that is often difficult to predict or influence. Most of the scholarship examining the role of research evidence in policymaking has focused narrowly on characteristics of the evidence and the interactions between scientists and government officials. The real-life context in which policymakers are situated and make decisions also is crucial to the development of evidence-informed policy. Methods: This qualitative study expands on other studies of research utilization at the state level through interviews with twenty-eight state legislators and administrators about their real-life experiences incorporating evidence into policymaking. The interviews were coded inductively into the following categories: (1) the important or controversial issue or problem being addressed, (2) the information that was used, (3) facilitators, and (4) hindrances. Findings: Hindrances to evidence-informed policymaking included institutional features; characteristics of the evidence supply, such as research quantity, quality, accessibility, and usability; and competing sources of influence, such as interest groups. The policymakers identified a number of facilitators to the use of evidence, including linking research to concrete impacts, costs, and benefits; reframing policy issues to fit the research; training to use evidence-based skills; and developing research venues and collaborative relationships in order to generate relevant evidence. Conclusions: Certain hindrances to the incorporation of research into policy, like limited budgets, are systemic and not readily altered. However, some of the barriers and facilitators of evidence-informed health policymaking are amenable to change. Policymakers could benefit from evidence-based skills training to help them identify and evaluate high-quality information. Researchers and policymakers thus could collaborate to develop networks for generating and sharing relevant evidence for policy. [source]

    The Politics of Disciplining Water Rights

    Rutgerd Boelens
    ABSTRACT This article examines how the legal systems of Andean countries have dealt with the region's huge plurality of local water rights, and how official policies to ,recognize' local rights and identities harbour increasingly subtle politics of codification, confinement and disciplining. The autonomy and diversity of local water rights are a major hindrance for water companies, elites and formal rule-enforcers, since State and market institutions require a predictable, uniform playing field. Complex local rights orders are seen as irrational, ill-defined and disordered. Officialdom cannot simply ignore or oppress the ,unruliness and disobedience' of local rights systems: rather it ,incorporates' local normative orders that have the capacity to adequately respond to context-based needs. This article examines a number of evolving, overlapping legal domination strategies, such as the ,marrying' of local and official legal systems in ways that do not challenge the legal and power hierarchy; and reviews the ways in which official regulation and legal strategies deny or take into consideration local water rights repertoires, and the politics of recognition that these entail. Post-colonial recognition policies are not simply responses to demands by subjugated groups for greater autonomy. Rather, they facilitate the water bureaucracy's political control and help neoliberal sectors to incorporate local water users' rights and organizations into the market system , even though many communities refuse to accept these policies of recognition and politics of containment. [source]

    Attachment of Amine- and Maleimide-Containing Ferrocene Derivatives onto Self-Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross-Linking Efficiencies and Surface Coverage of Electroactive Groups

    ELECTROANALYSIS, Issue 21 2004
    Yunfeng Wang
    Abstract Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N -hydrosuccinimidyl (NHS)-terminated alkanethiol self-assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross-linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N -hydrosuccinimidyl (NHS)-terminated alkanethiol SAM. The covalent attachment of ,-ferrocenylethylamine onto a 11,11,-dithio-bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross-linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael-type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross-linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross-linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups. [source]

    The structure of the human allo-ligand HLA-B*3501 in complex with a cytochrome p450 peptide: Steric hindrance influences TCR allo-recognition

    Abstract Virus-specific T cell populations have been implicated in allo-recognition. The subdominant T cell receptor JL12 recognizes both HLA-B*0801 presenting the Epstein,Barr virus-derived peptide FLRGRAYGL and also HLA-B*3501 presenting the cytochrome p450 self peptide KPIVVLHGY. This cross-reactivity could promote the rejection of HLA-B*3501-positive cells in Epstein,Barr virus-exposed HLA-B*0801 recipients. LC13, the dominant TCR against the HLA-B*0801:FLRGRAYGL complex, fails to recognize HLA-B*3501:KPIVVLHGY. We report the 1.75-Angstrom resolution crystal structure of the human allo-ligand HLA-B*3501:KPIVVLHGY. Similarities between this structure and that of HLA-B*0801:FLRGRAYGL may facilitate cross-recognition by JL12. Moreover, the elevated peptide position in HLA-B*3501:KPIVVLHGY would provide steric hindrance to LC13, preventing it from interacting in the manner in which it interacts with HLA-B*0801:FLRGRAYGL. These findings are relevant to understanding the basis of T cell cross-reactivity in allo-recognition, optimal transplant donor-recipient matching and developing specific molecular inhibitors of allo-recognition. [source]

    Ruthenium-to-Platinum Interactions in ,6,,1 NCN-Pincer Arene Heterobimetallic Complexes: An Experimental and Theoretical Study

    Sylvestre Bonnet
    Abstract A series of ,6,,1 -heterobimetallic complexes have been prepared in which a [Ru(,6 -arene)(C5R5)]+ fragment (R = H or Me) and an ,1 -NCN-pincer platinum fragment are combined within the same molecule. In complexes [2]+ and [3]+, the ruthenium and platinum centers are ,6 and ,1 coordinated, respectively, to the same arene ring, whereas in [4A]+ and [5A]+ they are coordinated to two different arene rings that are linked with a covalent bond ([4A]+) or an ethyl bridge ([5A]+). Upon changing the organic manifold between both metal centers, very strong ([2]+) to very weak ([5A]+) ruthenium-to-platinum interactions are obtained. Experimentally, X-ray crystal structures show an increaing steric hindrance when the Ru,Pt distance diminishes, and electrochemical and 195Pt NMR spectroscopic studies show a decreasing electron density on platinum from [5A]+ to [2]+. Theoretical DFT calculations were undertaken, which show an increasing charge on platinum from [5A]+ to [2]+. Our theoretical analysis shows that the particularly strong ruthenium-to-platinum electronic interactions in [2]+ and [3]+ do not come from binding of ruthenium to platinum, but from the pincer Cipso sharing its electron density between both metal centers, which decreases the , donation to platinum, and from increased backdonation of the platinum d electrons into the , system of the arene ring. [source]

    Synthesis, Characterization, and Protonation Reactions of Ar-BIAN and Ar-BICAT Diimine Platinum Diphenyl Complexes

    Jerome Parmene
    Abstract PtII diphenyl complexes (N,N)PtPh2 [N,N = diimines Ar,N=C(An)C=N,Ar with Ar = substituted aryl groups] have been prepared and characterized by 1H, 13C, and 195Pt NMR spectroscopy. The 195Pt NMR spectroscopic data establish the electronic influence exerted by substituents at the backbone of the diimine ligand system to the metal center. When compared to diimines Ar,N=CMe,CMe=N,Ar, the electron-withdrawing ability of the Ar-BIAN ligand and the electron-donating ability of the O,O-heterocyclic Ar-BICAT systems are demonstrated. Trends in 195Pt NMR chemical shifts suggest that electronic tuning of the metal center is better achieved through variations of the diimine backbone substituents rather than variation of the substituents at the N-Aryl groups. Protonation of (N,N)PtPh2 in dichloromethane/acetonitrile at ,78 °C furnishes the corresponding PtIV hydrides (N,N)PtPh2H(NCMe)+. The PtIV hydrides liberate benzene with the formation of (N,N)PtPh(NCMe)+ when the temperature is raised. A second protonation and rapid benzene elimination produces the dicationic PtII species (N,N)Pt(NCMe)22+ at approximately 50 °C. Protonation of (N,N)PtPh2 in the absence of acetonitrile results in the clean formation of (N,N)PtPh(,2 -C6H6)+ at temperatures that depend on the steric hindrance provided by the alkyl substituents at the diimine N-aryl groups. These findings support the notion that the metal is the kinetically preferred site of protonation. The results qualitatively agree with a recent mechanistic study of protonation-induced reactions of (diimine)PtPh2 complexes that bear simple methyl substituents at the diimine backbone. Several compounds have been crystallographically characterized. All complexes have the expected square planar environment at the metal. Modest variations in the metric parameters suggest that the Ar-BICAT system has a weaker trans influence than the Ar-BIAN and Ar-DAB systems. [source]

    A Polymer-Bound Oxidovanadium(IV) Complex Prepared from an L -Cysteine-Derived Ligand for the Oxidative Amination of Styrene

    Mannar R. Maurya
    Abstract The ligand H2sal-cys (I) derived from salicylaldehyde and L -cysteine has been covalently bonded to chloromethylated polystyrene cross-linked with 5,% divinylbenzene. Upon treatment with [VO(acac)2] in dimethylformamide (DMF) the polystyrene-bound ligand PS-H2sal-cys (II) gave the oxidovanadium(IV) complex, PS-[VO(sal-cys)·DMF] (1). The corresponding neat complex, [VO(sal-eta)]2 (2), has also been prepared similarly in methanol. These complexes have been characterised by IR, electronic, EPR spectroscopic studies, magnetic susceptibility measurements and thermal as well as scanning electron micrographs studies. Complex [VO(sal-eta)]2 exhibits a medium intensity band at 980 cm,1 in the IR spectrum due to ,(V=O) stretch. Broad features of the EPR spectrum for the neat complex along with magnetic susceptibility studies suggest the presence of antiferromagnetic exchange interaction between two vanadium centers in close proximity. Both complexes catalyze the oxidative amination of styrene, in mild basic conditions, with secondary amines (diethylamine, imidazole, and benzimidazole) and gave a mixture of two aminated products in good yields. Amongst the two aminated products, the anti-Markovnikov product is favored over the Markovnikov one due to the steric hindrance posed by the secondary amines. The polymer-anchored heterogeneous catalyst is free from leaching during catalytic action and recyclable.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Reactions of GaCp* with a Hemilabile Derivative of Rh6(CO)16 , Synthesis and Structural Characterization of Two Novel Heterometallic Clusters: Rh6(CO)13(,,,3 -Ph2PC2H3)(,3 -GaCp*) and Rh6(CO)13(,1 -Ph2PC2H3)(,3 -GaCp*)2

    Elena V. Grachova
    Abstract The reaction of Rh6(CO)14(Ph2PCH=CH2) with a twofold excess of GaCp* (Cp* = C5Me5) affords two novel heterometallic clusters Rh6(CO)13(,,,3 -Ph2PC2H3)(,3 -GaCp*) (1) and Rh6(CO)13(,1 -Ph2PC2H3)(,3 -GaCp*)2 (2) in a moderate yield. The hemilabile behavior of coordinated diphenylvinylphosphane in the starting cluster allows the easy insertion of GaCp* into the cluster coordination sphere, which eventually gives the cluster 1 with face-bridging coordination of the gallium fragment and the phosphane ligand recoordinated in an edge-bridging position. The further substitution of the vinyl function in 1 for the second GaCp* ligand affords cluster 2, where recoordination of the double bond is blocked by steric hindrance from adjacent GaCp* fragments. Both clusters were structurally characterized by X-ray crystallography in the solid state and by IR and NMR spectroscopy in solution. Hemilability of diphenylvinylphosphane in 1 was independently confirmed by the cluster reaction with CO, which gives the Rh6(CO)14(,1 -Ph2PC2H3)(GaCp*) adduct (3) with the phosphane ligand only coordinated through the phosphorus atom. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

    Bulky-Hindrance-Controlled Ligand Transformation from Linked Bis(amidinate) to Linked Imido-Amidinate Promoted by a Mono(cyclopentadienyl)titanium Group

    Sheng-Di Bai
    Abstract A novel class of silyl-linked bis(amidinate) ligands [SiMe2{NC(Ph)N(2,6-R2Ph)Li}2] [L1 (R = H), L2 (R = Me), and L3 (R = iPr)] reacted with TiCl3(C5H5) to produce the half-sandwich titanium complexes 1, 2, and 3. The molecular structures of 1,3 were confirmed successfully by X-ray crystallography. An unprecedented intramolecular ligand transformation from the linked bis(amidinate) configuration to the linked imido-amidinate configuration took place in the cases of L1 and L2. It was found that the rearrangement process was related to the steric hindrance of the terminal substituents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    Polymers or Supramolecules Generated From a New V-Shaped Bis-monodentate Ligand and the Effect of Steric Hindrance on Coordination Modes of the Ligand

    Caihua Zhou
    Abstract A new V-shaped bis-monodentate ligand L (L = 2,3,-dipyridylamine) (1) has been designed and synthesized by alkylation reaction of pyridylamine. An investigation of the charge distributions of the coordination atoms and single-point energy calculations of four conformers of ligand L based on the geometry of conformers optimized by the DFT (density functional theory) method was carried out. The results show that the four conformers of ligand L take on two stable and two less stable configurations. Theory forecasts that two relatively stable configurations present in complexes as probable coordination motifs of the ligand, and that steric hindrance of pyridine nitrogen atoms in isomers will affect its coordination ability together with the electronic factor. This forecast has been demonstrated by the coordination chemistry of ligand L, that is, configuration (a) and (b) of the ligand occur in the following reported complexes, which combines with AgI or CuII through two coordination modes (bidentate bridging or a monodentate mode) resulting in coordination polymers {[Ag (L)2]NO3}n (2), [Cu2(L)2(maa)4]n (maa = methacrylic acid) (3), and the mononuclear molecule [Cu(L)4](ClO4)2·2CH3CH2OH (4). The ligand assumes different coordination modes in the three complexes because of different levels of steric hindrance of the pyridine nitrogen atoms in the conformers. Interestingly, polymers 2 and 3 assume a 1D helical structure and a linear framework, respectively, and 4 has a 2D supramolecular architecture induced from hydrogen bond interactions. In addition, the magnetic properties of 3 have been explored, which shows a strong antiferromagnetic interaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    Structural, Photophysical and Chiro-Optical Properties of Lanthanide Complexes with a Bis(benzimidazole)pyridine-Based Chiral Ligand

    Gilles Muller
    Abstract The neutral LnIII 1:1 nitrato complexes with the chiral ligand 2,6-bis(1- S -neopentylbenzimidazol-2-yl)pyridine (L11) have been synthesised and their stability constants measured in acetonitrile (log K1 = 4.0,6.4). The crystal and molecular structure of [Eu(NO3)3(L11)(MeCN)] shows the typical meridional planar coordination of L11 to the metal ion and low symmetry of the coordination polyhedron. The influence of the steric hindrance generated by the substituent at R2 on the crystal packing and bond lengths is discussed. Photophysical measurements show that ligand L11 induces a 3,,*-to-Ln energy-transfer process in the EuIII complex, while the TbIII compound is ten times less luminescent. Addition of a second molecule of L11 to give [Ln(ClO4)2(L11)2]+ leads to a large quenching of the EuIII luminescence (140-fold) due to several factors: a less efficient 1,,*,3,,* transfer (ca. fourfold), a smaller intrinsic quantum yield QEu (ca. threefold), and a substantially less efficient ligand-to-metal transfer (ca. 12-fold). In the case of the TbIII complex, the decrease in the energy of the triplet state reduces further the TbIII emission through increased back transfer. The specific rotary dispersion of the 1:1 and 1:2 complexes points to the chirality of the complexes arising mainly from the ligand, while the circularly polarized luminescence of these complexes with EuIII and TbIII displays a weak effect, pointing to a small diastereomeric excess in solution. Altogether, this study demonstrates that electronic, thermodynamic and photophysical properties of lanthanide complexes with aromatic terdentate ligands can be tuned by modifying the number and the arrangement of the ligands, as well as their substituents, particularly those in the R2 and R3 positions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

    Synthesis and Molecular Structures of Nickel(II) and Cobalt(III) Complexes with 2-(Arylimino)-3-(hydroxyimino)butane

    Ennio Zangrando
    Abstract We report new series of NiII and CoIII complexes with nitrogen-donor chelating ligands of the (E,E)-2-(arylimino)-3-(hydroxyimino)butane type (Ar,N,N,OH). These ligands are characterized by a hydrophilic (OH group) and a hydrophobic region (aryl group). NiII derivatives were obtained either as trimers of formula [Ni3(Ar,N,N,OH)3Br4(OH)][Br], with the hydrophobic groups oriented on the same side, or as bis(chelated) derivatives with cis geometry, depending on the steric hindrance of the aryl groups. CoIII complexes were obtained only as bis(chelated) derivatives, with the two ligands coplanar. The "iso -oriented" arrangement of ligands in bis(chelated) CoIII complexes is favored by weak interactions between the two ligands, namely O,H···O hydrogen bond and stacking interactions between the aryl groups. CoIII complexes might find application as catalysts for living or controlled radical polymerization of polar olefins, and preliminary results are reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

    Novel Synthetic Strategy towards the Efficient Synthesis of Substituted Bis(pyrazolyl)(2-pyridyl)methane Ligands,

    Alexander Hoffmann
    Abstract A general one-pot synthesis of new substituted heteroscorpionate ligands is presented. These mixed-functionality ligands were obtained in a catalyzed Peterson rearrangement starting from the substituted pyrazole, thionyl chloride, and an aldehyde. Thus, the synthesis of polyfunctional tridentate ligands is enabled, and they contain, besides the two pyrazole groups, other functionalities relevant for coordination chemistry. Additionally, the steric hindrance is easily defined in the ligands by the substitution of the pyrazole rings. By combination of the versatility in donor function and steric demand, a systematic tuning of the properties of the bis(pyrazolyl)methane ligands is possible. The synthesis and full characterization of 11 bis(pyrazolyl)methane ligands are reported. Two of these were structurally characterized as well. [source]

    Theoretical Study on Hetero-Diels,Alder Reaction of Butadiene with Benzaldehyde Catalyzed by Chiral InIII Complexes

    Xiuli Cao
    Abstract The mechanism of the hetero-Diels,Alder reaction of butadiene with benzaldehyde catalyzed by chiral N,N, -dioxide/In(OTf)3 complexes was studied theoretically by using density functional theory (DFT) and model system. The computational results indicate that the catalyzed reaction proceeded through a concerted mechanism via a highly zwitterionic transition state. The lowest energy barrier was 11.8 kJ,mol,1, which is 63.0 kJ,mol,1 lower than that of the uncatalyzed reaction. The results indicate that the endo approach is advantageous over the exo approach, because exo transitions states suffer from more steric hindrance than the endo transitions states as a result of interactions among the substrates, the trifluoromethanesulfonic group and the R4 groups of the ligand. The (S) configuration was observed predominantly over the (R) form, because there is no distinguishable repulsion between butadiene and the exo amino side or the endo amino side of the ligand. Besides, the interactions between the terminal hydrogen atoms of butadiene and the oxygen atoms of the trifluoromethanesulfonic group make the structure more stable. Thus, the experimental results were explained well by calculation of the chiral N,N, -dioxide/In(OTf)3 complex catalyzed hetero-Diels,Alder reaction at the molecular level. [source]

    Aliphatic Thiocarbonyl Ylides and Thiobenzophenone: Experimental Study of Regiochemistry and Methylene Transfer in Cycloadditions

    Rolf Huisgen
    Abstract 1,3-Dipolar cycloadditions of aliphatic or alicyclic thiocarbonyl ylides 3A-D , sterically hindered at least at one terminus , with thiobenzophenone produce both regioisomeric 1,3-dithiolanes 4 and 5. According to quantum-chemical calculations (preceding paper), a concerted cycloaddition furnishing 2,4-substituted dithiolanes 4 competes with the formation of an intermediate C,C -biradical 9 which cyclizes to the more crowded 4,5-substituted dithiolanes 5. When steric hindrance of 3 increases, the cycloaddition is superseded by ,methylene transfer', i.e., the transfer of the less hindered terminus of 3E-J to the S-atom of thiobenzophenone. The thiobenzophenone S -alkylide 11, thus formed, rapidly reacts with a second molecule ofthiobenzophenone to generate the 4,4,5,5-tetraphenyl-1,3-dithiolane 12 via the highly stabilized C,C -biradical 10. Methylene transfer occurs when the cyclization of the mixed C,C -biradical 9 requires a higher activation barrier than its dissociation to aliphatic thioketone + 11; the threshold is surprisingly well reproduced by calculations. The structural assignment of sixteen 1,3-dithiolanes is based on their formation from corresponding reactant pairs as well as on 1H and 13C chemical shifts. X-ray diffraction analyses of three spiro-1,3-dithiolanes reveal the van der Waals strain in non-bonded interactions, folding angles, shearing forces, and bond lengths. Comparison of the mass spectra of many 1,3-dithiolanes allows the reconstruction of major fragmentation pathways. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    An Investigation on the Synthesis of New Molecular Architectures from the Cyclotrimerisation of exo - and endo -Benzotricyclo[,5]nonene

    Arif Dastan
    Abstract We have performed an investigation on the cyclotrimerisation of molecules having exo - and endo -benzotricyclo[,5]nonene skeletons (3 and 4) with the aim of producing their respective cyclotrimers 2 that feature unusual geometries and electronic properties. Activation towards the cyclotrimerisation reaction was performed using the vic -bromostannyl vinyl derivatives and was accomplished under copper-mediated or palladium-catalysed reaction conditions. While the exo isomer 3 proved to be quite reactive and afforded variable amounts of the syn and anti cyclotrimers, the endo isomer 4 turned out to be quite resistant to cyclotrimerisation because of steric hindrance. Only dimers and acyclic trimers were obtained from reactions using this substrate. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    Mechanisms of blister induction by autoantibodies

    Cassian Sitaru
    Abstract:, Autoimmune diseases are characterized by defined self-antigens, organ specificity, autoreactive T cells and/or autoantibodies that can transfer disease. Autoimmune blistering diseases are organ-specific autoimmune diseases associated with an immune response directed to structural proteins mediating cell,cell and cell,matrix adhesion in the skin. While both autoreactive T and B cells have been detected and characterized in patients with autoimmune blistering diseases, current evidence generally supports a pathogenic role of autoantibodies for blister formation. The immunopathology associated with blisters induced by autoantibodies relies on several mechanisms of action. Autoantibodies from patients with pemphigus diseases can exert a direct effect just by binding to their target mediated by steric hindrance and/or by triggering the transduction of a signal to the cell. In most subepidermal autoimmune blistering conditions, in addition to the binding to their target antigen, autoantibodies need to interact with factors of the innate immune system, including the complement system and inflammatory cells, in order to induce blisters. Generally, decisive progress has been made in the characterization of the mechanisms of blister formation in autoimmune skin diseases. However, various aspects, including the exact contribution of steric hindrance and signal transduction for pemphigus IgG-induced acantholysis or the fine tuning of the inflammatory cascade triggered by autoantibodies in some subepidermal blistering diseases, still need to be addressed. Understanding the mechanisms by which autoantibodies induce blisters should facilitate the development of more specific therapeutic strategies of autoimmune blistering diseases. [source]

    Post-translational cleavage of recombinantly expressed nitrilase from Rhodococcus rhodochrous J1 yields a stable, active helical form

    FEBS JOURNAL, Issue 8 2007
    R. Ndoria Thuku
    Nitrilases convert nitriles to the corresponding carboxylic acids and ammonia. The nitrilase from Rhodococcus rhodochrous J1 is known to be inactive as a dimer but to become active on oligomerization. The recombinant enzyme undergoes post-translational cleavage at approximately residue 327, resulting in the formation of active, helical homo-oligomers. Determining the 3D structure of these helices using electron microscopy, followed by fitting the stain envelope with a model based on homology with other members of the nitrilase superfamily, enables the interacting surfaces to be identified. This also suggests that the reason for formation of the helices is related to the removal of steric hindrance arising from the 39 C-terminal amino acids from the wild-type protein. The helical form can be generated by expressing only residues 1,327. [source]

    Highly Stable Au Nanoparticles with Tunable Spacing and Their Potential Application in Surface Plasmon Resonance Biosensors

    Shuyan Gao
    Abstract Colloidal Au-amplified surface plasmon resonance (SPR), like traditional SPR, is typically used to detect binding events on a thin noble metal film. The two major concerns in developing colloidal Au-amplified SPR lie in 1) the instability, manifested as a change in morphology following immersion in organic solvents and aqueous solutions, and 2) the uncontrollable interparticle distance, determining probe spacing and inducing steric hindrance between neighboring probe molecules. This may introduce uncertainties into such detecting techniques, degrade the sensitivity, and become the barricade hampering colloidal Au-based transducers from applications in sensing. In this paper, colloidal Au-amplified SPR transducers are produced by using ultrathin Au/Al2O3 nanocomposite films via a radio frequency magnetron co-sputtering method. Deposited Au/Al2O3 nanocomposite films exhibit superior stability, and average interparticle distances between Au nanoparticles with similar average sizes can be tuned by changing surface coverage. These characteristics are ascribed to the spacer function and rim confinement of dielectric Al2O3 and highlight their advantages for application in optimal nanoparticle-amplified SPR, especially when the probe size is smaller than the target molecule size. This importance is demonstrated here for the binding of protein (streptavidin) targets to the probe (biotin) surface. In this case, the dielectric matrix Al2O3 is a main contributor, behaving as a spacer, tuning the concentration of Au nanoparticles, and manipulating the average interparticle distance, and thus guaranteeing an appropriate number of biotin molecules and expected near-field coupling to obtain optimal sensing performance. [source]

    Tuning the Thermal Relaxation of a Photochromic Dye in Functionalized Mesoporous Silica

    Lea A. Mühlstein
    Abstract In this study, it is shown that the kinetics of the back-switching reaction of a photochromic spirooxazine dye encapsulated in mesoporous silica materials can be significantly influenced both by the space available to the dye molecules and by the functionalization of the silica wall. Steric hindrance of the ring-closing process due to high dye content or small pore size leads to a slow fading speed of the irradiated dye species. Further, the density of surface silanol-groups present at the silica walls has an effect on the switching behavior of the dye because of their ability to stabilize the zwitterionic merocyanine isomers, thereby slowing the fading process from the open to the closed form. This stabilization effect is further enhanced in the presence of acidic functional groups, while, in contrast, basic functional groups reduce the stabilization of the open-from dye isomers, and thus a faster decay of the irradiated species is observed. Control over the fading speed of photochromic dyes is interesting for applications requiring a particularly fast or slow fading speed. [source]

    Barriers to provision of dental care in long-term care facilities: the confrontation with ageing and death

    GERODONTOLOGY, Issue 3 2005
    Ina Nitschke
    Objective:, The aim of the study was to reveal barriers to providing dental care for residents in long-term care (LTC) facilities. Design:, Participants were selected randomly from the dentist register in Berlin and Saxony, Germany. The sample consisted of 60 self-employed and 60 employed dentists, a further 60 dentists worked in their own dental practice but also part-time in an LTC facility. In semi-structured interviews a questionnaire with 36 statements concerning working conditions, administration and cost, insecurity concerning treatment decisions as well as confrontation with ageing and death was employed. Subsequently, the study participants were asked to rank the four dimensions concerning their impact on the decision against providing dental care in an LTC facility. Results:, The random sample was representative in age and gender for the dental register in Berlin and Saxony. Fifty-six per cent of the participants (63% of the men and 51% of the women; 52% of the self-employed, 60% of the employed and 56% of the consultant dentists) indicated unfavourable working conditions as biggest obstacle in providing dental care in an LTC-facility. Thirty-two per cent of participants rated administration and cost, 7% the insecurity in treatment decisions as major hindrance. Only 5% of the participants rated the confrontation with age and death as substantial barrier. There were no age and gender differences. Dentists in Berlin seemed more concerned about administration and cost of a consultancy activity and less secure in their therapy decisions than the colleagues from Saxony (p < 0.001). Dentists who work partly in LTC facilities were the least concerned about the confrontation with ageing and death (not significant), employed dentist showed the least secure in their treatment decisions (p > 0.001). Conclusion:, It can be concluded that the awareness of infra-structural and financial aspects in providing dental care in LTC facilities should be raised with health politicians and that these aspects should be considered when inaugurating or re-structuring the consultancy services to LTC facilities. Further it would be desirable to establish more postgraduate training programmes to increase clinical and ethical competence in the area of gerodontology. [source]

    Assessment of alaryngeal speech using a sound-producing voice prosthesis in relation to sex and pharyngoesophageal segment tonicity

    M. van der Torn MD
    Abstract Background. A pneumatic artificial sound source incorporated in a regular tracheoesophageal shunt valve may improve alaryngeal voice quality. Methods. In 20 laryngectomees categorized for sex and pharyngoesophageal segment tonicity, a prototype sound-producing voice prosthesis (SPVP) is evaluated for a brief period and compared with their regular tracheoesophageal shunt speech. Results. Perceptual voice evaluation by an expert listener and acoustical analysis demonstrate a uniform rise of vocal pitch when using the SPVP. Female laryngectomees with an atonic pharyngoesophageal segment gain vocal strength with the SPVP. Exerted tracheal pressure and airflow rate are equivalent to those required for regular tracheoesophageal shunt valves. However, communicative suitability and speech intelligibility deteriorate by the SPVP for most patients. Tracheal phlegm clogging the SPVP is a hindrance for most patients. Conclusions. The SPVP raises vocal pitch. Female laryngectomees with an atonic or severely hypotonic pharyngoesophageal segment can benefit from a stronger voice with the SPVP. © 2005 Wiley Periodicals, Inc. Head Neck28: 400,412, 2006 [source]

    Stable, Low-Temperature Discotic Nematic Superstructures by Incorporating a Laterally Substituted Sidearm in Hexakis(phenylethynyl)benzene Discogens,

    S.-C. Chien
    Abstract A new strategy for preparing room-temperature discotic nematic materials is proposed and developed stepwise to improve various discotic nematic properties. This approach replaces one sidearm of the discotic hexakis(4-hexyloxyphenylethynyl)benzene with a tris(alkoxy) sidearm, 1-ethynyl-2,3,4-tris(hexyloxybenzene), to lower transition temperatures, manipulate discotic nematic ranges, and preserve the discotic nematic superstructure while preventing column formation. Although symmetry considerations play a role in the resulting mesogenic properties, steric hindrance and van,der,Waals interactions are ascribed to the superior discotic nematic properties of the newly prepared discogens containing a laterally substituted sidearm. [source]

    Chemical Approach for the Study of the ,Kissing Complex' of Moloney murine leukaemia Virus

    Sébastien Porcher
    Abstract The replication of Moloney murine leukaemia virus relies on the formation of a stable homodimeric ,kissing complex' of a GACG tetraloop interacting through only two C,G base pairs flanked of 5,-adjacent unpaired adenosines A9. Previous NMR investigations of a model stem loop 1 has not permitted to reveal the origin of this interaction. Therefore, with the aim of deeper comprehension of the phenomena, the model sequence 10 was prepared where position 9 has been substituted for a nucleoside offering a wider , -stacking. In this context, the wyosine phosphoramidite building block 2 was prepared and incorporated by adapting the conditions of the automated synthesis and developing original templated enzymatic ligation. However, no ,kissing interaction' has been observed for this model sequence 10 due to steric hindrance as confirmed by computational simulation. Consequently, several other model sequences, 18, 23,26, containing modified nucleosides were prepared. Finally, the importance of the cross-loop H-bond between G8 and G11 nucleobases was revealed by preparing a 18mer RNA hairpin 27, where the guanosine G8 has been substituted for inosine. The latter, which does not possess a C3 amino function compared to guanosine, is unable to form any ,kissing complex' demonstrating the importance of this secondary interaction in the formation of the complex. [source]

    Reactions of Di(tert -butyl)diazomethane with Acceptor-Substituted Ethylenes,

    Rolf Huisgen
    Abstract Di(tert- butyl)diazomethane (4) is a nucleophilic 1,3-dipole with strong steric hindrance at one terminus. In its reaction with 2,3-bis(trifluoromethyl)fumaronitrile ((E)- BTE), a highly electrophilic tetra-acceptor-substituted ethene, an imino-substituted cyclopentene 9 is formed as a 1,:,2 product. The open-chain zwitterion 10, assumed as intermediate, adds the second molecule of (E)- BTE. The 19F- and 13C-NMR spectra allow the structural assignment of two diastereoisomers, 9A and 9B. The zwitterion 10 can also be intercepted by dimethyl 2,3-dicyanofumarate (11) and furnishes diastereoisomeric cyclopentenes 12A and 12B; an X-ray-analysis of 12B confirms the ,mixed' 1,:,1,:,1 product. Competing is an (E)- BTE -catalyzed decomposition of 4 to give 2,3,4,4-tetramethylpent-1-ene (7)+N2; the reaction of (E)- BTE with a trace of water appears to be responsible for the chain initiation. The H2SO4 -catalyzed decomposition of diazoalkane 4, indeed, produced the alkene 7 in high yield. The attack on the hindered diazoalkane 4 by 11 is slower than that by (E)- BTE; the zwitterionic intermediate 21 undergoes cyclization and furnishes the tetrasubstituted furan 22. In fumaronitrile, electrophilicity and steric demand are diminished, and a 1,3-cycloaddition produces the 4,5-dihydro-1H -pyrazole derivative 25. The reaction of 4 with dimethyl acetylenedicarboxylate leads to pyrazole 29+isobutene. [source]

    Diradical-Promoted Two-Carbon Ring-Expansion Reactions by Thermal Isomerization: Synthesis of Functionalized Macrocyclic Ketones

    Georg Rüedi
    A new method for the smooth and highly efficient preparation of functionalized macrocyclic ketones has been developed. Pyrolysis of medium- and large-ring 3-vinylcycloalkanones by dynamic gas-phase thermo-isomerization (DGPTI) at 600,630° yielded, under insertion of a previously attached vinyl side chain by means of a 1,3-C shift, the corresponding ,,, -unsaturated cycloalkanones. The yield of the two-carbon ring-expanded ketones greatly depended on the relative ring strains of substrate and product (5,87%, cf. Table,5). The formation of minor amounts of one-carbon ring-expanded cycloalkenes (<10%) can be ascribed to a subsequent decarbonylation step. A reaction mechanism involving initial cleavage of the weakest single bond in the molecule has been established (cf. Scheme,6). Recombination within the generated diradical intermediate in terminal vinylogous position led to the observed products, while reclosure gave recovered starting material. Substituents on the vinyl moiety were transferred locospecifically into the ring-expanded products. An isopropenyl group did not significantly affect the isomerization process, whereas substrates bearing a prop-1-enyl group in , -position enabled competing intramolecular H-abstraction reactions, leading to acyclic dienones (cf. Schemes,9,11). DGPTI of the 13-membered analogue directly yielded 4-muscenone, which, upon hydrogenation, led to the valuable musk odorant (±)-muscone. Increasing the steric hindrance on the vinyl moiety gave rise to diminishing amounts of the desired ,,, -unsaturated cycloalkanones. This novel two-carbon ring-expansion protocol was also successfully applied to 3-ethynylcycloalkanones, giving rise to the corresponding ring-expanded cyclic allenes (cf. Scheme,13). [source]

    Factors Influencing the Course of the Macrocyclization of ,,, -Diamines with Esters of ,,, -Dicarboxylic Acids

    Dorota Gryko
    The efficient synthesis of eight new macrocyclic amides (lactams) via reaction of diesters with diamines under normal dilution conditions is described. The role of intermolecular H-bond formation and steric hindrance is discussed based on 1H- and 15N-NMR studies of appropriate model compounds. Principles for the optimal choice of esters that can be efficiently transformed into diamides have been developed. [source]