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  • self-assembly  (23)
  • NMR spectroscopy  (22)
  • 1
    ISSN: 0947-6539
    Keywords: electron transfer ; iron-sulfur proteins ; NMR spectroscopy ; proteins ; solution structures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the 1H NMR spectrum of the oxidized form of the high-potential iron-sulfur protein (HiPIPI) from Ectothiorhodospira halophila, 91% of the total proton resonances and 100% of the residues have been assigned. The standard COSY, NOESY, and TOCSY sequences have been optimized for the paramagnetism of the molecule. Extensive assignment of the 15N NMR spectrum has been obtained through HMQC spectra. With 1437 dipolar connectivities, of which about 10% involved fast-relaxing protons, a family of 18 structures was generated with an RMSD of 0.65 Å by using the programs developed by Wüthrich. The family of structures was further refined by various calculation steps; the final RMSD was 0.48 Å. The structures appear to be very similar but not equal to the structures of the reduced protein. Despite the similarity in structure, significant variations in the chemical shifts are observed. A similar behavior was observed for the homologous protein from Chromatium vinosum. It is concluded that NMR is a sensitive tool to monitor differences between oxidized and reduced proteins; however, the detailed structural variations should be evaluated with caution at the present level of resolution, which roughly corresponds to a resolution of 2.5 Å in an X-ray structure.
    Additional Material: 12 Ill.
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  • 2
    ISSN: 0947-6539
    Keywords: exchange processes ; kinetics ; NMR spectroscopy ; zirconium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have shown that it is possible to inhibit the transfer of magnetization in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis- and trans-ZrCl4L2 with excess free ligand L (L = (CH3O)3PO). The forward and backward reaction rates involving two selected sites can be studied while the effect of competing exchange processes is “quenched”. This can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval, or alternatively by inversion of the magnetization of all other sites in the exchange network. The rate of exchange from the free to the cis site was determined to be kcis←free = 0.018 s-1. In the usual methods, this process would tend to be overshadowed by the almost two hundred times faster competing exchange process from the cis to the trans site (Ktrans←cis = 3.32 s-1).
    Additional Material: 6 Ill.
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  • 3
    ISSN: 0947-6539
    Keywords: catenanes ; polycatenanes ; polyrotaxanes ; rotaxanes ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of three bis[2]catenanes and a bis[2]rotaxane, by two complementary strategies, is reported. A synthetic route to derivatives of bis-para-phenylene[34]crown-10 (BPP34C10) and 1,5-naphtho-para-phenylene[36]-crown-10 (1/5NPP36C10) containing a fused five-membered ring with a secondary amine function is described. These intermediate N-allylimido macrocyclic polyethers undergo template-directed reactions with 1,1′-[1,4-phenylenebis-(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) and 1,4-bis(bromo-methyl)benzene to produce [2]catenanes containing an N-allyl functionality. The N-allylimido macrocyclic polyethers have also been reduced and deprotected to afford macrocycles possessing a free NH group, which are then linked through a 4,4′-biphenyldicarbonyl spacer to produce bis(crown ether)s, in which each crown ether moiety has two recognition sites. These ditopic BPP34C10 and 1/5NPP36C10 derivatives are capable of sustaining self-assembly reactions at both recognition sites to yield bis[2]catenanes. The self-assembly of a complementary bis[2]catenane, in which two tetracationic cyclophanes are linked together with a flexible hexyl chain, has also been achieved by treating 1,1′-[1,4-phenylenebis(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) with a compound containing two linked 1,4-bis(bromomethyl)benzene units in the presence of BPP34C10. Replacing BPP34C10 with a dumbbell-shaped compound containing a linear polyether unit intercepted by a naphthalene residue and terminated by two bulky adamantoyl groups has led to the self-assembly of a bis[2]rotaxane. The X-ray crystal structures of one of the catenanes and its associated crown ether component are reported, together with solution state dynamic 1H NMR spectroscopic studies, showing that there is substantial degree of order characterizing the molecular structure of the catenanes.
    Additional Material: 10 Ill.
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  • 4
    ISSN: 0947-6539
    Keywords: lactam ; macrocycles ; rotaxanes ; self-assembly ; template syntheses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two new rotaxane types, the [2]rotaxanes 12a,b and the [3]rotaxane 13, have been synthesised by treating the “axle” 7 with the stopper component 9 in the presence of the “wheel” 8. A nonionic template intermediate of type 10 is proposed. The [3]rotaxane 13 was only obtained when the chain of the “axle” had a certain length: with n = 1 only [2]rotaxane 12 a was isolated, whereas with n = 2 the [3]rotaxane 13 was formed besides the [2]rotaxane 12 b. This suggests that more extended rotaxanes and polyrotaxanes can be synthesised by the template strategy.
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  • 5
    ISSN: 0947-6539
    Keywords: NMR spectroscopy ; peptides ; protein structures ; structure elucidation ; zinc ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Besides its functional role in many hydrolytic metalloenzymes, zinc acts as a structural component by being attached to bis(cysteinyl) protein sequences in some of the same enzymes, and in other metalloproteins and zinc fingers, and by being an essential constituent in metallothioneins. It is not always obvious whether the zinc-binding proteins are pre-organized for the incorporation of the metal or whether the zinc ion provides the structurizing power and stability for the observed peptide conformations. We have addressed the coordination chemistry aspects of this question by synthesizing zinc complexes of small model peptides and by determining their structures in solution by 2D NMR spectroscopy. The peptides chosen were of the terminally protected bis(cysteinyl) type: Cys-Cys, Cys-Gly-Cys, Cys-Phe-Cys, and Cys-Gly-Ile-Cys. The zinc ions fold these peptides into structures that can be superimposed on those of the natural proteins.
    Additional Material: 8 Ill.
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  • 6
    ISSN: 0947-6539
    Keywords: hydrogen bonds ; LFER ; molecular recognition ; NMR spectroscopy ; nucleobases ; thermodynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Association energies ΔGt in chloroform, in part also in carbon tetrachloride, were determined by NMR titrations of suitably substituted nucleosides and several synthetic analogues. Based on these and on many literature data, two simple free energy increments were derived describing the ΔGt values of 58 complexes within 1.8 kJ mol-1. With chloroform as solvent the increment for the primary interaction between donor and acceptor is 7.9 kJ mol-1, for the secondary one 2.9 kJ mol-1, irrespective of whether the latter is attractive or repulsive. Addition of only 1% methanol to CCl4 led to a decrease in association constants by a factor of 25. Calorimetric titrations of G-C nucleoside derivatives in CCl4 showed substantial contributions from G dimers, in line with NMR titrations, and surprisingly small decreases in entropy. Preliminary NOE measurements allowed us to single out some of the possible association modes; they are also in line with expected self- and triple-association modes of the nucleobases. These modes are generally in accord with nucleobase associations predicted by MM calculations in the literature, which in turn agree with predictions based solely on the increments derived in the present work.
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  • 7
    ISSN: 0947-6539
    Keywords: boron compounds ; fluxionality ; iron complexes ; NMR spectroscopy ; rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The fluxional behavior of [1-4-η4 - exo - 7 - dipropylborylcyclohepta - 1,3,5-triene]tricarbonyliron (14) has been studied by dynamic NMR. The 2 D 1H and 13C EXSY NMR experiments carried out at various temperatures revealed that three independent fluxional processes occur in 14. The fastest rearrangement is a [1,7] B sigmatropic shift accompanied by [1,2] Fe migration (EA = 71.2±2.3 kJ mol-1, ln A = 32±1). Two other rearrangements also take place: [1,3] B sigmatropic and [1,3] Fe haptotropic migrations. The analysis of the observed selectivity towards [1,j] B sigmatropic shifts ([1,3] and [1,7] B shifts are observed, but [1,5] B shifts are not) in terms of theoretical predictions gives reason to assume that the [1,j] boron migrations occur with inversion of configuration at the migrating atom.
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  • 8
    ISSN: 0947-6539
    Keywords: alcohols ; cracking ; dehydrations ; isomerizations ; NMR spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The hydrocarbon products that are formed upon dehydration at 296-673 K of tert-butyl alcohol (tBuOH), adsorbed on H-ZSM-5 zeolite in concentrations equal to that to active Al—OH—Si sites in the catalyst, have been analyzed by 13C solid-state MAS NMR and GC-MS. To facilitate 13C NMR analysis, the alcohol selectively labeled with 13C isotope in the COH group was used. It was found that tBuOH transforms to the adsorbed C8 butene dimers plus a trace amount of alkanes at 296 K. Butene dimers exist inside H—ZSM-5 pores in the form of interconverting adsorbed octene, octyl silyl ether, and octyl carbenium ion; octyl silyl ether is the main adsorption state. Fluxionality of the carbenium ion form provides a pathway for isomerization of the highly branched hydrocarbon skeleton of the intial alcohol to the predominantly linear one in the adsorbed butene dimer. The driving force for the isomerization into the linear structure is the shape selectivity induced by the small size of the zeolite channels. At 373 K the adsorbed butene dimers further crack into species that contain an average of about 6.5 carbon atoms, in addition to further alkanes. At 448 K the adsorbed C3-C7+ paraffins become the predominant hydrocarbon products observed with both in situ 13C NMR and ex situ GC-MS. Simultaneously, a mixture of adsorbed polyenes is formed. According to 13C CP/MAS NMR, polyenes exist in the zeolite pores in the form of rather stable cyclopentenyl cations. At 573-673 K adsorbed cyclopentenyl cations further transform into a mixture of condensed and simple aromatics and then into xylenes and toluene. Simultaneously, paraffins crack further to give mainly C3-C4 paraffinic species at 573 K and propane at 673 K.
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  • 9
    ISSN: 0947-6539
    Keywords: alkylthiols ; gold ; nanoparticles ; phase transitions ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfaces and as modulators of metal surface properties. Sensitivity restrictions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films in molecular terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-defined. The characterization of the thermal behaviour of SAMs is important not only for the design of stable, well-ordered organic superlattices, but also for the fundamental understanding of the factors that drive molecular interactions in two dimensions. Phase properties in SAMs have been addressed here through the synthesis of gold nanoparticles of 20-30 Å in diameter and fully covered with alkylthiol chains. These thiolmodified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by transmission FT-IR spectroscopy, NMR spectroscopy, and differential scanning calorimetry. Our studies reveal that for long-chain thiols (≥ C16), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25°C. Furthermore, calorimetry, variable temperature transmission FT-IR spectroscopy, and solid-state 13C NMR studies have established that a cooperative chain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the extended chain conformation and the geometry of the spherical nanoparticles. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this suggests that chain ordering arises from an interdigitation of chains on neighboring particles. The thermotropic behavior is sensitive to the alkyl chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much-studied planar SAMs.
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  • 10
    ISSN: 0947-6539
    Keywords: bridging ligands ; chelate ligands ; copper complexes ; self-assembly ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The “self-assembly” reaction between bischelating 2,2′-bipyrimidine, metallic copper, Cu2+, and two equivalents of bidentate Ph2P(CH2)nPPh2 produces crystallographically characterized dinuclear complexes with a [Cu1(μ-bpym)-Cu1]2+ core and chelating (n = 5; type I) or bridging diphosphine ligands (n = 6, 8; type II). Structures II may be designated as “inverse cryptate” arrangements. The structural data of the solids and the spectroscopic results for the dissolved species indicate a strained configuration in the case of complex [(μ-bpym)Cu2{μ-Ph2P-(CH2)n]2+ (6) with n = 6. Once the polymethylene chain length is reduced one notch further to n = 5 (5), the diphosphines can no longer bridge but help to form eight-membered chelate rings (structure I). In this complex there is a distortion towards a trigonal-pyramidal coordination geometry at the copper(I) centers, which is supported by the formation of an “organic sandwich” arrangement between the central bpym acceptor and two phosphino phenyl rings.
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