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51.
van Dijken A Bastiaansen JJ Kiggen NM Langeveld BM Rothe C Monkman A Bach I Stössel P Brunner K 《Journal of the American Chemical Society》2004,126(24):7718-7727
A carbazole homopolymer and carbazole copolymers based on 9,9'-dialkyl-[3,3']-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output. 相似文献
52.
Kim K Webster S Levi N Carroll DL Pinto MR Schanze KS 《Langmuir : the ACS journal of surfaces and colloids》2005,21(11):5207-5211
We describe the preparation and characterization of a photonic crystal filled with a luminescent conjugated polyelectrolyte, sulfonated poly(phenylene ethynylene). The conjugated polymer was coated onto the nanospheres by the layer-by-layer method and assembled directly into a fluorescent opal structure avoiding the defects associated with post-filling schemes. These structures exhibit strong angle-dependent luminescent properties. By using multiple layers, we further demonstrate control over the emissive bands of the opal. 相似文献
53.
The molecular recognition of peptides and proteins in aqueous solution by designed molecules remains an elusive goal with broad implications for basic biochemical research and for sensors and separations technologies. This paper describes the recognition of N-terminal tryptophan in aqueous solution by the synthetic host cucurbit[8]uril (Q8). Q8 is known to form 1:1:1 heteroternary complexes with methyl viologen (MV) and a second aromatic guest. Here, the complexes of Q8.MV with (i) the four natural aromatic alpha-amino acids, (ii) four singly charged tryptophan derivatives, and (iii) four tryptophan-containing tripeptides were characterized by isothermal titration calorimetry, mass spectrometry, and UV-visible, fluorescence, and (1)H NMR spectroscopy. We find that Q8.MV binds Trp-Gly-Gly with high affinity (K(a) = 1.3 x 10(5) M(-1)), with 6-fold specificity over Gly-Trp-Gly, and with 40-fold specificity over Gly-Gly-Trp. Analysis of the nine indole-containing compounds suggests that peptide recognition is mediated by the electrostatic charge(s) proximal to the indole, and that the mode of binding is consistent for these compounds. Complex formation is accompanied by the growth of a visible charge-transfer band and the quenching of indole fluorescence. These optical properties, combined with the stability and selectivity of this system, are promising for applications in sensing and separating specific peptides. 相似文献
54.
Kempf JG Jung JY Sampson NS Loria JP 《Journal of the American Chemical Society》2003,125(40):12064-12065
Current solution NMR experiments for characterizing conformational exchange processes in large proteins are limited to exchange rates ca. 500-3000 s-1. A TROSY-based constant relaxation time (R1rho - R1) experiment is designed to extend this capability to measure motion with rates up to 105 s-1 in large macromolecules. The experiment combines off-resonance spin-lock rf fields, which provide access to the faster time-scale dynamics, with TROSY coherence selection, which extends the molecular-weight range available for study. When implemented on the 53-kDa dimeric enzyme triosephosphate isomerase, the experiment yielded substantial gains in signal-to-noise (up to 60%) over current experiments at modest static magnetic fields (14.1 T). The TROSY (R1rho - R1) experiment should therefore be of general utility for investigation of fast conformational exchange events in large proteins. 相似文献
55.
Peng S Okeley NM Tsai AL Wu G Kulmacz RJ van der Donk WA 《Journal of the American Chemical Society》2002,124(36):10785-10796
Prostaglandin H synthase (PGHS) catalyzes the conversion of arachidonic acid to prostaglandin G(2) in the cyclooxygenase reaction. The first step of the mechanism has been proposed to involve abstraction of the pro-S hydrogen atom from C13 to generate a pentadienyl radical spanning C11-C15. We report here the synthesis of six site-specifically deuterated arachidonic acids to investigate the structure of the radical intermediate. The preparation of these compounds was achieved using a divergent scheme that involved one advanced intermediate for all targets. The synthetic design introduced the label late in the routes and allowed the utilization of common synthetic intermediates in the preparation of various targets. Both 13(R)- and 13(S)-deuterium-labeled arachidonic acids were synthesized in high enantiomeric purity as deduced from soybean lipoxygenase assays and mass spectrometric analysis of the resulting enzymatic products. Each synthetic compound was reacted under anaerobic conditions with the wide singlet tyrosyl radical of PGHS-2 to generate a radical intermediate that was analyzed by EPR. Deuterium substitution at positions 11, 13(S), and 15 resulted in the loss of one hyperfine interaction, indicating that the protons at these positions interact with the unpaired electron. Simulation of the spectra was achieved with one set of parameters that are consistent with the assignment of a pentadienyl radical. Use of 16-[(2)H(2)]-arachidonic acid indicated that only one of the protons at C16 gives rise to a strong hyperfine interaction. The findings are discussed in the context of two proposed mechanisms for the cyclooxygenase reaction. 相似文献
56.
Zimmermann N Meggers E Schultz PG 《Journal of the American Chemical Society》2002,124(46):13684-13685
We have generated a novel silver(I)-mediated unnatural DNA base pair consisting of two 2,6-bis(ethylthiomethyl)pyridine nucleobases SPy. This metallo-base pair has a remarkably high pairing stability and selectivity which rivals that of the natural base pairs dA:dT and dC:dG. UV-melting experiments revealed that the dSPy:dSPy self-pair can replace natural base pairs at multiple sites and still form stable DNA duplexes. 相似文献
57.
Co(2+)cobalmain (Co(2+)Cbl) is implicated in the catalytic cycles of all adenosylcobalamin (AdoCbl)-dependent enzymes, as in each case catalysis is initiated through homolytic cleavage of the cofactor's Co-C bond. The rate of Co-C bond homolysis, while slow for the free cofactor, is accelerated by 12 orders of magnitude when AdoCbl is bound to the protein active site, possibly through enzyme-mediated stabilization of the post-homolysis products. As an essential step toward the elucidation of the mechanism of enzymatic Co-C bond activation, we employed electronic absorption (Abs), magnetic circular dichroism (MCD), and resonance Raman spectroscopies to characterize the electronic excited states of Co(2+)Cbl and Co(2+)cobinamide (Co(2+)Cbi(+), a cobalamin derivative that lacks the nucleotide loop and 5,6-dimethylbenzimazole (DMB) base and instead binds a water molecule in the lower axial position). Although relatively modest differences exist between the Abs spectra of these two Co(2+)corrinoid species, MCD data reveal that substitution of the lower axial ligand gives rise to dramatic changes in the low-energy region where Co(2+)-centered ligand field transitions are expected to occur. Our quantitative analysis of these spectral changes within the framework of time-dependent density functional theory (TD-DFT) calculations indicates that corrin-based pi --> pi transitions, which dominate the Co(2+)corrinoid Abs spectra, are essentially insulated from perturbations of the lower ligand environment. Contrastingly, the Co(2+)-centered ligand field transitions, which are observed here for the first time using MCD spectroscopy, are extremely sensitive to alterations in the Co(2+) ligand environment and thus may serve as excellent reporters of enzyme-induced perturbations of the Co(2+) state. The power of this combined spectroscopic/computational methodology for studying Co(2+)corrinoid/enzyme active site interactions is demonstrated by the dramatic changes in the MCD spectrum as Co(2+)Cbi(+) binds to the adenosyltransferase CobA. 相似文献
58.
Li M Bonnet D Bill E Neese F Weyhermüller T Blum N Sellmann D Wieghardt K 《Inorganic chemistry》2002,41(13):3444-3456
Two new pentadentate, pendent arm macrocyclic ligands of the type 1-alkyl-4,7-bis(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane where alkyl represents an isopropyl, (L(Pr))(2-), or an ethyl group, (L(Et))(2-), have been synthesized. It is shown that they bind strongly to ferric ions generating six-coordinate species of the type [Fe(L(alk))X]. The ground state of these complexes is governed by the nature of the sixth ligand, X: [Fe(III)(L(Et))Cl] (2) possesses an S = 5/2 ground state as do [Fe(III)(L(Et))(OCH(3))] (3) and [Fe(III)(L(Pr))(OCH(3))] (4). In contrast, the cyano complexes [Fe(III)(L(Et))(CN)] (5) and [Fe(III)(L(Pr))(CN)] (6) are low spin ferric species (S = 1/2). The octahedral [FeNO](7) nitrosyl complex [Fe(L(Pr))(NO)] (7) displays spin equilibrium behavior S = 1/2<==>S = (3)/(2) in the solid state. Complexes [Zn(L(Pr))] (1), 4.CH(3)OH, 5.0.5toluene.CH(2)Cl(2), and 7.2.5CH(2)Cl(2) have been structurally characterized by low-temperature (100 K) X-ray crystallography. All iron complexes have been carefully studied by zero- and applied-field M?ssbauer spectroscopy. In addition, Sellmann's complexes [Fe(pyS(4))(NO)](0/1+) and [Fe(pyS(4))X] (X = PR(3), CO, SR(2)) have been studied by EPR and M?ssbauer spectroscopies and DFT calculations (pyS(4) = 2,6-bis(2-mercaptophenylthiomethyl)pyridine(2-)). It is concluded that the electronic structure of 7 with an S = 1/2 ground state is low spin ferrous (S(Fe) = 0) with a coordinated neutral NO radical (Fe(II)-NO) whereas the S = 3/2 state corresponds to a high spin ferric (S(Fe) = 5/2) antiferromagnetically coupled to an NO(-) anion (S = 1). The S = 1/2<==>S = 3/2 equilibrium is then that of valence tautomers rather than that of a simple high spin<==>low spin crossover. 相似文献
59.
Michele Daney Henry Bouas-Laurent Bernard Calas Louis Giral Nicole Platzer 《Journal of organometallic chemistry》1980,188(3):277-291
The structure of the 9,10-dihydroanthracenyl anion and of a series of 9-alkyl-10-lithio-9-10-dihydroanthracenes (9-R-10-LiDHA, I–V where R = H, Me, Et, i-Pr, t-Bu) was studied in solution by electronic absortion spectrometry and proton magnetic resonance. Our electronic absorption results, in addition to those of other authors, show that the contact ion pairs (c.i.p.) have an absorption at λmax}- 400 nm (I–III) and 415 nm (V) whereas the loose ion pairs (l.i.p.) absorb at λmax}- 450 nm (I–V). In the NMR the chemical shift of the proton para with respect to the carbanionic center was examined as a function of solvent (THF, THF/HMPA, and in some cases ether or pure HMPA) and temperature (+20 to ?40°C). The para proton is shielded significantly with regard to the aromatic protons of the hydrocarbon (Δδ(Hpara) ca. 1–1.7 ppm). The weakest shielding was observed in ether, in agreement with the existence of c.i.p. The largest shielding (THF/HMPA or pure HMPA) is in connection with the presence of l.i.p. where the negative charge is less localised at position 10. Moreover, in the same solvent, and at the same temperature, Δδ(Hpara) was observed to increase with the substituent bulk, up to the point that there are only l.i.p. present. As found previously (namely for the fluorenyl anion) the l.i.p./c.i.p. ratio increases when temperature decreases. The results of this structural study allow to rationalize the protonation stereochemistry of 9-alkyl-10-lithio-9,10-dihydroanthracenes in the above-mentioned solvents. 相似文献
60.
The thermodynamics and dynamics of a model S(N)1 reaction: t-BuCl --> t-Bu+ + Cl- is studied at the water liquid/vapor interface using molecular-dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and in several locations at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface. Reactive flux correlation function calculations show significant deviation of the rate constant from the transition-state theory: The transmission coefficients range from 0.49 in the bulk to 0.05 above the Gibbs surface. The low transmission coefficient at the interface despite the lower friction is shown to be due to slow vibrational relaxation. 相似文献