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31.
Die erste Röntgenstrukturanalyse eines 3,7-Dehydrotropon ( 1 )-Derivates, nämlich von 2-Diisopropylammo-3,7-dehydrotropon ( 4 ) zeigt das Ringsystem in 4 als planaren Bicyclus mit nahezu C2V-Symmetrie. Dementsprechend enthalten Dehydrotropone formell zwei trans-konfigurierte Doppelbindungen in einem 7-gliedrigen Ring; auch das O-Atom und das N-Atom mit seinen Ligandatomen liegen in derselben Ebene wie die Ringatome. Die Bindung zwischen den beiden C-Atomen, welche die Heteroatome tragen, (C(1) and C(2)), ist wesentlich langer (1,56 Å) als die anderen Bindungen im Ringsystem (1,37-1,46 Å). Dies impliziert eine ‘push-pull’-π-Elektronen-Delokalisierung, in der auch das O-und das N-Atom involviert sind, und macht eine ‘aromatische’ Ring-Delokalisierung weniger wahr-scheinlich. Im Gegensatz zu 2-(t-Butyl)-3,7-dehydrotropon ( 3 ) existiert das 2-Diiso-propylamino-3,7-dehydrotropon ( 4 ) im Kristall als Monomeres, was dem stabili-sierenden Einfluss der ‘push-pull’-Delokausierung zugeschrieben werden kann. 相似文献
32.
To explore tertiary contact formation in alpha-synuclein, a natively unfolded protein implicated in Parkinson's disease, we have measured the rates of reaction between a powerful electron donor, the tryptophan (W) triplet excited state, and an acceptor, 3-nitro-tyrosine (Y(NO2)) in six different variants, probing loop sizes between 15 and 132 residues. Electron transfer rates decrease with loop size with the fastest contact time of 140 ns for the N-terminal pair and the slowest of 1.2 mus for the N- to C-terminal pair. Diffusion coefficients ranging from approximately 2 x 10-6 to approximately 10-5 cm2 s-1 were extracted from simultaneous fits of the W to Y(NO2) electron (triplet excited state) and energy transfer (singlet excited state) kinetics. 相似文献
33.
The kinetics of the exchange between56Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β− activity acquired by the solution. The results of the exchange between a chemical γ MnO2 and a divalent M2+ ion (M=Mn, Co, Cu or Zn) or a trivalent M3+ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that
M2+ ions exchange with Mn2+ ions and M3+ ions exchange with Mn3+ ions in MnO2. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies.
It seems that the γ MnO2 studied contains more Mn3+ than Mn2+ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange
between Co2+ ions and MnO2 was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples
of MnO2 exchanged with a given cation depends on the type and not on the surface area of the sample. 相似文献
34.
Wu X Ding S Ding Q Gray NS Schultz PG 《Journal of the American Chemical Society》2002,124(49):14520-14521
Purmorphamine, which is a 2,6,9-trisubstituted purine compound, was discovered through cell-based high-throughput screening from a heterocycle combinatorial library. It differentiates multipotent mesenchymal progenitor cells into an osteoblast lineage. It will serve as a unique chemical tool to study the molecular mechanisms of osteogenesis of stem cells and bone development. 相似文献
35.
Miller JE Grădinaru C Crane BR Di Bilio AJ Wehbi WA Un S Winkler JR Gray HB 《Journal of the American Chemical Society》2003,125(47):14220-14221
Near-UV irradiation of structurally characterized [Re(I)(CO)3(1,10-phenanthroline)(Q107H)](W48F/Y72F/H83Q/Y108W)AzM(II) [Az = Pseudomonas aeruginosa azurin, M = Cu, Zn]/[Co(NH3)5Cl]Cl2 produces a tryptophan radical (W108*) with unprecedented kinetic stability. After rapid formation (k = 2.8 x 106 s-1), the radical persists for more than 5 h at room temperature in the folded ReAzM(II) structure. The absorption spectrum of ReAz(W108*)M(II) exhibits maxima at 512 and 536 nm. Oxidation of K4[Mo(CN)8] by ReAz(W108*)Zn(II) places the W108*/W108 reduction potential in the protein above 0.8 V vs NHE. 相似文献
36.
[formula: see text] A highly diastereoselective, microwave-induced Claisen rearrangement of an appropriately substituted propargylic enol ether allows the formation of the sterically congested C8-C14 bond of azadirachtin. When combined with a radical-mediated cyclization of the corresponding allene, this sequence offers rapid entry to the framework of azadirachtin. 相似文献
37.
Gray M Cuello AO Cooke G Rotello VM 《Journal of the American Chemical Society》2003,125(26):7882-7888
Two receptors, a diaminotriazine derivative (DAT) and diamidopyridine (DAP), are complementary to the electroactive naphthalimide (N) through three-point hydrogen bonding. The association constants of the two receptors were evaluated for both the fully oxidized and the radical anion forms of N. In the oxidized state, the two receptors displayed identical binding constants. Diamidopyridine, however, lowers the reduction potential of naphthalimide to a far greater extent than does diaminotriazine, indicating a greater affinity for diamidopyridine by naphthalimide in the radical anion form. This behavior was mirrored by EPR experiments that showed small deviations from the hyperfine coupling pattern of N(red) in the presence of DAT, with greater effects seen for the N(red).DAP complex. Computational simulations using the UB3LYP/6-311+G(d,p)//UHF/6-31G(d) hybrid gave theoretical hyperfine constants in good quantitative agreement with the experimental results. Using this correlation, we determined that electrostatics and hydrogen bond polarizability play key roles in controlling redox-modulated molecular recognition. 相似文献
38.
A range of mixed ether-esters of cellulose was prepared from partially substituted ethylcellulose and methylcellulose. The 13C-NMR analysis of ethylcellulose with a DS of 2.5 indicated that the hydroxyl groups at carbon six of anhydroglucose units were completely substituted. Acetylation of the ethylcellulose under different conditions yielded (acetyl) (ethyl) cellulose (AEC) samples with acetyl degree of substitution ranging from 0 to 0.5. Fully substituted (propionyl) (ethyl) cellulose (PEC) and (acetyl) (methyl) cellulose (AMC) were also prepared. Chiral nematic liquid crystals were formed in these mixed ester/ethers of cellulose in concentrated solutions of acidic solvents. The critical concentration for the phase separation of the cellulosic solutions depended on the nature of the substituent, the degree of substitution, and the solvent at a given temperature. Methylcellulose solutions in trifluoroacetic acid and dichloroacetic acid form chiral nematic liquid crystals with a left-handed helicoidal structure. The acetylated methyl cellulose samples did not show the reversal of handedness with increasing acetyl content that was previously observed for the corresponding ethylcellulose samples. © 1994 John Wiley & Sons, Inc. 相似文献
39.
Fulton R Jensen T Johnson DR Kagan H Kass R Morrow F Whitmore J Wilson P Bortoletto D Chen W Dominick J McIlwain RL Miller DH Ng CR Schaffner SF Shibata EI Shipsey IP Yao W Battle M Sparks K Thorndike EH Wang C Alam MS Kim IJ Li WC Romero V Sun CR Wang P Zoeller MM Goldberg M Haupt T Horwitz N Jain V Mestayer MD Moneti GC Rozen Y Rubin P Sharma V Skwarnicki T Thulasidas M Zhu G Csorna SE Letson T Alexander J Artuso M Bebek C Berkelman K Browder T Cassel DG Cheu E Coffman DM Crawford G DeWire JW 《Physical review D: Particles and fields》1991,43(3):651-663
40.