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71.
T.
ivkovi M. Randi D. J. Klein H.-Y. Zhu N. Trinajsti 《Journal of computational chemistry》1995,16(4):517-526
We consider rigorous evaluation of conjugated-circuit resonance energies for families of structurally related benzenoid hydrocarbons of increasing size. Local and global aromatic properties of such molecules are investigated with particular interest in modeling high polymers. Using the algebra of large numbers, exact formulas for contributions from individual benzene rings of polymers with up to 25,000 repeating units (close to half a million carbon atoms) were derived. All arithmetic procedures were carried out in terms of whole numbers retaining all digits, of which there were sometimes more than 105. © 1995 by John Wiley & Sons, Inc. 相似文献
72.
Abstract— The influence of excitation wavelength, pH, oxygen and solvents, upon fluorescence quantum yields, were measured for insole Indole in neutral aqueous solution exhibits the same wavelength dependence as tryptophan, which indicates that COO- absorption is not responsible for the effect. Parameters such as pH and oxygen influence only the absolute fluorescence quantum yields but not their relative variation with wavelength, indicating competition with fluorescence emission for S1 deactivation. without any influence upon deactivation of higher excited states. In contrast, solvents exhibit a specific influence upon the wavelength dependence; for indole, the decrease of fluorescence yield excited around 215 nm, compared with the yield in the first absorption band is about 40% in water, 10% in acetonitrile, 70% in n-hexane and cyclohexane, whereas no appreciable decrease occurs in methanol, ethanol or n-butanol. These observations, together with the Stokes shifts of the emission spectra, may be well correlated with Kosower's Z-values, expressing microscopic solvent-solute interactions. 相似文献
73.
Astuti Y Topoglidis E Briscoe PB Fantuzzi A Gilardi G Durrant JR 《Journal of the American Chemical Society》2004,126(25):8001-8009
In this paper, we report a spectroelectrochemical investigation of proton-coupled electron transfer in flavodoxin D. vulgaris Hildenborough (Fld). Poly-L-lysine is used to promote the binding of Fld to the nanocrystalline, mesoporous SnO(2) electrodes. Two reversible redox couples of the immobilized Fld are observed electrochemically and are assigned by spectroelectrochemistry to the quinone/semiquinone and semiquinone/hydroquinone couples of the protein's flavin mononucleotide (FMN) redox cofactor. Comparison with control data for free FMN indicates no contamination of the Fld data by dissociated FMN. The quinone/semiquinone and semiquinone/hydroquinone midpoint potentials (E(q/sq) and E(sq/hq)) at pH 7 were determined to be -340 and -585 mV vs Ag/AgCl, in good agreement with the literature. E(q/sq) exhibited a pH dependence of 51 mV/pH. The kinetics of these redox couples were studied using cyclic voltammetry, cyclic voltabsorptometry, and chronoabsorptometry. The semiquinone/quinone reoxidation is found to exhibit slow, potential-independent but pH-sensitive kinetics with a reoxidation rate constant varying from 1.56 s(-)(1) at pH 10 to 0.0074 s(-)(1) at pH 5. The slow kinetics are discussed in terms of a simple kinetics model and are assigned to the reoxidation process being rate limited by semiquinone deprotonation. It is proposed that this slow deprotonation step has the physiological benefit of preventing the undesirable loss of reducing equivalents which results from semiquinone oxidation to quinone. 相似文献
74.
[reaction: see text] A stereoselective synthesis of (-)-ilimaquinone (4) is presented. The synthetic strategy is based on a novel radical decarboxylation and quinone addition methodology that produces quinone 7 from reaction of thiohydroxamic acid derivative 8 with benzoquinone (9). Final functionalization of 7 to ilimaquinone (4) is achieved by exploring the electronic effects of the residual thiopyridyl group. 相似文献
75.
Michael Klein 《Tetrahedron》2004,60(5):1087-1092
The synthesis of an enediyne sulfonamide by alkylidene carbene rearrangement is reported. The compound cyclizes thermally to give the Bergman product, which was prepared independently for comparison. Like other σ-acceptor substituents at the enediyne alkyne termini, such as fluoride, oxonium or ammonium groups, the sulfonamide moiety enhances the reactivity for thermal Bergman cyclization as shown by the cyclization kinetic of the title compound. 相似文献
76.
Heinz-Peter Klein Ulf Thewalt Karl Döppert Roberto Sanchez-Delgado 《Journal of organometallic chemistry》1982,236(2):189-195
The trinuclear titanium(IV) complex (π-C5H5)2TiClOTi(π-C5H5)ClOTiCl(π-C5H5)2 · CHCl3 is formed by hydrolysis of (π-C5H5)2TiCl2 at pH > 3.5 and can be isolated in the crystalline state from the reaction of (π-C5H5)2TiCl2 with Ag2O and water in chloroform. Its structure is determined by X-ray analysis. 相似文献
77.
The spontaneous decarboxylation of 0.5 m aqueous α‐alanine solutions as a function of pH (1–9 at 320°C, where neutrality is approximately 6) was determined with a flow reactor at 280–330°C and 275 bar by FT‐IR spectroscopy. The kinetics for the cationic and anionic forms have not been previously reported. The rate constants for the cationic form [CH3(NH3+)CHCO2H], the anionic form [CH3(NH2)CHCO2?], and the zwitterion form [CH3(NH3+)CHCO2?] were obtained and followed the first‐order rate law. The rate of decarboxylation of the zwitterion is three times greater than that of the cationic and anionic forms in the temperature and pH ranges of study. The corresponding Arrhenius parameters were determined and compared with previously reported data. The addition of KCl (1 and 2 m) at the natural pH of α‐alanine resulted in a reduction of the decarboxylation rate, suggesting that the transition state is less polar than the zwitterion and/or that the activity of the zwitterion has been reduced. The α‐alanine solution is therefore somewhat more robust in solutions of high ionic strength, such as seawater, than it is in pure water. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 271–277, 2002 相似文献
78.
Arsenidostannates with [SnAs] Nets Isostructural to Grey Arsenic: Synthesis and Crystal Structure of Na[Sn2As2], Na0.3Ca0.7[Sn2As2], Na0.4Sr0.6[Sn2As2], Na0.6Ba0.4[Sn2As2], and K0.3Sr0.7[Sn2As2] The metallic lustrous compounds Na[Sn2As2], Na0.3Ca0.7[Sn2As2], Na0.4Sr0.6[Sn2As2], Na0.6Ba0.4[Sn2As2] and K0.3Sr0.7[Sn2As2] were prepared from melts of mixtures of the elements. The compounds crystallize in the trigonal system (space group R3 m, No. 166, Z = 3) with lattice constants see in “Inhaltsübersicht”. The structures are isotypic to Sr[Sn2As2] containing puckered [SnAs] nets which are stacked with a sequence of six layers. The E(I)/E(II) atoms are located between each second [SnAs] layer in trigonal antiprismatic interstices formed by As atoms. In the resulting [Sn2As2] double layers the ∞2[SnAs] nets are stacked in such a way that additional Sn—Sn contacts arise. 相似文献
79.
Detection of a biomarker for Alzheimer's disease from synthetic and clinical samples using a nanoscale optical biosensor 总被引:9,自引:0,他引:9
Haes AJ Chang L Klein WL Van Duyne RP 《Journal of the American Chemical Society》2005,127(7):2264-2271
A nanoscale optical biosensor based on localized surface plasmon resonance (LSPR) spectroscopy has been developed to monitor the interaction between the antigen, amyloid-beta derived diffusible ligands (ADDLs), and specific anti-ADDL antibodies. Using the sandwich assay format, this nanosensor provides quantitative binding information for both antigen and second antibody detection that permits the determination of ADDL concentration and offers the unique analysis of the aggregation mechanisms of this putative Alzheimer's disease pathogen at physiologically relevant monomer concentrations. Monitoring the LSPR-induced shifts from both ADDLs and a second polyclonal anti-ADDL antibody as a function of ADDL concentration reveals two ADDL epitopes that have binding constants to the specific anti-ADDL antibodies of 7.3 x 10(12) M(-1) and 9.5 x 10(8) M(-1). The analysis of human brain extract and cerebrospinal fluid samples from control and Alzheimer's disease patients reveals that the LSPR nanosensor provides new information relevant to the understanding and possible diagnosis of Alzheimer's disease. 相似文献
80.
Carine Livage Ahmad Safari Lisa C. Klein 《Journal of Sol-Gel Science and Technology》1994,2(1-3):605-609
A new sol-gel system using ethylene glycol was developed for the fabrication of PZT thin films with compositions near the morphotropic phase boundary Pb(Zr0.52Ti0.48)O3. Ethylene glycol was used as both a chelating agent and a solvent to replace the highly toxic methoxyethanol used in previous formulations. Thin films were deposited by spin coating the solutions onto platinized silicon substrates. Films were completely crystallized by about 600°C and contained the ferroelectric perovskite phase. A dielectric constant of about 750–800 at 1 KHz was obtained for thin films of 0.3 µm thickness. The hysteresis measurements revealed a remanent polarization of 15 mC/cm2 with a coercive field of 60 kV/cm. 相似文献