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591.
Maximilian Bradler Frederik D. Nielsen Carl Elias Eckert Eberhard Riedle 《Applied physics. B, Lasers and optics》2014,116(4):875-882
Broadly tunable picosecond pulses in the UV for nonlinear microscopy and lifetime measurements are not yet readily available. Complex synchronously pumped optical parametric oscillators with subsequent frequency doubling are typically used. We show that direct second harmonic generation of a visible picosecond supercontinuum source at 78 MHz renders pulses easily tunable from 250 to 430 nm. We find that an unexpectedly large numerical aperture and the use of thick crystals increase the efficiency of the frequency doubling process dramatically. The observed spectral width and efficiency are nearly two orders of magnitude larger than predicted by conventional theory. With broadband achromatic doubling, a 130 nm wide spectrum is achieved. Pulse durations of 17–35 ps are found in the UV and an average power between 1 and 70 μW. This qualifies the setup for most UV-based microscopic investigations. As first application, the fluorescence lifetime of two differing conformations of 2-(2′-hydroxyphenyl) benzothiazole is measured. 相似文献
592.
Hooff GP Volmer DA Wood WG Müller WE Eckert GP 《Analytical and bioanalytical chemistry》2008,392(4):673-680
Farnesyl- and geranylgeranylpyrophosphate (FPP and GGPP) are isoprenoid intermediates in the mevalonate pathway. They play a crucial role in cell survival, growth and differentiation due to their attachment (isoprenylation) to small GTPases (Ras, Rho, etc.). Isoprenoid formation seems to be tightly regulated within the mevalonate pathway and its perturbation has been linked to certain diseases (e.g., cancer, Alzheimer's disease), but tissue levels are unknown. It is therefore of the utmost importance to quantify these isoprenoids in diseased tissue or in tissue after drug administration. The current work describes an isolation procedure utilizing a combination of Extrelut(R) liquid/liquid and reversed-phase solid-phase extraction (SPE) for homogenized human frontal cortex tissue. In addition, after a careful validation of an HPLC-fluorescence method, this assay allowed the determination of nanomolar concentrations of endogenous FPP and GGPP levels (4.5 and 10.6 ng/mg protein, respectively) in human brain tissue. The method is selective, precise (<15% RSD), accurate (<15% relative error) and sensitive over a linear range of 10-400 ng/mL for FPP and 50-1000 ng/mL for GGPP according to the current FDA criteria for bioanalytical method validation. Overall, this new method introduces the ability to simultaneously quantify FPP and GGPP in human brain tissue, and is potentially applicable to several other tissues and species. 相似文献
593.
594.
Wiegand T Eckert H Ekkert O Fröhlich R Kehr G Erker G Grimme S 《Journal of the American Chemical Society》2012,134(9):4236-4249
Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of "frustrated Lewis pairs" (FLPs) (mainly substituted vinylene linked intramolecular phosphane-borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the (11)B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron-phosphorus internuclear distances. The principal component V(zz) of the (11)B electric field gradient tensor is tilted slightly away (~20°) from the boron-phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary (31)P{(1)H}-CPMAS experiments reveal significant (31)P-(11)B scalar spin-spin interactions ((1)J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, (11)B{(31)P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin-spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding. 相似文献
595.
I. Kaban P. Jóvári T. Wágner M. Bartoš M. Frumar B. Beuneu W. Hoyer N. Mattern J. Eckert 《Journal of Non》2011,357(19-20):3430-3434
(As0.33S0.67)100-xAgx (0 ≤ x ≤ 28) bulk glasses showing micro-phase separation in a wide concentration range have been studied by X-ray diffraction, neutron diffraction and extended X-ray absorption fine structure measurements. The AsAgS2 composition, which forms a homogeneous glass, is modeled with the reverse Monte-Carlo simulation technique. It is established that Ag prefers the environment of S; Ag―As bonding cannot be observed. Similarly to the AsAgS2 crystalline modifications smithite and trechmannite, the main structural units of the glass are AsS3 pyramids. The covalent network of As and S atoms becomes fragmented in the glassy AsAgS2 unlike in the glassy AsS2. The environment of Ag atoms in the AsAgS2 glass differs from that in the crystalline state. In average, each Ag atom has four nearest neighbors, three of them being S and one being Ag. 相似文献
596.
Shukla CL Hallett JP Popov AV Hernandez R Liotta CL Eckert CA 《The journal of physical chemistry. B》2006,110(47):24101-24111
Local solvation and transport effects in gas-expanded liquids (GXLs) are reported based on molecular simulation. GXLs were found to exhibit local density enhancements similar to those seen in supercritical fluids, although less dramatic. This approach was used as an alternative to a multiphase atomistic model for these mixtures by utilizing experimental results to describe the necessary fixed conditions for a locally (quasi-) stable molecular dynamics model of the (single) GXL phase. The local anisotropic pair correlation function, orientational correlation functions, and diffusion rates are reported for two systems: CO2-expanded methanol and CO2-expanded acetone at 298 K and pressures up to 6 MPa. 相似文献
597.
598.
Thomas Harmening Adel Al Alam Samir F. Matar Hellmut Eckert Rainer Pöttgen 《Solid State Sciences》2009,11(7):1239-1245
The silicide Sc2RuSi2 was synthesized from the elements by arc-melting. The structure was refined on the basis of single crystal X-ray diffractometer data: Zr2CoSi2 type, C2/m, a = 1004.7 (2), b = 406.8 (1), c = 946.6 (2) pm, β = 117.95 (2), wR2 = 0.0230, 743 F2 values, and 32 variables. The structure consists of a rigid three-dimensional [RuSi2] network in which the two crystallographically independent scandium atoms fill larger cages of coordination numbers 16 and 15, respectively. The [RuSi2] network shows short Ru–Si distances (234–247 pm) and two different Si2 pairs: Si1–Si1 at 247 and Si2–Si2 at 243 pm. Each silicon atom has trigonal prismatic Sc6 (for Si2) or Sc4Ru2 (for Si1) coordination. These building units are condensed via common edges and faces. The various Sc–Sc distances between the prisms range from 327 to 361 pm. From electronic structure investigation within DFT, chemical bonding shows a major role of Ru–Si bonding and the presence of strong electron localization around Si–Si pairs pointing to a polyanionic silicide network [RuSi2]δ?. The 45Sc MAS-NMR spectra recorded at 11.7 and 9.4 T clearly resolve the two distinct scandium sites. The large electric field gradients present at both scandium sites result in typical line shapes arising from second-order quadrupole perturbation effects. 相似文献
599.
I Kaban P Jóvári RP Wang B Luther-Davies N Mattern J Eckert 《J Phys Condens Matter》2012,24(38):385802
The structure of Ge(5)As(x)Se(95-x) (x?=?10, 20, 30, 38?at.%) and Ge(15)As(x)Se(85-x) (x?=?10, 25, 34?at.%) glasses has been investigated by high-energy x-ray diffraction and extended x-ray absorption fine structure measurements. The experimental datasets have been modelled using the reverse Monte Carlo simulation technique. The model atomic configurations have been analysed in detail. It has been found that the homonuclear Ge-Ge, As-As, Se-Se and heteronuclear Ge-As bonds play an important role in the structure formation of the Ge-As-Se glasses. The total number of these bonds decreases quite slowly with the mean coordination number similarly to the nonlinear refractive index. 相似文献
600.
Köllner T. Schwarzenberger K. Eckert K. Boeck T. 《The European physical journal. Special topics》2015,224(2):261-276
The European Physical Journal Special Topics - We study Marangoni roll cell convection in a ternary mass transfer system composed of cyclohexanol, water and butanol (transferred species) placed in... 相似文献