Herstellung und 29Si-NMR-Untersuchung von Polymeren mit definierten Kieselsäurebaueinheiten

Durch Additionsreaktion der käfigartigen Doppelvierring-(D4R-)kieselsäurederivate [(CH3)2Hsi]Si8O20 mit dem zweifach ungesättigten Divinyltetramethyldisiloxan bzw. den mehrfachfunktionellen Tetramethylcyclotetrasiloxan und Polymethylhydrogensiloxan im molaren Verhältnis der funktionellen Gruppen von 1:1 wurden 3 in organischen Lösungsmitteln unlösliche Polymere hergestellt. 29Si-Festkörper-NMR-Untersuichungen zeigen, daß sich die Polymere aus D4R-Kieselsäureeinheiten aufbauen, die über ketten- oder ringförmige Siloxanbrücken miteinander vernetzt sind. Mit zunehmender Zabl funktioneller Gruppen in den Reaktionspartnern der D4R-Derivate wächst die räumliche Behinderung der Vernetzungsreaktionen. Die Polymere weisen nur eine kleine Oberfläche zwischen 1 und 8 m2/g auf

Solid state 29Si NMR investigation of inorganic-organic polymers with defined silicic acid units

By thermal analysis and 29Si NMR spectroscopy the thermal behaviour and structural changes of two different types of inorganic-organic polymers with defined double four-ring silicic acid units (Si8O20) were characterized. Polymer 1, prepared from the organic silicic acid precursors [(CH3)2HSi]8Si8O20 (Q8M8H) and [CH2=CH(CH3)2Si]8Si8O20 (Q8M8V), preserves the double four-ring structure up to 350°C Higher temperatures lead to structural reorganizations of the SiO4 tetrahedra forming at 900°C a structure similiar to amorphous silica. Polymer 2, synthesized by condensation reaction of the new organic silicic acid precursor [CH3O)3Si(CH2)3OC(O)CH(CH3)CH2Si(CH3)2]8Si8O20, shows with 37% uncondensed SiOH groups a remarkable high content of silanol groups caused by sterical hindrances

Quantum key distribution using non-classical photon number correlations in macroscopic light pulses

We propose a new scheme for quantum key distribution using macroscopic non-classical pulses of light having of the order 10^6 photons per pulse. Sub-shot-noise quantum correlation between the two polarization modes in a pulse gives the necessary sensitivity to eavesdropping that ensures the security of the protocol. We consider pulses of two-mode squeezed light generated by a type-II seeded parametric amplification process. We analyze the security of the system in terms of the effect of an eavesdropper on the bit error rates for the legitimate parties in the key distribution system. We also consider the effects of imperfect detectors and lossy channels on the security of the scheme.Comment: Modifications:added new eavesdropping attack, added more references Submitted to Physical Review A [email protected]

A simple low-SAR technique for chemical-shift selection with high-field spin-echo imaging

We have discovered a simple and highly robust method for removal of chemical shift artifact in spin-echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin-echo echo-planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice-select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin-echo echo-planar imaging. Moreover, the method can be generally applied to any sequence involving slice-selective excitation and at least one slice-selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B0)

George Biersack Selected to Serve on the Nominators Committee of the Emerson College

News release announces that Chairman of Communication Arts at the University of Dayton, George Biersack, has been selected to serve on the Nominators Committee of the Emerson College

Charakterisierung von Organokieselsäurepolymeren durch thermoanalytische Untersuchungen

Four organosilicate polymers synthesized by addition of vinyl- and H-substituted double four-ring silicic acid derivatives were characterized using DSC and simultaneous TG-DTA measurements. Thermooxidative decomposition proceeds in several steps. Oxidation of (Si-H9 groups, oxidation of (Si-Ch2-CH2-Si) bridges, and oxidation of (Si-CH3) groups, with formation of new (Si-O-si) bonds. 29Si-NMR-spectroscopic emeasurements confirm this interpretation. Thermoanalytical methods of investigation proved useful to characterize these organosilicate polymers

Ion detection in the photoionization of a Rb Bose-Einstein condensate

Two-photon ionization of Rubidium atoms in a magneto-optical trap and a Bose-Einstein condensate (BEC) is experimentally investigated. Using 100 ns laser pulses, we detect single ions photoionized from the condenstate with a 35(10)% efficiency. The measurements are performed using a quartz cell with external electrodes, allowing large optical access for BECs and optical lattices.Comment: 14 pages, 7 figure

Structural basis for c-di-AMP–dependent regulation of the bacterial stringent response by receptor protein DarB

The bacterial second messenger c-di-AMP controls essential cellular processes, including potassium and osmolyte homeostasis. This makes synthesizing enzymes and components involved in c-di-AMP signal transduction intriguing as potential targets for drug development. The c-di-AMP receptor protein DarB of Bacillus subtilis binds the Rel protein and triggers the Rel-dependent stringent response to stress conditions; however, the structural basis for this trigger is unclear. Here, we report crystal structures of DarB in the ligand-free state and of DarB complexed with c-di-AMP, 3′3′-cGAMP, and AMP. We show that DarB forms a homodimer with a parallel, head-to-head assembly of the monomers. We also confirm the DarB dimer binds two cyclic dinucleotide molecules or two AMP molecules; only one adenine of bound c-di-AMP is specifically recognized by DarB, while the second protrudes out of the donut-shaped protein. This enables DarB to bind also 3′3′-cGAMP, as only the adenine fits in the active site. In absence of c-di-AMP, DarB binds to Rel and stimulates (p)ppGpp synthesis, whereas the presence of c-di-AMP abolishes this interaction. Furthermore, the DarB crystal structures reveal no conformational changes upon c-di-AMP binding, leading us to conclude the regulatory function of DarB on Rel must be controlled directly by the bound c-di-AMP. We thus derived a structural model of the DarB–Rel complex via in silico docking, which was validated with mass spectrometric analysis of the chemically crosslinked DarB–Rel complex and mutagenesis studies. We suggest, based on the predicted complex structure, a mechanism of stringent response regulation by c-di-AMP

Lifetime determination of excited states in Cd-106

Two separate experiments using the Differential Decay Curve Method have been performed to extract mean lifetimes of excited states in 106 Cd. The inedium-spin states of interest were populated by the Mo-98(C-12, 4n) Cd-106 reaction performed at the Wright Nuclear Structure Lab., Yale University. From this experiment, two isomeric state mean lifetimes have been deduced. The low-lying states were populated by the Mo-96(C-13, 3n)Cd-106 reaction performed at the Institut fur Kernphysik, Universitat zu Koln. The mean lifetime of the I-pi = 2(1)(+) state was deduced, tentatively, as 16.4(9) ps. This value differs from the previously accepted literature value from Coulomb excitation of 10.43(9) ps