96,168 research outputs found
Novel "green" catalysts for controlled ring-opening polymerization of lactide
Syntéza polylaktidu (PLA) polymerací za otevření kruhu cyklického monomeru (ROP) může být uskutečněna různými způsoby. Literatura uvádí více než 100 katalytických systémů, jejichž pomocí lze polylaktid a jiné biodegradabilní alifatické polyestery získat. Například organokovové katalyzátory na bázi Sn, Zn, Al atd. se po splnění své polymerační funkce stávají kontaminanty a pro humánní implantáty je použití takového materiálu diskutabilní. V současné době jsou v centru výzkumné pozornosti nové N-heterocyklické karbenové katalyzátory. Tyto „metal-free“ katalytické struktury jsou schopné reprodukovatelně řídit syntézu polymerů předem definované molekulové hmotnosti s definovanými koncovými skupinami a nízkou polydisperzitou, která je charakteristická pro živý průběh polymerace. Nabízí se možnost syntézy blokových kopolymerů a různorodých makromolekulárních architektur. Předložená diplomová práce se zabývá studiem polymerace cyklického monomeru D,L-laktidu katalyzované N-heterocyklickým karbenem. Polymerace byly vedeny v přítomnosti benzylalkoholu jako iniciátoru v roztoku THF. Byl sledován vliv složení reakčního systému monomer – iniciátor – katalyzátor. Dále byly připraveny polymery opticky čistého L-laktidu s makroiniciátory PEG s Mn = 1000 a 2000 g/mol. Střední číselná molekulová hmotnost (Mn) a polydisperzita (PDI) byly stanoveny pomocí GPC. Definovatelnost koncových skupin vybraných polymerů byla prokázána pomocí 1H NMR.The synthesis of polylactide (PLA) by ring-opening polymerization (ROP) of cyclic monomer can be realized by different routes. More than 100 catalysts for the synthesis of polylactide and other biodegradable aliphatic polyesters are published in the literature. For example organometallic catalysts based on Sn, Zn, Al etc. after finishing polymerization function became contaminants and using obtained polymer material in human body is controversial. At present, the research is focused on novel N-hererocyclic carbene catalysts. These metal-free catalysts are able to produce polymers with controlled molecular weight, narrow polydispersity, end-group fidelity with high reproducibility as well as to synthesize the block copolymers and complex macromolecular architectures, which is characteristic for living polymerization system. This diploma thesis is focused on study of polymerization of cyclic monomer D,L-lactide catalyzed by N-hererocyclic carbene. Polymerizations were carried out at the presence of benzylalcohol as initiator at THF. We were focused on the influence of composition of reaction system monomer – initiator – catalyst. Polymers of optically pure L-lactide with macroinitiators PEG with Mn of 1000 a 2000 g/mol were prepared as well. Number average molecular weight (Mn) and polydispersity index (PDI) was determined by GPC. 1H NMR was used to prove end-group fidelity.
Pseudogap Formation in the Symmetric Anderson Lattice Model
We present self-consistent calculations for the self-energy and magnetic
susceptibility of the 2D and 3D symmetric Anderson lattice Hamiltonian, in the
fluctuation exchange approximation. At high temperatures, strong f-electron
scattering leads to broad quasiparticle spectral functions, a reduced
quasiparticle band gap, and a metallic density of states. As the temperature is
lowered, the spectral functions narrow and a pseudogap forms at the
characteristic temperature at which the width of the quasiparticle
spectral function at the gap edge is comparable to the renormalized activation
energy. For , the pseudogap is approximately equal to the
hybridization gap in the bare band structure. The opening of the pseudogap is
clearly apparent in both the spin susceptibility and the compressibility.Comment: RevTeX - 14 pages and 7 figures (available on request),
NRL-JA-6690-94-002
High temperature sensitivity is intrinsic to voltage-gated potassium channels.
Temperature-sensitive transient receptor potential (TRP) ion channels are members of the large tetrameric cation channels superfamily but are considered to be uniquely sensitive to heat, which has been presumed to be due to the existence of an unidentified temperature-sensing domain. Here we report that the homologous voltage-gated potassium (Kv) channels also exhibit high temperature sensitivity comparable to that of TRPV1, which is detectable under specific conditions when the voltage sensor is functionally decoupled from the activation gate through either intrinsic mechanisms or mutations. Interestingly, mutations could tune Shaker channel to be either heat-activated or heat-deactivated. Therefore, high temperature sensitivity is intrinsic to both TRP and Kv channels. Our findings suggest important physiological roles of heat-induced variation in Kv channel activities. Mechanistically our findings indicate that temperature-sensing TRP channels may not contain a specialized heat-sensor domain; instead, non-obligatory allosteric gating permits the intrinsic heat sensitivity to drive channel activation, allowing temperature-sensitive TRP channels to function as polymodal nociceptors
Block of NMDA receptor channels by endogenous neurosteroids: implications for the agonist induced conformational states of the channel vestibule
N-methyl-D-aspartate receptors (NMDARs) mediate synaptic plasticity, and their dysfunction is implicated in multiple brain disorders. NMDARs can be allosterically modulated by numerous compounds, including endogenous neurosteroid pregnanolone sulfate. Here, we identify the molecular basis of the use-dependent and voltage-independent inhibitory effect of neurosteroids on NMDAR responses. The site of action is located at the extracellular vestibule of the receptor's ion channel pore and is accessible after receptor activation. Mutations in the extracellular vestibule in the SYTANLAAF motif disrupt the inhibitory effect of negatively charged steroids. In contrast, positively charged steroids inhibit mutated NMDAR responses in a voltage-dependent manner. These results, in combination with molecular modeling, characterize structure details of the open configuration of the NMDAR channel. Our results provide a unique opportunity for the development of new therapeutic neurosteroid-based ligands to treat diseases associated with dysfunction of the glutamate system
Temperature dependence of the ``0.7'' 2(e^2)/h quasi plateau in strongly confined quantum point contacts
We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in
some of the most strongly confined point contacts so far reported. This strong
confinement is obtained by a combination of shallow etching and metal gate
deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting
subband separations are up to 20 meV, and as a consequence the quantized
conductance can be followed at temperatures up to 30 K, an order of magnitude
higher than in conventional split gate devices. We observe pronounced quasi
plateaus at several of the lowest conductance steps all the way from their
formation around 1 K to 30 K, where the entire conductance quantization is
smeared out thermally. We study the deviation of the conductance from ideal
integer quantization as a function of temperature, and we find an activated
behavior, exp(-T_a/T), with a density dependent activation temperature T_a of
the order of 2 K. We analyze our results in terms of a simple theoretical model
involving scattering against plasmons in the constriction.Comment: RevTex (4 pages) including 2 postscript figures. To appear in Physica
B, 199
Wide band X-ray Imager (WXI) and Soft Gamma-ray Detector (SGD) for the NeXT Mission
The NeXT mission has been proposed to study high-energy non-thermal phenomena
in the universe. The high-energy response of the super mirror will enable us to
perform the first sensitive imaging observations up to 80 keV. The focal plane
detector, which combines a fully depleted X-ray CCD and a pixellated CdTe
detector, will provide spectra and images in the wide energy range from 0.5 keV
to 80 keV. In the soft gamma-ray band up to ~1 MeV, a narrow field-of-view
Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe
detector will provide precise spectra with much higher sensitivity than present
instruments. The continuum sensitivity will reach several times 10^(-8)
photons/s/keV/cm^(2) in the hard X-ray region and a few times10^(-7)
photons/s/keV/cm^(2) in the soft gamma-ray region.Comment: 12 pages, 13 figures, to be published in the SPIE proceedings 5488,
typo corrected in sec.
Soft Gamma-ray Detector for the ASTRO-H Mission
ASTRO-H is the next generation JAXA X-ray satellite, intended to carry
instruments with broad energy coverage and exquisite energy resolution. The
Soft Gamma-ray Detector (SGD) is one of ASTRO-H instruments and will feature
wide energy band (40-600 keV) at a background level 10 times better than the
current instruments on orbit. SGD is complimentary to ASTRO-H's Hard X-ray
Imager covering the energy range of 5-80 keV. The SGD achieves low background
by combining a Compton camera scheme with a narrow field-of-view active shield
where Compton kinematics is utilized to reject backgrounds. The Compton camera
in the SGD is realized as a hybrid semiconductor detector system which consists
of silicon and CdTe (cadmium telluride) sensors. Good energy resolution is
afforded by semiconductor sensors, and it results in good background rejection
capability due to better constraints on Compton kinematics. Utilization of
Compton kinematics also makes the SGD sensitive to the gamma-ray polarization,
opening up a new window to study properties of gamma-ray emission processes.
The ASTRO-H mission is approved by ISAS/JAXA to proceed to a detailed design
phase with an expected launch in 2014. In this paper, we present science
drivers and concept of the SGD instrument followed by detailed description of
the instrument and expected performance.Comment: 17 pages, 15 figures, Proceedings of the SPIE Astronomical
Instrumentation "Space Telescopes and Instrumentation 2010: Ultraviolet to
Gamma Ray
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