3,905 research outputs found
Optical conductivity of wet DNA
Motivated by recent experiments we have studied the optical conductivity of
DNA in its natural environment containing water molecules and counter ions. Our
density functional theory calculations (using SIESTA) for four base pair B-DNA
with order 250 surrounding water molecules suggest a thermally activated doping
of the DNA by water states which generically leads to an electronic
contribution to low-frequency absorption. The main contributions to the doping
result from water near DNA ends, breaks, or nicks and are thus potentially
associated with temporal or structural defects in the DNA.Comment: 4 pages, 4 figures included, final version, accepted for publication
in Phys. Rev. Let
preclinical evidence from C57BL/6 mice
Chemotherapy-induced central nervous system (CNS) neurotoxicity presents an
unmet medical need. Patients often report a cognitive decline in temporal
correlation to chemotherapy, particularly for hippocampus-dependent verbal and
visuo-spatial abilities. We treated adult C57Bl/6 mice with 12 × 20 mg kg−1
paclitaxel (PTX), mimicking clinical conditions of dose-dense chemotherapy,
followed by a pulse of bromodesoxyuridine (BrdU) to label dividing cells. In
this model, mice developed visuo-spatial memory impairments, and we measured
peak PTX concentrations in the hippocampus of 230 nm l−1, which was sevenfold
higher compared with the neocortex. Histologic analysis revealed a reduced
hippocampal cell proliferation. In vitro, we observed severe toxicity in
slowly proliferating neural stem cells (NSC) as well as human neuronal
progenitor cells after 2 h exposure to low nanomolar concentrations of PTX. In
comparison, mature post-mitotic hippocampal neurons and cell lines of
malignant cells were less vulnerable. In PTX-treated NSC, we observed an
increase of intracellular calcium levels, as well as an increased activity of
calpain- and caspase 3/7, suggesting a calcium-dependent mechanism. This cell
death pathway could be specifically inhibited with lithium, but not glycogen
synthase kinase 3 inhibitors, which protected NSC in vitro. In vivo,
preemptive treatment of mice with lithium prevented PTX-induced memory
deficits and abnormal adult hippocampal neurogenesis. In summary, we
identified a molecular pathomechanism, which invokes PTX-induced cytotoxicity
in NSC independent of cell cycle status. This pathway could be
pharmacologically inhibited with lithium without impairing paclitaxel’s
tubulin-dependent cytostatic mode of action, enabling a potential
translational clinical approach
Strong electron correlations in cobalt valence tautomers
We have examined cobalt based valence tautomer molecules such as
Co(SQ)(phen) using density functional theory (DFT) and variational
configuration interaction (VCI) approaches based upon a model Hamiltonian. Our
DFT results extend earlier work by finding a reduced total energy gap (order
0.6 eV) between high temperature and low temperature states when we fully relax
the coordinates (relative to experimental ones). Futhermore we demonstrate that
the charge transfer picture based upon formal valence arguments succeeds
qualitatively while failing quantitatively due to strong covalency between the
Co 3 orbitals and ligand orbitals. With the VCI approach, we argue that
the high temperature, high spin phase is strongly mixed valent, with about 30 %
admixture of Co(III) into the predominantly Co(II) ground state. We confirm
this mixed valence through a fit to the XANES spectra. Moreover, the strong
electron correlations of the mixed valent phase provide an energy lowering of
about 0.2-0.3 eV of the high temperature phase relative to the low temperature
one. Finally, we use the domain model to account for the extraordinarily large
entropy and enthalpy values associated with the transition.Comment: 10 pages, 4 figures, submitted to J. Chem. Phy
Aviram-Ratner rectifying mechanism for DNA base pair sequencing through graphene nanogaps
We demonstrate that biological molecules such as Watson-Crick DNA base pairs
can behave as biological Aviram-Ratner electrical rectifiers because of the
spatial separation and weak hydrogen bonding between the nucleobases. We have
performed a parallel computational implementation of the ab-initio
non-equilibrium Green's function (NEGF) theory to determine the electrical
response of graphene---base-pair---graphene junctions. The results show an
asymmetric (rectifying) current-voltage response for the Cytosine-Guanine base
pair adsorbed on a graphene nanogap. In sharp contrast we find a symmetric
response for the Thymine-Adenine case. We propose applying the asymmetry of the
current-voltage response as a sensing criterion to the technological challenge
of rapid DNA sequencing via graphene nanogaps
Fragmentation and systematics of the Pygmy Dipole Resonance in the stable N=82 isotones
The low-lying electric dipole (E1) strength in the semi-magic nucleus 136Xe
has been measured which finalizes the systematic survey to investigate the
so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with
the method of nuclear resonance fluorescence using real photons in the entrance
channel. In all cases, a fragmented resonance-like structure of E1 strength is
observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation
of the strength reveals that the degree of fragmentation decreases towards the
proton-deficient isotones while the total integrated strength increases
indicating a dependence of the total strength on the neutron-to-proton ratio.
The experimental results are compared to microscopic calculations within the
quasi-particle phonon model (QPM). The calculation includes complex
configurations of up to three phonons and is able to reproduce also the
fragmentation of the E1 strength which allows to draw conclusions on the
damping of the PDR. Calculations and experimental data are in good agreement in
the degree of fragmentation and also in the integrated strength if the
sensitivity limit of the experiments is taken into account
Possible experimental signature of octupole correlations in the 0 states of the actinides
= 0 states have been investigated in the actinide nucleus
Pu up to an excitation energy of 3 MeV with a high-resolution (p,t)
experiment at = 24 MeV. To test the recently proposed = 0
double-octupole structure, the phenomenological approach of the
spdf-interacting boson model has been chosen. In addition, the total 0
strength distribution and the strength fragmentation have been compared
to the model predictions as well as to the previously studied (p,t) reactions
in the actinides. The results suggest that the structure of the 0 states
in the actinides might be more complex than the usually discussed pairing
isomers. Instead, the octupole degree of freedom might contribute
significantly. The signature of two close-lying 0 states below the
2-quasiparticle energy is presented as a possible manifestation of strong
octupole correlations in the structure of the 0 states in the actinides.Comment: 6 pages, 5 figures, published in Phys. Rev. C 88, 041303(R) (2013
The decay of quadrupole-octupole states in Ca and Ce
Background: Two-phonon excitations originating from the coupling of two
collective one-phonon states are of great interest in nuclear structure
physics. One possibility to generate low-lying excitations is the coupling
of quadrupole and octupole phonons.
Purpose: In this work, the -decay behavior of candidates for the
state in the doubly-magic nucleus Ca and in
the heavier and semi-magic nucleus Ce is investigated.
Methods: experiments have been carried out at the
High Intensity -ray Source (HIS) facility in combination with
the high-efficiency -ray spectroscopy setup consisting of
HPGe and LaBr detectors. The setup enables the acquisition of
- coincidence data and, hence, the detection of direct decay
paths.
Results: In addition to the known ground-state decays, for Ca the
decay into the state was observed, while for Ce the direct
decays into the and the state were detected. The experimentally
deduced transition strengths and excitation energies are compared to
theoretical calculations in the framework of EDF theory plus QPM approach and
systematically analyzed for isotones. In addition, negative parities for
two states in Ca were deduced simultaneously.
Conclusions: The experimental findings together with the theoretical
calculations support the two-phonon character of the excitation in the
light-to-medium-mass nucleus Ca as well as in the stable even-even
nuclei.Comment: 11 pages, 6 figures, as accepted in Phys. Rev.
One particle interchain hopping in coupled Hubbard chains
Interchain hopping in systems of coupled chains of correlated electrons is
investigated by exact diagonalizations and Quantum-Monte-Carlo methods. For two
weakly coupled Hubbard chains at commensurate densities (e.g. n=1/3) the
splitting at the Fermi level between bonding and antibonding bands is strongly
reduced (but not suppressed) by repulsive interactions extending to a few
lattice spacings. The magnitude of this reduction is directly connected to the
exponent of the 1D Luttinger liquid. However, we show that the
incoherent part of the single particle spectral function is much less affected
by the interchain coupling. This suggests that incoherent interchain hopping
could occur for intermediate values.Comment: 4 pages, LaTeX 3.0, 7 PostScript figures in uuencoded for
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