7,805 research outputs found
Confinement Effects on the Kinetics and Thermodynamics of Protein Dimerization
In the cell, protein complexes form relying on specific interactions between
their monomers. Excluded volume effects due to molecular crowding would lead to
correlations between molecules even without specific interactions. What is the
interplay of these effects in the crowded cellular environment? We study
dimerization of a model homodimer both when the mondimers are free or tethered
to each other. We consider a structured environment: Two monomers first diffuse
into a cavity of size and then fold and bind within the cavity. The folding
and binding are simulated using molecular dynamics based on a simplified
topology based model. The {\it confinement} in the cell is described by an
effective molecular concentration . A two-state coupled folding
and binding behavior is found. We show the maximal rate of dimerization
occurred at an effective molecular concentration M which is a
relevant cellular concentration. In contrast, for tethered chains the rate
keeps at a plateau when .
For both the free and tethered cases, the simulated variation of the rate of
dimerization and thermodynamic stability with effective molecular concentration
agrees well with experimental observations. In addition, a theoretical argument
for the effects of confinement on dimerization is also made
Optogenetics and deep brain stimulation neurotechnologies
Brain neural network is composed of densely packed, intricately wired neurons whose activity patterns ultimately give rise to every behavior, thought, or emotion that we experience. Over the past decade, a novel neurotechnique, optogenetics that combines light and genetic methods to control or monitor neural activity patterns, has proven to be revolutionary in understanding the functional role of specific neural circuits. We here briefly describe recent advance in optogenetics and compare optogenetics with deep brain stimulation technology that holds the promise for treating many neurological and psychiatric disorders
Spin swap gate in the presence of qubit inhomogeneity in a double quantum dot
We study theoretically the effects of qubit inhomogeneity on the quantum
logic gate of qubit swap, which is an integral part of the operations of a
quantum computer. Our focus here is to construct a robust pulse sequence for
swap operation in the simultaneous presence of Zeeman inhomogeneity for quantum
dot trapped electron spins and the finite-time ramp-up of exchange coupling in
a double dot. We first present a geometric explanation of spin swap operation,
mapping the two-qubit operation onto a single-qubit rotation. We then show that
in this geometric picture a square-pulse-sequence can be easily designed to
perform swap in the presence of Zeeman inhomogeneity. Finally, we investigate
how finite ramp-up times for the exchange coupling negatively affect the
performance of the swap gate sequence, and show how to correct the problems
numerically.Comment: published versio
Dimensional Crossover in the Effective Second Harmonic Generation of Films of Random Dielectrics
The effective nonlinear response of films of random composites consisting of
a binary composite with nonlinear particles randomly embedded in a linear host
is theoretically and numerically studied. A theoretical expression for the
effective second harmonic generation susceptibility, incorporating the
thickness of the film, is obtained by combining a modified effective-medium
approximation with the general expression for the effective second harmonic
generation susceptibility in a composite. The validity of the thoretical
results is tested against results obtained by numerical simulations on random
resistor networks. Numerical results are found to be well described by our
theory. The result implies that the effective-medium approximation provides a
convenient way for the estimation of the nonlinear response in films of random
dielectrics.Comment: 9 pages, 2 figures; accepted for publication in Phys. Rev.
Large Magnetoresistance Ratio in Ferromagnetic Single-Electron Transistors in the Strong Tunneling Regime
We study transport through a ferromagnetic single-electron transistor. The
resistance is represented as a path integral, so that systems where the tunnel
resistances are smaller than the quantum resistance can be investigated. Beyond
the low order sequential tunneling and co-tunneling regimes, a large
magnetoresistance ratio at sufficiently low temperatures is found. In the
opposite limit, when the thermal energy is larger than the charging energy, the
magnetoresistance ratio is only slightly enhanced.Comment: updated versio
Survivin as a therapeutic target in Sonic hedgehog-driven medulloblastoma.
Medulloblastoma (MB) is a highly malignant brain tumor that occurs primarily in children. Although surgery, radiation and high-dose chemotherapy have led to increased survival, many MB patients still die from their disease, and patients who survive suffer severe long-term side effects as a consequence of treatment. Thus, more effective and less toxic therapies for MB are critically important. Development of such therapies depends in part on identification of genes that are necessary for growth and survival of tumor cells. Survivin is an inhibitor of apoptosis protein that regulates cell cycle progression and resistance to apoptosis, is frequently expressed in human MB and when expressed at high levels predicts poor clinical outcome. Therefore, we hypothesized that Survivin may have a critical role in growth and survival of MB cells and that targeting it may enhance MB therapy. Here we show that Survivin is overexpressed in tumors from patched (Ptch) mutant mice, a model of Sonic hedgehog (SHH)-driven MB. Genetic deletion of survivin in Ptch mutant tumor cells significantly inhibits proliferation and causes cell cycle arrest. Treatment with small-molecule antagonists of Survivin impairs proliferation and survival of both murine and human MB cells. Finally, Survivin antagonists impede growth of MB cells in vivo. These studies highlight the importance of Survivin in SHH-driven MB, and suggest that it may represent a novel therapeutic target in patients with this disease
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