929 research outputs found
Biological activities of solubilized surface antigens of embryonic and polyoma-virus-transformed cells.
Various antigenic activities of polyoma virus-transformed and embryonic mouse cells were retained after 3M KCl solubilization of surface components. Particularly, transplantation antigen (TSTA) demonstrated by homograft rejection, and surface (S) antigen, detected by inhibition of immunofluorescence on polyoma-virus-transformed mouse cells, could be demonstrated. The crude soluble extracts were partially purified by salting out with (NH4)2SO4. In the case of polyoma-virus-transformed cells, TSTA and a part of S antigen activity were found in the same fraction (60% (NH4)2SO4 saturation) while another part of S antigen was salted out at 80% saturation. By chromatography, S antigen activity was found in 2 zones for transformed cells and in one zone for embryonic cells. One of these zones was common to both cell extracts
A linear Stark shift in dressed atoms as a signal to measure a nuclear anapole moment with a cold atom fountain or interferometer
We demonstrate theoretically the existence of a linear dc Stark shift of the
individual substates of an alkali atom in its ground state, dressed by a
circularly polarized laser field. It arises from the electroweak nuclear
anapole moment violating P but not T. It is characterized by the pseudoscalar
equal to the mixed product formed with the photon angular momentum and static
electric and magnetic fields. We derive the relevant left-right asymmetry with
its complete signature in a field configuration selected for a precision
measurement with cold atom beams. The 3,3 to 4,3 Cs hyperfine-transition
frequency shift amounts to 7 Hz for a laser power of about 1 kW at 877 nm,
E=100 kV/cm and B larger than 0.5 G.Comment: Article, 4 pages, 2 figure
CENP-A Is Dispensable for Mitotic Centromere Function after Initial Centromere/Kinetochore Assembly
Human centromeres are defined by chromatin containing the histone H3 variant CENP-A assembled onto repetitive alphoid DNA sequences. By inducing rapid, complete degradation of endogenous CENP-A, we now demonstrate that once the first steps of centromere assembly have been completed in G1/S, continued CENP-A binding is not required for maintaining kinetochore attachment to centromeres or for centromere function in the next mitosis. Degradation of CENP-A prior to kinetochore assembly is found to block deposition of CENP-C and CENP-N, but not CENP-T, thereby producing defective kinetochores and failure of chromosome segregation. Without the continuing presence of CENP-A, CENP-B binding to alphoid DNA sequences becomes essential to preserve anchoring of CENP-C and the kinetochore to each centromere. Thus, there is a reciprocal interdependency of CENP-A chromatin and the underlying repetitive centromere DNA sequences bound by CENP-B in the maintenance of human chromosome segregation
How glassy are neural networks?
In this paper we continue our investigation on the high storage regime of a
neural network with Gaussian patterns. Through an exact mapping between its
partition function and one of a bipartite spin glass (whose parties consist of
Ising and Gaussian spins respectively), we give a complete control of the whole
annealed region. The strategy explored is based on an interpolation between the
bipartite system and two independent spin glasses built respectively by
dichotomic and Gaussian spins: Critical line, behavior of the principal
thermodynamic observables and their fluctuations as well as overlap
fluctuations are obtained and discussed. Then, we move further, extending such
an equivalence beyond the critical line, to explore the broken ergodicity phase
under the assumption of replica symmetry and we show that the quenched free
energy of this (analogical) Hopfield model can be described as a linear
combination of the two quenched spin-glass free energies even in the replica
symmetric framework
Mean-field cooperativity in chemical kinetics
We consider cooperative reactions and we study the effects of the interaction
strength among the system components on the reaction rate, hence realizing a
connection between microscopic and macroscopic observables. Our approach is
based on statistical mechanics models and it is developed analytically via
mean-field techniques. First of all, we show that, when the coupling strength
is set positive, the model is able to consistently recover all the various
cooperative measures previously introduced, hence obtaining a single unifying
framework. Furthermore, we introduce a criterion to discriminate between weak
and strong cooperativity, based on a measure of "susceptibility". We also
properly extend the model in order to account for multiple attachments
phenomena: this is realized by incorporating within the model -body
interactions, whose non-trivial cooperative capability is investigated too.Comment: 25 pages, 4 figure
Phosphorylation of CENP-A on serine 7 does not control centromere function
CENP-A is the histone H3 variant necessary to specify the location of all eukaryotic centromeres via its CENP-A targeting domain and either one of its terminal regions. In humans, several post-translational modifications occur on CENP-A, but their role in centromere function remains controversial. One of these modifications of CENP-A, phosphorylation on serine 7, has been proposed to control centromere assembly and function. Here, using gene targeting at both endogenous CENP-A alleles and gene replacement in human cells, we demonstrate that a CENP-A variant that cannot be phosphorylated at serine 7 maintains correct CENP-C recruitment, faithful chromosome segregation and long-term cell viability. Thus, we conclude that phosphorylation of CENP-A on serine 7 is dispensable to maintain correct centromere dynamics and function
Characterization of the S = 9 excited state in Fe8Br8 by Electron Paramagnetic Resonance
High Frequency electron paramagnetic resonance has been used to observe the
magnetic dipole, M = 1, transitions in the excited
state of the single molecule magnet FeBr. A Boltzmann analysis of the
measured intensities locates it at 24 2 K above the ground
state, while the line positions yield its magnetic parameters D = -0.27 K, E =
0.05 K, and B = -1.3 10 K. D is thus smaller by 8%
and E larger by 7% than for . The anisotropy barrier for is
estimated as 22 K,which is 25% smaller than that for (29 K). These
data also help assign the spin exchange constants(J's) and thus provide a basis
for improved electronic structure calculations of FeBr.Comment: 7 pages, Figs included in text, submitted to PR
Properties of low-lying states in some high-nuclearity Mn, Fe and V clusters: Exact studies of Heisenberg models
Using an efficient numerical scheme that exploits spatial symmetries and spin
parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for the high nuclearity spin clusters, Mn_{12}, Fe_8 and V_{15}.
The largest calculation involves the Mn_{12} cluster which spans a Fock space
of a hundred million. Our results show that the earlier estimates of the
exchange constants need to be revised for the Mn_{12} cluster to explain the
level ordering of low-lying eigenstates. In the case of the Fe_8 cluster,
correct level ordering can be obtained which is consistent with the exchange
constants for the already known clusters with butterfly structure. In the
V_{15} cluster, we obtain an effective Hamiltonian that reproduces exactly, the
eight low-lying eigenvalues of the full Hamiltonian.Comment: Revtex, 12 pages, 16 eps figures; this is the final published versio
Optimizing picene molecular assembling by supersonic molecular beam deposition
Here we report an investigation of the growth of picene by supersonic molecular beam deposition on thermal silicon oxide and on a self-assembled monolayer of hexamethyldisiloxane (HMDS). In both cases film morphology shows a structure with very sharp island edges and well-separated islands which size and height depend on the deposition conditions. Picene films growth on bare silicon covered with hydrophobic HDMS shows islands characterized by large regular crystallites of several micrometers; on the other hand, films growth on silicon oxide shows smaller and thicker islands. We analyzed the details of the growth model and describe it as a balancing mechanism involving the weak interaction between molecules and surface and the strong picene-picene interaction that leads to a different Schwoebel-Ehrlich barrier in the first layer with respect to the successive one. Finally, we study the charge transport properties of these films by fabricating field-effect transistors devices in both top and bottom contact configuration. We notice that substrate influences the electrical properties of the device and we obtained a maximum mobility value of 1.2 cm2 V-1 s-1 measured on top contact devices in air. © 2012 American Chemical Society
Exactly solvable model of quantum diffusion
We study the transport property of diffusion in a finite translationally
invariant quantum subsystem described by a tight-binding Hamiltonian with a
single energy band and interacting with its environment by a coupling in terms
of correlation functions which are delta-correlated in space and time. For weak
coupling, the time evolution of the subsystem density matrix is ruled by a
quantum master equation of Lindblad type. Thanks to the invariance under
spatial translations, we can apply the Bloch theorem to the subsystem density
matrix and exactly diagonalize the time evolution superoperator to obtain the
complete spectrum of its eigenvalues, which fully describe the relaxation to
equilibrium. Above a critical coupling which is inversely proportional to the
size of the subsystem, the spectrum at given wavenumber contains an isolated
eigenvalue describing diffusion. The other eigenvalues rule the decay of the
populations and quantum coherences with decay rates which are proportional to
the intensity of the environmental noise. On the other hand, an analytical
expression is obtained for the dispersion relation of diffusion. The diffusion
coefficient is proportional to the square of the width of the energy band and
inversely proportional to the intensity of the environmental noise because
diffusion results from the perturbation of quantum tunneling by the
environmental fluctuations in this model. Diffusion disappears below the
critical coupling.Comment: Submitted to J. Stat. Phy
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