1,385 research outputs found
Topology and energy transport in networks of interacting photosynthetic complexes
We address the role of topology in the energy transport process that occurs
in networks of photosynthetic complexes. We take inspiration from light
harvesting networks present in purple bacteria and simulate an incoherent
dissipative energy transport process on more general and abstract networks,
considering both regular structures (Cayley trees and hyperbranched fractals)
and randomly-generated ones. We focus on the the two primary light harvesting
complexes of purple bacteria, i.e., the LH1 and LH2, and we use
network-theoretical centrality measures in order to select different LH1
arrangements. We show that different choices cause significant differences in
the transport efficiencies, and that for regular networks centrality measures
allow to identify arrangements that ensure transport efficiencies which are
better than those obtained with a random disposition of the complexes. The
optimal arrangements strongly depend on the dissipative nature of the dynamics
and on the topological properties of the networks considered, and depending on
the latter they are achieved by using global vs. local centrality measures. For
randomly-generated networks a random arrangement of the complexes already
provides efficient transport, and this suggests the process is strong with
respect to limited amount of control in the structure design and to the
disorder inherent in the construction of randomly-assembled structures.
Finally, we compare the networks considered with the real biological networks
and find that the latter have in general better performances, due to their
higher connectivity, but the former with optimal arrangements can mimic the
real networks' behaviour for a specific range of transport parameters. These
results show that the use of network-theoretical concepts can be crucial for
the characterization and design of efficient artificial energy transport
networks.Comment: 14 pages, 16 figures, revised versio
Structure and dynamics of the E. coli chemotaxis core signaling complex by cryo-electron tomography and molecular simulations
To enable the processing of chemical gradients, chemotactic bacteria possess large arrays of transmembrane chemoreceptors, the histidine kinase CheA, and the adaptor protein CheW, organized as coupled core-signaling units (CSU). Despite decades of study, important questions surrounding the molecular mechanisms of sensory signal transduction remain unresolved, owing especially to the lack of a high-resolution CSU structure. Here, we use cryo-electron tomography and sub-tomogram averaging to determine a structure of the Escherichia coli CSU at sub-nanometer resolution. Based on our experimental data, we use molecular simulations to construct an atomistic model of the CSU, enabling a detailed characterization of CheA conformational dynamics in its native structural context. We identify multiple, distinct conformations of the critical P4 domain as well as asymmetries in the localization of the P3 bundle, offering several novel insights into the CheA signaling mechanism
Exact and asymptotic computations of elementary spin networks: classification of the quantum-classical boundaries
Increasing interest is being dedicated in the last few years to the issues of
exact computations and asymptotics of spin networks. The large-entries regimes
(semiclassical limits) occur in many areas of physics and chemistry, and in
particular in discretization algorithms of applied quantum mechanics. Here we
extend recent work on the basic building block of spin networks, namely the
Wigner 6j symbol or Racah coefficient, enlightening the insight gained by
exploiting its self-dual properties and studying it as a function of two
(discrete) variables. This arises from its original definition as an
(orthogonal) angular momentum recoupling matrix. Progress also derives from
recognizing its role in the foundation of the modern theory of classical
orthogonal polynomials, as extended to include discrete variables. Features of
the imaging of various regimes of these orthonormal matrices are made explicit
by computational advances -based on traditional and new recurrence relations-
which allow an interpretation of the observed behaviors in terms of an
underlying Hamiltonian formulation as well. This paper provides a contribution
to the understanding of the transition between two extreme modes of the 6j,
corresponding to the nearly classical and the fully quantum regimes, by
studying the boundary lines (caustics) in the plane of the two matrix labels.
This analysis marks the evolution of the turning points of relevance for the
semiclassical regimes and puts on stage an unexpected key role of the Regge
symmetries of the 6j.Comment: 15 pages, 11 figures. Talk presented at ICCSA 2012 (12th
International Conference on Computational Science and Applications, Salvador
de Bahia (Brazil) June 18-21, 2012
Imprint of Reionization on the Cosmic Microwave Background Bispectrum
We study contributions to the cosmic microwave background (CMB) bispectrum
from non-Gaussianity induced by secondary anisotropies during reionization.
Large-scale structure in the reionized epoch both gravitational lenses CMB
photons and produces Doppler shifts in their temperature from scattering off
electrons in infall. The resulting correlation is potentially observable
through the CMB bispectrum. The second-order Ostriker-Vishniac also couples to
a variety of linear secondary effects to produce a bispectrum. For the
currently favored flat cosmological model with a low matter content and small
optical depth in the reionized epoch \tau \la 0.3, however, these bispectrum
contributions are well below the detection threshold of MAP and at or below
that of Planck, given their cosmic and noise variance limitations. At the upper
end of this range, they can serve as an extra source of noise for measurements
with Planck of either primordial nongaussianity or that induced by the
correlation of gravitational lensing with the integrated Sachs-Wolfe and the
thermal Sunyaev-Zel'dovich effects. We include a discussion of the general
properties of the CMB bispectrum, its configuration dependence for the various
effects, and its computation in the Limber approximation and beyond.Comment: 17 pages, 10 figures (with emulateapj.sty); submitted to Ap
Symmetric angular momentum coupling, the quantum volume operator and the 7-spin network: a computational perspective
A unified vision of the symmetric coupling of angular momenta and of the
quantum mechanical volume operator is illustrated. The focus is on the quantum
mechanical angular momentum theory of Wigner's 6j symbols and on the volume
operator of the symmetric coupling in spin network approaches: here, crucial to
our presentation are an appreciation of the role of the Racah sum rule and the
simplification arising from the use of Regge symmetry. The projective geometry
approach permits the introduction of a symmetric representation of a network of
seven spins or angular momenta. Results of extensive computational
investigations are summarized, presented and briefly discussed.Comment: 15 pages, 10 figures, presented at ICCSA 2014, 14th International
Conference on Computational Science and Application
Implementation of self-organizing neural networks for visuo-motorcontrol of an industrial robot
Walter JA, Schulten K. Implementation of self-organizing neural networks for visuo-motorcontrol of an industrial robot. IEEE Transactions on Neural Networks. 1993;4(1):86-96.The implementation of two neural network algorithms for visuo-motor control of an industrial robot (Puma 562) is reported. The first algorithm uses a vector quantization technique, the "neural-gas" network, together with an error correction scheme based on a Widrow-Hoff-type learning rule. The second algorithm employs an extended self-organizing feature map algorithm. Based on visual information provided by two cameras, the robot learns to position its end effector without an external teacher. Within only 3000 training steps, the robot-camera system is capable of reducing the positioning error of the robot's end effector to approximately 0.1% of the linear dimension of the work space. By employing adaptive feedback the robot succeeds in compensating not only slow calibration drifts, but also sudden changes in its geometry. Hardware aspects of the robot-camera system are discusse
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