21,183 research outputs found
Microscopic Theory of Protein Folding Rates.I: Fine Structure of the Free Energy Profile and Folding Routes from a Variational Approach
A microscopic theory of the free energy barriers and folding routes for
minimally frustrated proteins is presented, greatly expanding on the
presentation of the variational approach outlined previously [J. J. Portman, S.
Takada, P. G. Wolynes, Phys. Rev. Lett. {\bf 81}, 5237 (1998)]. We choose the
-repressor protein as an illustrative example and focus on how the
polymer chain statistics influence free energy profiles and partially ordered
ensembles of structures. In particular, we investigate the role of chain
stiffness on the free energy profile and folding routes. We evaluate the
applicability of simpler approximations in which the conformations of the
protein molecule along the folding route are restricted to have residues that
are either entirely folded or unfolded in contiguous stretches. We find that
the folding routes obtained from only one contiguous folded region corresponds
to a chain with a much greater persistence length than appropriate for natural
protein chains, while the folding route obtained from two contiguous folded
regions is able to capture the relatively folded regions calculated within the
variational approach. The free energy profiles obtained from the contiguous
sequence approximations have larger barriers than the more microscopic
variational theory which is understood as a consequence of partial ordering.Comment: 16 pages, 11 figure
High-Accuracy Calculations of the Critical Exponents of Dyson's Hierarchical Model
We calculate the critical exponent gamma of Dyson's hierarchical model by
direct fits of the zero momentum two-point function, calculated with an Ising
and a Landau-Ginzburg measure, and by linearization about the Koch-Wittwer
fixed point. We find gamma= 1.299140730159 plus or minus 10^(-12). We extract
three types of subleading corrections (in other words, a parametrization of the
way the two-point function depends on the cutoff) from the fits and check the
value of the first subleading exponent from the linearized procedure. We
suggest that all the non-universal quantities entering the subleading
corrections can be calculated systematically from the non-linear contributions
about the fixed point and that this procedure would provide an alternative way
to introduce the bare parameters in a field theory model.Comment: 15 pages, 9 figures, uses revte
Quantum annealing and the Schr\"odinger-Langevin-Kostin equation
We show, in the context of quantum combinatorial optimization, or quantum
annealing, how the nonlinear Schr\"odinger-Langevin-Kostin equation can
dynamically drive the system toward its ground state. We illustrate, moreover,
how a frictional force of Kostin type can prevent the appearance of genuinely
quantum problems such as Bloch oscillations and Anderson localization which
would hinder an exhaustive search.Comment: 5 pages, 4 figures. To appear on Physical Review
A pseudo-spectral approach to inverse problems in interface dynamics
An improved scheme for computing coupling parameters of the
Kardar-Parisi-Zhang equation from a collection of successive interface
profiles, is presented. The approach hinges on a spectral representation of
this equation. An appropriate discretization based on a Fourier representation,
is discussed as a by-product of the above scheme. Our method is first tested on
profiles generated by a one-dimensional Kardar-Parisi-Zhang equation where it
is shown to reproduce the input parameters very accurately. When applied to
microscopic models of growth, it provides the values of the coupling parameters
associated with the corresponding continuum equations. This technique favorably
compares with previous methods based on real space schemes.Comment: 12 pages, 9 figures, revtex 3.0 with epsf style, to appear in Phys.
Rev.
The double mass hierarchy pattern: simultaneously understanding quark and lepton mixing
The charged fermion masses of the three generations exhibit the two strong
hierarchies m_3 >> m_2 >> m_1. We assume that also neutrino masses satisfy
m_{nu 3} > m_{nu 2} > m_{nu 1} and derive the consequences of the hierarchical
spectra on the fermionic mixing patterns. The quark and lepton mixing matrices
are built in a general framework with their matrix elements expressed in terms
of the four fermion mass ratios m_u/m_c, m_c/m_t, m_d/m_s, and m_s/m_b and
m_e/m_mu, m_mu/m_tau, m_{nu 1}/m_{nu 2}, and m_{nu 2}/m_{nu 3}, for the quark
and lepton sector, respectively. In this framework, we show that the resulting
mixing matrices are consistent with data for both quarks and leptons, despite
the large leptonic mixing angles. The minimal assumption we take is the one of
hierarchical masses and minimal flavour symmetry breaking that strongly follows
from phenomenology. No special structure of the mass matrices has to be assumed
that cannot be motivated by this minimal assumption. This analysis allows us to
predict the neutrino mass spectrum and set the mass of the lightest neutrino
well below 0.01 eV. The method also gives the 1 sigma allowed ranges for the
leptonic mixing matrix elements. Contrary to the common expectation, leptonic
mixing angles are found to be determined solely by the four leptonic mass
ratios without any relation to symmetry considerations as commonly used in
flavor model building. Still, our formulae can be used to build up a flavor
model that predicts the observed hierarchies in the masses---the mixing follows
then from the procedure which is developed in this work.Comment: 28 pages, 3 figures, 4 tables; v2: references added, Appendix C
added, additional clarification and explanations in Sec. 2; matches version
accepted by Nucl. Phys.
Quantum Cognition based on an Ambiguous Representation Derived from a Rough Set Approximation
Over the last years, in a series papers by Arrechi and others, a model for
the cognitive processes involved in decision making has been proposed and
investigated. The key element of this model is the expression of apprehension
and judgement, basic cognitive process of decision making, as an inverse Bayes
inference classifying the information content of neuron spike trains. For
successive plural stimuli, it has been shown that this inference, equipped with
basic non-algorithmic jumps, is affected by quantum-like characteristics. We
show here that such a decision making process is related consistently with
ambiguous representation by an observer within a universe of discourse. In our
work ambiguous representation of an object or a stimuli is defined by a pair of
maps from objects of a set to their representations, where these two maps are
interrelated in a particular structure. The a priori and a posteriori
hypotheses in Bayes inference are replaced by the upper and lower
approximation, correspondingly, for the initial data sets each derived with
respect to a map. We show further that due to the particular structural
relation between the two maps, the logical structure of such combined
approximations can only be expressed as an orthomodular lattice and therefore
can be represented by a quantum rather than a Boolean logic. To our knowledge,
this is the first investigation aiming to reveal the concrete logic structure
of inverse Bayes inference in cognitive processes.Comment: 23 pages, 8 figures, original research pape
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