2,733 research outputs found
A non-equilibrium dynamic mechanism for the allosteric effect
Allosteric regulation is often viewed as thermodynamic in nature. However
protein internal motions during an enzymatic reaction cycle can be slow hopping
processes over numerous potential barriers. We propose that regulating
molecules may function by modifying the nonequilibrium protein dynamics. The
theory predicts that an enzyme under the new mechanism has different
temperature dependence, waiting time distribution of the turnover cycle, and
dynamic fluctuation patterns with and without effector. Experimental tests of
the theory are proposed.Comment: accepted by Phys. Rev. Lett. Major revisions were made to fit the
style. 4 pages, 2 figure
Low-field microwave absorption in epitaxial La-Sr-Mn-O films resulting from the angle-tuned ferromagnetic resonance in the multidomain state
We studied magnetic-field induced microwave absorption in 100-200 nm thick
LaSrMnO films on SrTiO substrate and found a
low-field absorption with a very peculiar angular dependence: it appears only
in the oblique field and is absent both in the parallel and in the
perpendicular orientations. We demonstrate that this low-field absorption
results from the ferromagnetic resonance in the multidomain state (domain-mode
resonance). Its unusual angular dependence arises from the interplay between
the parallel component of the magnetic field that drives the film into
multidomain state and the perpendicular field component that controls the
domain width through its effect on domain wall energy. The low-field microwave
absorption in the multidomain state can be a tool to probe domain structure in
magnetic films with in-plane magnetization.Comment: 9 pages, 9 Figure
Automatic generation of generalised regular factorial designs
Open Access for this article was paid for by the French Research Agency (ANR), project Escapade (ANR-12-AGRO-0003).The R package planor enables the user to search for, and construct, factorial designs satisfying given conditions. The user specifies the factors and their numbers of levels, the factorial terms which are assumed to be non-zero, and the subset of those which are to be estimated. Both block and treatment factors can be allowed for, and they may have either fixed or random effects, as well as hierarchy relationships. The designs are generalised regular designs, which means that each one is constructed by using a design key and that the underlying theory is that of finite abelian groups. The main theoretical results and algorithms on which planor is based are developed and illustrated, with the emphasis on mathematical rather than programming details. Sections 3–5 are dedicated to the elementary case, when the numbers of levels of all factors are powers of the same prime. The ineligible factorial terms associated with users’ specifications are defined and it is shown how they can be used to search for a design key by a backtrack algorithm. Then the results are extended to the case when different primes are involved, by making use of the Sylow decomposition of finite abelian groups. The proposed approach provides a unified framework for a wide range of factorial designs.Publisher PDFPeer reviewe
An information-bearing seed for nucleating algorithmic self-assembly
Self-assembly creates natural mineral, chemical, and biological structures of great complexity. Often, the same starting materials have the potential to form an infinite variety of distinct structures; information in a seed molecule can determine which form is grown as well as where and when. These phenomena can be exploited to program the growth of complex supramolecular structures, as demonstrated by the algorithmic self-assembly of DNA tiles. However, the lack of effective seeds has limited the reliability and yield of algorithmic crystals. Here, we present a programmable DNA origami seed that can display up to 32 distinct binding sites and demonstrate the use of seeds to nucleate three types of algorithmic crystals. In the simplest case, the starting materials are a set of tiles that can form crystalline ribbons of any width; the seed directs assembly of a chosen width with >90% yield. Increased structural diversity is obtained by using tiles that copy a binary string from layer to layer; the seed specifies the initial string and triggers growth under near-optimal conditions where the bit copying error rate is 17 kb of sequence information. In sum, this work demonstrates how DNA origami seeds enable the easy, high-yield, low-error-rate growth of algorithmic crystals as a route toward programmable bottom-up fabrication
Spin wave resonances in La_{0.7}Sr_{0.3}MnO_{3} films: measurement of spin wave stiffness and anisotropy field
We studied magnetic field dependent microwave absorption in epitaxial
LaSrMnO films using an X-band Bruker ESR spectrometer. By
analyzing angular and temperature dependence of the ferromagnetic and spin-wave
resonances we determine spin-wave stiffness and anisotropy field. The spin-wave
stiffness as found from the spectrum of the standing spin-wave resonances in
thin films is in fair agreement with the results of inelastic neutron
scattering studies on a single crystal of the same composition [Vasiliu-Doloc
et al., J. Appl. Phys. \textbf{83}, 7343 (1998)].Comment: 15 pages, 7 figures (now figure captions are included
Two Gap State Density in MgB: A True Bulk Property or A Proximity Effect?
We report on the temperature dependence of the quasiparticle density of
states (DOS) in the simple binary compound MgB2 directly measured using
scanning tunneling microscope (STM). To achieve high quality tunneling
conditions, a small crystal of MgB2 is used as a tip in the STM experiment. The
``sample'' is chosen to be a 2H-NbSe2 single crystal presenting an atomically
flat surface. At low temperature the tunneling conductance spectra show a gap
at the Fermi energy followed by two well-pronounced conductance peaks on each
side. They appear at voltages V mV and V mV. With rising temperature both peaks disappear at the Tc of the bulk
MgB2, a behavior consistent with the model of two-gap superconductivity. The
explanation of the double-peak structure in terms of a particular proximity
effect is also discussed.Comment: 4 pages, 3 figure
Nonresonant microwave absorption in epitaxial La-Sr-Mn-O films and its relation to colossal magnetoresistance
We study magnetic-field-dependent nonresonant microwave absorption and
dispersion in thin LaSrMnO films and show that it
originates from the colossal magnetoresistance. We develop the model for
magnetoresistance of a thin ferromagnetic film in oblique magnetic field. The
model accounts fairly well for our experimental findings, as well as for
results of other researchers. We demonstrate that nonresonant microwave
absorption is a powerful technique that allows contactless measurement of
magnetic properties of thin films, including magnetoresistance, anisotropy
field and coercive field.Comment: 20 pages, 11 figure
A core genetic module : the Mixed Feedback Loop
The so-called Mixed Feedback Loop (MFL) is a small two-gene network where
protein A regulates the transcription of protein B and the two proteins form a
heterodimer. It has been found to be statistically over-represented in
statistical analyses of gene and protein interaction databases and to lie at
the core of several computer-generated genetic networks. Here, we propose and
mathematically study a model of the MFL and show that, by itself, it can serve
both as a bistable switch and as a clock (an oscillator) depending on kinetic
parameters. The MFL phase diagram as well as a detailed description of the
nonlinear oscillation regime are presented and some biological examples are
discussed. The results emphasize the role of protein interactions in the
function of genetic modules and the usefulness of modelling RNA dynamics
explicitly.Comment: To be published in Physical Review
Scale-free Networks from Optimal Design
A large number of complex networks, both natural and artificial, share the
presence of highly heterogeneous, scale-free degree distributions. A few
mechanisms for the emergence of such patterns have been suggested, optimization
not being one of them. In this letter we present the first evidence for the
emergence of scaling (and smallworldness) in software architecture graphs from
a well-defined local optimization process. Although the rules that define the
strategies involved in software engineering should lead to a tree-like
structure, the final net is scale-free, perhaps reflecting the presence of
conflicting constraints unavoidable in a multidimensional optimization process.
The consequences for other complex networks are outlined.Comment: 6 pages, 2 figures. Submitted to Europhysics Letters. Additional
material is available at http://complex.upc.es/~sergi/software.ht
- …