2,716 research outputs found
Enzyme kinetics for a two-step enzymic reaction with comparable initial enzyme-substrate ratios
We extend the validity of the quasi-steady state assumption for a model double intermediate enzyme-substrate reaction to include the case where the ratio of initial enzyme to substrate concentration is not necessarily small. Simple analytical solutions are obtained when the reaction rates and the initial substrate concentration satisfy a certain condition. These analytical solutions compare favourably with numerical solutions of the full system of differential equations describing the reaction. Experimental methods are suggested which might permit the application of the quasi-steady state assumption to reactions where it may not have been obviously applicable before
Scale-Free topologies and Activatory-Inhibitory interactions
A simple model of activatory-inhibitory interactions controlling the activity
of agents (substrates) through a "saturated response" dynamical rule in a
scale-free network is thoroughly studied. After discussing the most remarkable
dynamical features of the model, namely fragmentation and multistability, we
present a characterization of the temporal (periodic and chaotic) fluctuations
of the quasi-stasis asymptotic states of network activity. The double (both
structural and dynamical) source of entangled complexity of the system temporal
fluctuations, as an important partial aspect of the Correlation
Structure-Function problem, is further discussed to the light of the numerical
results, with a view on potential applications of these general results.Comment: Revtex style, 12 pages and 12 figures. Enlarged manuscript with major
revision and new results incorporated. To appear in Chaos (2006
Quasiparticle Chirality in Epitaxial Graphene Probed at the Nanometer Scale
Graphene exhibits unconventional two-dimensional electronic properties
resulting from the symmetry of its quasiparticles, which leads to the concepts
of pseudospin and electronic chirality. Here we report that scanning tunneling
microscopy can be used to probe these unique symmetry properties at the
nanometer scale. They are reflected in the quantum interference pattern
resulting from elastic scattering off impurities, and they can be directly read
from its fast Fourier transform. Our data, complemented by theoretical
calculations, demonstrate that the pseudospin and the electronic chirality in
epitaxial graphene on SiC(0001) correspond to the ones predicted for ideal
graphene.Comment: 4 pages, 3 figures, minor change
Michaelis-Menten dynamics in protein subnetworks
To understand the behaviour of complex systems it is often necessary to use
models that describe the dynamics of subnetworks. It has previously been
established using projection methods that such subnetwork dynamics generically
involves memory of the past, and that the memory functions can be calculated
explicitly for biochemical reaction networks made up of unary and binary
reactions. However, many established network models involve also
Michaelis-Menten kinetics, to describe e.g. enzymatic reactions. We show that
the projection approach to subnetwork dynamics can be extended to such
networks, thus significantly broadening its range of applicability. To derive
the extension we construct a larger network that represents enzymes and enzyme
complexes explicitly, obtain the projected equations, and finally take the
limit of fast enzyme reactions that gives back Michaelis-Menten kinetics. The
crucial point is that this limit can be taken in closed form. The outcome is a
simple procedure that allows one to obtain a description of subnetwork
dynamics, including memory functions, starting directly from any given network
of unary, binary and Michaelis-Menten reactions. Numerical tests show that this
closed form enzyme elimination gives a much more accurate description of the
subnetwork dynamics than the simpler method that represents enzymes explicitly,
and is also more efficient computationally
Dynamics of gene expression and the regulatory inference problem
From the response to external stimuli to cell division and death, the
dynamics of living cells is based on the expression of specific genes at
specific times. The decision when to express a gene is implemented by the
binding and unbinding of transcription factor molecules to regulatory DNA.
Here, we construct stochastic models of gene expression dynamics and test them
on experimental time-series data of messenger-RNA concentrations. The models
are used to infer biophysical parameters of gene transcription, including the
statistics of transcription factor-DNA binding and the target genes controlled
by a given transcription factor.Comment: revised version to appear in Europhys. Lett., new titl
Stub model for dephasing in a quantum dot
As an alternative to Buttiker's dephasing lead model, we examine a dephasing
stub. Both models are phenomenological ways to introduce decoherence in chaotic
scattering by a quantum dot. The difference is that the dephasing lead opens up
the quantum dot by connecting it to an electron reservoir, while the dephasing
stub is closed at one end. Voltage fluctuations in the stub take over the
dephasing role from the reservoir. Because the quantum dot with dephasing lead
is an open system, only expectation values of the current can be forced to
vanish at low frequencies, while the outcome of an individual measurement is
not so constrained. The quantum dot with dephasing stub, in contrast, remains a
closed system with a vanishing low-frequency current at each and every
measurement. This difference is a crucial one in the context of quantum
algorithms, which are based on the outcome of individual measurements rather
than on expectation values. We demonstrate that the dephasing stub model has a
parameter range in which the voltage fluctuations are sufficiently strong to
suppress quantum interference effects, while still being sufficiently weak that
classical current fluctuations can be neglected relative to the nonequilibrium
shot noise.Comment: 8 pages with 1 figure; contribution for the special issue of J.Phys.A
on "Trends in Quantum Chaotic Scattering
Computation of saddle type slow manifolds using iterative methods
This paper presents an alternative approach for the computation of trajectory
segments on slow manifolds of saddle type. This approach is based on iterative
methods rather than collocation-type methods. Compared to collocation methods,
that require mesh refinements to ensure uniform convergence with respect to
, appropriate estimates are directly attainable using the method of
this paper. The method is applied to several examples including: A model for a
pair of neurons coupled by reciprocal inhibition with two slow and two fast
variables and to the computation of homoclinic connections in the
FitzHugh-Nagumo system.Comment: To appear in SIAM Journal of Applied Dynamical System
Compatibility of glass-guided recording microelectrodes in the brain stem of squirrel monkeys with high-resolution 3D MRI
Multiannual Seafloor Dynamics around a Subtidal Rocky Reef Habitat in the North Sea
Rocky reefs provide complex structures in the otherwise largely sand-dominated coastal North Sea. Therefore, these reefs are highly important natural habitats for the functioning of coastal ecosystems, as they provide shelter, refuge and nursery grounds for various mobile and sessile species. In the North Sea, the spatial distribution of these habitats has been intensively investigated over recent years. However, these studies generally provide static accounts of the current state of these reef systems, but limited data exist on the temporal variations in sediment dynamics at and around natural rocky reefs. In this study, we provide observations from a multiannual time series of hydroacoustic seafloor surveys conducted at an isolated rocky reef in the North Sea. We use multibeam bathymetry and side-scan sonar backscatter data in combination with video observations, sediment sampling, and sub-bottom profiler data to assess the long-term variations of the rocky reef system. The reef is located in water depths between 11 and 17 m with an areal extent of ~0.5 km2 and is surrounded by mobile sands. The topography of the rocky reef appears to create a distinct hydrodynamic system that permits mobile sands to settle or move into bathymetrical deeper parts of the reef. Our results suggest a very dynamic system surrounding the reef with large scale scouring, sediment reworking and transport, while the shallower central part of the reef remains stable over time. We demonstrate the importance of hydrodynamics and current scouring around reefs for the local variability in seafloor properties over time. These small-scale dynamics are likewise reflected in the spatial distribution of sessile species, which are less abundant in proximity to mobile sands. The hydroacoustic mapping and monitoring of seafloor dynamics at higher spatial and temporal resolutions presents an important future direction in the study of valuable coastal habitats
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