352 research outputs found
Effective adhesion strength of specifically bound vesicles
A theoretical approach has been undertaken in order to model the
thermodynamic equilibrium of a vesicle adhering to a flat substrate. The
vesicle is treated in a canonical description with a fixed number of sites. A
finite number of these sites are occupied by mobile ligands that are capable of
interacting with a discrete number of receptors immobilized on the substrate.
Explicit consideration of the bending energy of the vesicle shape has shown
that the problem of the vesicle shape can be decoupled from the determination
of the optimum allocation of ligands over the vesicle. The allocation of bound
and free ligands in the vesicle could be determined as a function of the size
of the contact zone, the ligand-receptor binding strength and the concentration
of the system constituents. Several approximate solutions for different regions
of system parameters are determined and in particular, the distinction between
receptor-dominated equilibria and ligand-dominated equilibria is found to be
important. The crossover between these two types of solutions is found to occur
at a critical size of the contact zone. The presented approach enables the
calculation of the effective adhesion strength of the vesicle and thus permits
meaningful comparisons with relevant experiments as well as connecting the
presented model with the proven success of the continuum approach for modeling
the shapes of adhering vesicles. The behavior of the effective adhesion
strength is analyzed in detail and several approximate expressions for it are
given.Comment: 19 pages, 6 figures. To appear in Phys. Rev.
Dynamic force spectroscopy on multiple bonds: experiments and model
We probe the dynamic strength of multiple biotin-streptavidin adhesion bonds
under linear loading using the biomembrane force probe setup for dynamic force
spectroscopy. Measured rupture force histograms are compared to results from a
master equation model for the stochastic dynamics of bond rupture under load.
This allows us to extract the distribution of the number of initially closed
bonds. We also extract the molecular parameters of the adhesion bonds, in good
agreement with earlier results from single bond experiments. Our analysis shows
that the peaks in the measured histograms are not simple multiples of the
single bond values, but follow from a superposition procedure which generates
different peak positions.Comment: to appear in Europhysics Letter
Influence of shear flow on vesicles near a wall: a numerical study
We describe the dynamics of three-dimensional fluid vesicles in steady shear
flow in the vicinity of a wall. This is analyzed numerically at low Reynolds
numbers using a boundary element method. The area-incompressible vesicle
exhibits bending elasticity. Forces due to adhesion or gravity oppose the
hydrodynamic lift force driving the vesicle away from a wall. We investigate
three cases. First, a neutrally buoyant vesicle is placed in the vicinity of a
wall which acts only as a geometrical constraint. We find that the lift
velocity is linearly proportional to shear rate and decreases with increasing
distance between the vesicle and the wall. Second, with a vesicle filled with a
denser fluid, we find a stationary hovering state. We present an estimate of
the viscous lift force which seems to agree with recent experiments of Lorz et
al. [Europhys. Lett., vol. 51, 468 (2000)]. Third, if the wall exerts an
additional adhesive force, we investigate the dynamical unbinding transition
which occurs at an adhesion strength linearly proportional to the shear rate.Comment: 17 pages (incl. 10 figures), RevTeX (figures in PostScript
Optoacoustic solitons in Bragg gratings
Optical gap solitons, which exist due to a balance of nonlinearity and
dispersion due to a Bragg grating, can couple to acoustic waves through
electrostriction. This gives rise to a new species of ``gap-acoustic'' solitons
(GASs), for which we find exact analytic solutions. The GAS consists of an
optical pulse similar to the optical gap soliton, dressed by an accompanying
phonon pulse. Close to the speed of sound, the phonon component is large. In
subsonic (supersonic) solitons, the phonon pulse is a positive (negative)
density variation. Coupling to the acoustic field damps the solitons'
oscillatory instability, and gives rise to a distinct instability for
supersonic solitons, which may make the GAS decelerate and change direction,
ultimately making the soliton subsonic.Comment: 5 pages, 3 figure
Rupture of multiple parallel molecular bonds under dynamic loading
Biological adhesion often involves several pairs of specific receptor-ligand
molecules. Using rate equations, we study theoretically the rupture of such
multiple parallel bonds under dynamic loading assisted by thermal activation.
For a simple generic type of cooperativity, both the rupture time and force
exhibit several different scaling regimes. The dependence of the rupture force
on the number of bonds is predicted to be either linear, like a square root or
logarithmic.Comment: 8 pages, 2 figure
Ligand-Receptor Interactions
The formation and dissociation of specific noncovalent interactions between a
variety of macromolecules play a crucial role in the function of biological
systems. During the last few years, three main lines of research led to a
dramatic improvement of our understanding of these important phenomena. First,
combination of genetic engineering and X ray cristallography made available a
simultaneous knowledg of the precise structure and affinity of series or
related ligand-receptor systems differing by a few well-defined atoms. Second,
improvement of computer power and simulation techniques allowed extended
exploration of the interaction of realistic macromolecules. Third, simultaneous
development of a variety of techniques based on atomic force microscopy,
hydrodynamic flow, biomembrane probes, optical tweezers, magnetic fields or
flexible transducers yielded direct experimental information of the behavior of
single ligand receptor bonds. At the same time, investigation of well defined
cellular models raised the interest of biologists to the kinetic and mechanical
properties of cell membrane receptors. The aim of this review is to give a
description of these advances that benefitted from a largely multidisciplinar
approach
Measurement of double beta decay of ¹⁰⁰Mo to excited states in the NEMO 3 experiment
The double beta decay of ¹⁰⁰Mo to the 0_{1}^{+} and 2_{1}^{+} excited states of ¹⁰⁰Ru is studied using the NEMO 3 data. After the analysis of 8024 h of data the half-life for the two-neutrino double beta decay of ¹⁰⁰Mo to the excited 0_{1}^{+} state is measured to be T_{1/2}^{2v} = [5.7_{-0.9}^{+1.3} (stat.) ± 0.8 (syst.)] x 10²⁰ y. The signal-to-background ratio is equal to 3. Information about energy and angular distributions of emitted electrons is also obtained. No evidence for neutrinoless double beta decay to the excited 0_{1}^{+} state has been found. The corresponding half-life limit is T_{1/2}^{0v} (0⁺→0_{1}^{+}) > 8.9 x 10²² y (at 90% C.L.). The search for the double beta decay to the 2_{1}^{+} excited state has allowed the determination of limits on the half-life for the two neutrino mode T_{1/2}^{0v} (0⁺→2_{1}^{+}) > 1.1 x 10²¹ y (at 90% C.L.) and for the neutrinoless mode T_{1/2}^{0v} (0⁺→2_{1}^{+}) > 1.6 x 10²³ y (at 90% C.L.)
Results of the BiPo-1 prototype for radiopurity measurements for the SuperNEMO double beta decay source foils
The development of BiPo detectors is dedicated to the measurement of
extremely high radiopurity in Tl and Bi for the SuperNEMO
double beta decay source foils. A modular prototype, called BiPo-1, with 0.8
of sensitive surface area, has been running in the Modane Underground
Laboratory since February, 2008. The goal of BiPo-1 is to measure the different
components of the background and in particular the surface radiopurity of the
plastic scintillators that make up the detector. The first phase of data
collection has been dedicated to the measurement of the radiopurity in
Tl. After more than one year of background measurement, a surface
activity of the scintillators of (Tl) 1.5
Bq/m is reported here. Given this level of background, a larger BiPo
detector having 12 m of active surface area, is able to qualify the
radiopurity of the SuperNEMO selenium double beta decay foils with the required
sensitivity of (Tl) 2 Bq/kg (90% C.L.) with a six
month measurement.Comment: 24 pages, submitted to N.I.M.
Probing New Physics Models of Neutrinoless Double Beta Decay with SuperNEMO
The possibility to probe new physics scenarios of light Majorana neutrino
exchange and right-handed currents at the planned next generation neutrinoless
double beta decay experiment SuperNEMO is discussed. Its ability to study
different isotopes and track the outgoing electrons provides the means to
discriminate different underlying mechanisms for the neutrinoless double beta
decay by measuring the decay half-life and the electron angular and energy
distributions.Comment: 17 pages, 14 figures, to be published in E.P.J.
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