Entropic Stochastic Resonance

We present a novel scheme for the appearance of Stochastic Resonance when the dynamics of a Brownian particle takes place in a confined medium. The presence of uneven boundaries, giving rise to an entropic contribution to the potential, may upon application of a periodic driving force result in an increase of the spectral amplification at an optimum value of the ambient noise level. This Entropic Stochastic Resonance (ESR), characteristic of small-scale systems, may constitute a useful mechanism for the manipulation and control of single-molecules and nano-devices.Comment: 4 pages, 3 figure

Unifying thermodynamic and kinetic descriptions of single-molecule processes: RNA unfolding under tension

We use mesoscopic non-equilibrium thermodynamics theory to describe RNA unfolding under tension. The theory introduces reaction coordinates, characterizing a continuum of states for each bond in the molecule. The unfolding considered is so slow that one can assume local equilibrium in the space of the reaction coordinates. In the quasi-stationary limit of high sequential barriers, our theory yields the master equation of a recently proposed sequential-step model. Non-linear switching kinetics is found between open and closed states. Our theory unifies the thermodynamic and kinetic descriptions and offers a systematic procedure to characterize the dynamics of the unfolding processComment: 13 pages, 3 figure

Diffusion and Current of Brownian Particles in Tilted Piecewise Linear Potentials: Amplification and Coherence

Overdamped motion of Brownian particles in tilted piecewise linear periodic potentials is considered. Explicit algebraic expressions for the diffusion coefficient, current, and coherence level of Brownian transport are derived. Their dependencies on temperature, tilting force, and the shape of the potential are analyzed. The necessary and sufficient conditions for the non-monotonic behavior of the diffusion coefficient as a function of temperature are determined. The diffusion coefficient and coherence level are found to be extremely sensitive to the asymmetry of the potential. It is established that at the values of the external force, for which the enhancement of diffusion is most rapid, the level of coherence has a wide plateau at low temperatures with the value of the Peclet factor 2. An interpretation of the amplification of diffusion in comparison with free thermal diffusion in terms of probability distribution is proposed.Comment: To appear in PR

Review state-of-the-art and ongoing research and managerial activities related to emerging benthic HABs in ICES countries

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Changes in toxins, intracellular and dissolved free amino acids of the toxic dinoflagellate Gymnodinium catenatum in response to changes in inorganic nutrients and salinity

19 páginas, 7 figuras, 2 tablas.The paralytic shellfish poison prducing dinoflagellate Gymnodiniun catemrum was subjected to changes in salinity, phosphate, ammonium and nitrate using continuous culture and batch culture methods. In contrast with other algae, this species showed very slow changes in the concentration of intracellular amino acids, in the Gln:Glu ratio, and, in contrast with Alrsandnum spp., only slow changes in toxin content, during such events as N-feeding of Ndeprived cells or during nutrient deprivation. This organism was found to be very susceptible to disturbance; maximum growth rates around 0.25–0.3 day–1 with a minimum C:N mass ratio of 5.5, were attained when cultures were only disturbed by sampling once a day. P-deprived cells were larger (twice the usual C content of 4 ng C cell–1 and volume of 20 pl). The content of free amino acids was always low (5% of cell-N), with low contributions made by arginine (the precursor for paralytic shellfish toxins). Cells growing using ammonium had the lowest C:N ratios and the highest proportion of intracellular amino acids as arginine. The toxin profile (equal mole ratios of dcSTX, GTX5, dcGT2/3 C1 and C2, and half those values for C3 and C4) was stable and the toxin concentration varied between 0.2 and 1 mM STX equivalents (highest when ammonium was not limiting, lowest in P-deprived cells, though as the latter were larger toxin per cell was not so variable). Decreased salinity did not result in increases in toxin content. Significant amounts of amino acids (mainly serine and glycine, with a total often exceeding 4 µM) accumulated in the growth medium during batch growth even though the cultures were not bacteria free.This work was funded by the Natural Environment Research Council (UK) through grants to K.J.F. and a studentship to E.H J. We also acknowledge funding received from the Spanish CICYT: projects MAR95-1791 to B.R. and ALI95- 1012-C05-01 to J.M.F.; the IEO-ESF grant to M.I.R., and the scholarship from Xunta de Galicia which funded M.I.R.'s visit to Swansea.Peer reviewe

Phase space reduction of the one-dimensional Fokker-Planck (Kramers) equation

A pointlike particle of finite mass m, moving in a one-dimensional viscous environment and biased by a spatially dependent force, is considered. We present a rigorous mapping of the Fokker-Planck equation, which determines evolution of the particle density in phase space, onto the spatial coordinate x. The result is the Smoluchowski equation, valid in the overdamped limit, m->0, with a series of corrections expanded in powers of m. They are determined unambiguously within the recurrence mapping procedure. The method and the results are interpreted on the simplest model with no field and on the damped harmonic oscillator.Comment: 13 pages, 1 figur

Driven diffusion in a periodically compartmentalized tube: homogeneity versus intermittency of particle motion

We study the effect of a driving force F on drift and diffusion of a point Brownian particle in a tube formed by identical ylindrical compartments, which create periodic entropy barriers for the particle motion along the tube axis. The particle transport exhibits striking features: the effective mobility monotonically decreases with increasing F, and the effective diffusivity diverges as F → ∞, which indicates that the entropic effects in diffusive transport are enhanced by the driving force. Our consideration is based on two different scenarios of the particle motion at small and large F, homogeneous and intermittent, respectively. The scenarios are deduced from the careful analysis of statistics of the particle transition times between neighboring openings. From this qualitative picture, the limiting small-F and large-F behaviors of the effective mobility and diffusivity are derived analytically. Brownian dynamics simulations are used to find these quantities at intermediate values of the driving force for various compartment lengths and opening radii. This work shows that the driving force may lead to qualitatively different anomalous transport features, depending on the geometry design