Molecular sorting by stochastic resonance

Abstract

To sort a targeted species from a mixture, we introduce a procedure that relies on the enhancement of its effective diffusion coefficient. We use the formation of a host-guest complex between ␣-cyclodextrin and a dye to evidence the dye dispersion when the medium is submitted to an oscillating field. In particular, we demonstrate that the effective diffusion coefficient of the dye may be increased far beyond its intrinsic value by tuning the driving field frequency in the stochastic resonance regime. We use this effect to selectively sort from a mixture a dye that is addressed by its rate constants for association with ␣-cyclodextrin.S orting molecules from a mixture is an essential issue in chemistry (1). A radical answer consists in using ''universal'' chromatography columns, which would ideally resolve all of the mixture components. Then the counterpart is to reach a sufficient resolution. Affinity chromatography is another approach that can be envisaged when one is interested in extracting components that exhibit a given reactivity. For example, one can search in a mixture {C i } for a free molecule C i that targets a receptor P to give a bound state C i P according to the reactionIn the following, we consider association of dyes (C i ; i ϭ 1 or 2) with ␣-cyclodextrin (P) to form host-guest complexes (C i P) ( b are, respectively, associated to the forward and the backward reaction 1: Those components that do not react are easily discarded, whereas the others are separated according to their affinity for the target. With respect to ''universal'' columns, selective addressing with a chemical reaction reduces the mixture to a smaller pool, making separation less resolution-demanding.To enlarge selectivity and facilitate molecular sorting that implies control of the motion of the mixture components, we propose a chromatography procedure that is related to affinity chromatography but that determines sorting by the kinetics of both binding and release, as described by k i f and k i b . In fact, the rate constants of the mixture components often differ to a larger extent than their association constants. For example, the dyes C 1 and C 2 exhibit a similar affinity for the ␣-cyclodextrin P: K 1 ͞K 2 ϭ 0.34 at 283 K (2, 3). In contrast, k 1 f ͞k 2 f ϭ 0.021 and k 1 b ͞k 2 b ϭ 0.063 at the same temperature. A chromatography based on differences in rate constants may therefore improve selectivity in comparison with traditional approaches that rely on differences in equilibrium properties (1, 6) (e.g., association constants). In addition, discrimination among components that exhibit the same association constant but different rate constants becomes possible.To simultaneously emphasize on kinetic properties and induce molecular motion, we apply a uniform time-periodic field (e.g., an electric field) with zero average value having a period tuned to the dynamics of the reaction involved in selective addressing (here reaction 1) (7-9). We predicted that field-sensitive species C i submitted to reaction 1 with receptor P in...

Taylor dispersion with adsorption and desorption

“…This expression constitutes one of the main results of the present work. It extends in particular the results known in absence of adsorption and desorption at the wall [12] (recovered in the limits Y → 0 or Z → ∞) and in the case of an infinitely well stirred limit in the transverse direction (corresponding to X → 0) [10,12,14,15], with an adsorption rate 2k a /L.…”

“…However, in the important case where the components of the mixture have different kinetic rates k a and k d but similar partitioning coefficients k a /k d L, such a separation is impossible with a stationary flow, since P stat (b), hence x(t) , of all species are similar. As we now proceed to show, efficient sorting can nevertheless be achieved in this case by resorting to an oscillatory flow and exploiting the stochastic resonance between the exchange kinetics and the flow, thereby extending the idea put forward by Alcor et al in another context [14,15].…”

Accounting for adsorption and desorption in lattice Boltzmann simulations

“…Recently, the great interest in nanofluidic devices and in the transport in heterogeneous porous media has also raised the issue of the relevance of models which do not take into account these sorption processes. Moreover, it was recently shown that stochastic resonance between these processes and some external field may be of practical importance, e.g., for molecular sorting [5,8].…”

Photoswitching Kinetics and Phase‐Sensitive Detection Add Discriminative Dimensions for Selective Fluorescence Imaging

“…Herein, we exploit the unique discriminative kinetic parameters [13,14] of photoswitchable fluorescent probes to explore an alternative approach for selective and quantitative imaging. Our strategy relies on a periodically modulated monochromatic illumination with average light intensity and modulation frequency tuned so that the amplitude of the outof-phase fluorescence response extracted from phase-sensitive detection results only from a fluorescent probe of interest, thus enabling extraction of its signal from a complicated fluorescence signal composed of several emitters.…”