Permeation Of Neutral Molecules Research Articles

Role of Electroosmosis in the Permeation of Neutral Molecules: CymA and Cyclodextrin as an Example

To quantify the flow of small uncharged molecules into and across nanopores, one often uses ion currents. The respective ion-current fluctuations caused by the presence of the analyte make it possible to draw some conclusions about the direction and magnitude of the analyte flow. However, often this flow appears to be asymmetric with respect to the applied voltage. As a possible reason for this asymmetry, we identified the electroosmotic flow (EOF), which is the water transport associated with ions driven by the external transmembrane voltage. As an example, we quantify the contribution of the EOF through a nanopore by investigating the permeation of α-cyclodextrin through CymA, a cyclodextrin-specific channel from Klebsiella oxytoca. To understand the results from electrophysiology on a molecular level, all-atom molecular dynamics simulations are used to detail the effect of the EOF on substrate entry to and exit from a CymA channel in which the N-terminus has been deleted. The combined experimental and computational results strongly suggest that one needs to account for the significant contribution of the EOF when analyzing the penetration of cyclodextrins through the CymA pore. This example study at the same time points to the more general finding that the EOF needs to be considered in translocation studies of neutral molecules and, at least in many cases, should be able to help in discriminating between translocation and binding events.

The Permeation of Neutral Molecules, Ions, and Ionic Species Through Membranes: Brain Permeation as an Example

Known descriptors for ions and ionic species (deprotonated carboxylic acid anions and protonated base cations) can be combined with descriptors for neutral species to obtain a general equation for permeation from saline through the blood-brain barrier. The equation can be used to indicate the compound factors that influence permeation and to show how substituent effects on compounds may be altered to increase or decrease the rate of permeation. It is shown that log PS values for carboxylate anions are about 2.0 log units less than those for the neutral carboxylic acids, and that log PS values for protonated base cations are about 1.0 log units less than those for the neutral bases. It is possible to use the obtained general equation to calculate log PS for an acid or a base that is either totally ionized or partly ionized in the saline perfusate. The same method can be used to analyze water-artificial membrane partition coefficients of a set of compounds that include ionized species.

Permeation of macromolecules into polyelectrolyte microcapsules.

Polyelectrolyte microcapsules (PEMCs) have been prepared by coating red blood cells with the polyelectrolytes poly(styrenesulfonate), poly(allylamine hydrochloride), and dextran sulfate applying the layer-by-layer technique with subsequent dissolution of the core. The capsule permeability for human serum albumin (HSA) was studied as a function of the ionic strength and pH by means of confocal microscopy. PEMCs produced with dextran sulfate and poly(allylamine hydrochloride) show a significant increase in permeability for HSA at salt concentrations over 1 mM. For PEMCs prepared with poly(styrenesulfonate) and poly(allylamine hydrochloride) the limiting salt concentration is 5 mM. No pH dependence for permeation was observed. A correlation between the permeation and adsorption of HSA on the PEMC walls was investigated. Finally, a mechanism for the permeability, combining electrostatic interactions, and the presence of pores in the polymer layers is presented confirmed by the considerable increase of permeation of charged molecules in the presence of salt and the permeation of neutral molecules regardless of the ionic strength.

Characterization of organosilasesquioxane-intercalated-laponite-clay modified electrodes and (bio)electrochemical applications

Electrode surface modification by organo-inorganic layered coatings can be achieved readily by drying a completely delaminated laponite clay sol mixed with polycationic silasesquioxane oligomers. Oligosilsesquioxanes were synthesized by hydrolytic polycondensation of trialkoxysilanes bearing an alkylamino or alkyltrimethylammonium function. Films of excellent quality with exceptional adhesion and mechanical properties can be obtained. The intercalation of organosiloxane oligomers is accompanied by the expansion of the film and by the existence of a mesoporosity as shown by XRD measurements and N 2 adsorption experiments. The ion-exchange properties of the resulting coatings as well as permeation of neutral molecules were studied in aqueous and non-aqueous electrolytes as a function of the oligomer loading. For oligomer loadings higher than the cation-exchange capacity (cec) of laponite, the coatings behave as anion-exchangers which allows the binding a wide range of redox anions. Incorporated anions remain electroactive not only in aqueous but also in non aqueous electrolytes as a consequence of a fixed pore size and permanent interlayer spacing of oligomer-expanded laponite. On the other hand intercalation of oligomers allows us to modulate the permeability of the coating as shown by permeation experiments using neutral electroactive probes in non aqueous electrolytes. In the field of electroanalysis, amperometric biosensors made by the entrapment of glucose oxidase inside the hybrid material have been investigated successfully. Such enzymatic films exhibit enhanced analytical performances as compared with those obtained using native sodic laponite. The potential applications of this new hybrid material in the field of electrocatalysis have been exemplified by the electroprecipitation of catalytic nanoparticles such as Pt(0) obtained from the incorporation of the anionic precursor PtCl 4 2− .

Permeation of neutral molecules through calcium channel in sarcoplasmic reticulum vesicles.

Permeation of neutral molecules as well as Ca2+ through the Ca2+ channel in sarcoplasmic reticulum vesicles has been studied by the tracer and/or by the light scattering methods. In the absence of KCl, the Ca2+ channel was found not to be able to pass neutral molecules such as glucose, xylose, and glycine under the condition that the channel was open, although the channel could pass Ca2+. On the other hand, submolar concentrations of KCl made the channel become permeable to neutral molecules as well as Ca2+. Since the effect of KCl could be replaced by NaCl and KNO3, but not by sucrose and glucose, this effect of KCl is considered to be due to ionic strength and not to osmotic pressure. These results suggest that low ionic strength transforms the Ca2+ channel protein in such a manner as to block the permeation of neutral molecules without modifying the gating mechanism of the channel.