Kinetics of the iodine- and bromine-mediated transport of halide ions: demonstration of an interfacial complexation mechanism (original) (raw)
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Shape of the potential energy barrier of the iodine-mediated halide transport
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1995
Voltage-clamp experiments were performed on lipid bilayer membranes to study the voltage dependence of the iodine-mediated halide transport. Under all experimental conditions only one exponential current relaxation, apart from the capacitive spike, could be resolved up to a clamp voltage of 200 mV. The current relaxation could be described by an initial conductance, Go, the relaxation time constant, T, and the relaxation amplitude, a, that is the difference between the initial current, I0, and the steady state current, I~, divided by the steady state current. The occurrence of one single exponential relaxation suggested that one of the different transport steps involved in the carrier-mediated ion transport according to the L~iuger-model is always in equilibrium. This is most probably the transport of the free carriers across the membrane. The voltage dependence of Go, ~-, and of o~ were used to determine the voltage dependence of the translocation rate constants of the complexed carriers, kAs. In the case of the iodine-mediated iodide transport, Go, r and o~ were only mediate voltage-dependent, which means the voltage dependent translocation of the complex encounters a trapezoidal barrier shape. For the iodine-mediated bromide translocation Go, T and o~ exhibited no dependence on the applied clamp-voltage, which suggested that a square Nernst-Planck barrier limits the transport of the corresponding complex.
(Thio)urea Resorcinarene Cavitands. Complexation and Membrane Transport of Halide Anions
The Journal of Organic Chemistry, 1998
Reaction of aminomethylcavitands with iso(thio)cyanates gives (thio)urea-functionalized resorcinarene cavitands, which represent a novel class of neutral anion receptors. The complexation of halide anions has been studied both with infrared and 1 H NMR spectroscopy. The receptors have a small preference for chloride over the other halides; p-fluorophenylthiourea cavitand 8a gives the highest association constant (K ass) 4.7 × 10 5 M-1 with chloride in CDCl 3). A cooperative effect of the ligating (thio)urea moieties is indicated by the lower affinity of the corresponding tris(thio)urea-functionalized cavitands. For the first time facilitated membrane transport of halide anions through supported liquid membranes is achieved.
Facilitated transport of halides through Nafion ionomer membrane modified with lanthanide complexes
Journal of Membrane Science, 1998
Permeation of chloride and bromide through Na®on TM 117 modi®ed with hydrophobic metal complexes of Eu 3 and Pr 3 with thenoyl tri¯uoro acetone (TTA), -isopropyl tropolone (IPT) and 8-hydroxyquinoline (oxine) has been studied. The complexes were precipitated within the polymer bed with an aqueous±alcoholic solution of the reagents at a pH between 5 and 6. The permeation¯uxes of the halides have been calculated by measuring the concentrations of the anions in the receiving solutions using ion chromatography. The high¯ux values have been attributed to the direct coordination of the inorganic anions to the central metal ions in their complexes. The chloride ion having a smaller radius and higher free energy of hydration as compared to bromide, showed higher permeation. The cations associated with the corresponding anion is also transported along with the anion. The size of the accompanying cation has a strong in¯uence on anion permeation. # 1998 Published by Elsevier Science B.V.
Biophysical Journal, 1998
Two alternative mechanisms are frequently used to describe ionic permeation of lipid bilayers. In the first, ions partition into the hydrophobic phase and then diffuse across (the solubility-diffusion mechanism). The second mechanism assumes that ions traverse the bilayer through transient hydrophilic defects caused by thermal fluctuations (the pore mechanism). The theoretical predictions made by both models were tested for halide anions by measuring the permeability coefficients for chloride, bromide, and iodide as a function of bilayer thickness, ionic radius, and sign of charge. To vary the bilayer thickness systematically, liposomes were prepared from monounsaturated phosphatidylcholines (PC) with chain lengths between 16 and 24 carbon atoms. The fluorescent dye MQAE (N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide) served as an indicator for halide concentration inside the liposomes and was used to follow the kinetics of halide flux across the bilayer membranes. The observed permeability coefficients ranged from 10 Ϫ9 to 10 Ϫ7 cm/s and increased as the bilayer thickness was reduced. Bromide was found to permeate approximately six times faster than chloride through bilayers of identical thickness, and iodide permeated three to four times faster than bromide. The dependence of the halide permeability coefficients on bilayer thickness and on ionic size were consistent with permeation of hydrated ions by a solubility-diffusion mechanism rather than through transient pores. Halide permeation therefore differs from that of a monovalent cation such as potassium, which has been accounted for by a combination of the two mechanisms depending on bilayer thickness.